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Host specificity, release, and establishment of the gorse spider mite, Tetranychus lintearius Dufour (Acarina: Tetranychidae), for the biological control of gorse, Ulex europaeus L. (Fabaceae), in Australia
Biological Control 26 (2003) 117–127 www.elsevier.com/locate/ybcon
Host specificity, release, and establishment of the gorse spider mite, Tetranychus lintearius Dufour (Acarina: Tetranychidae), for the biological control of gorse, Ulex europaeus L. (Fabaceae), in Australia J.E. Ireson,a,* A.H. Gourlay,b R.M. Kwong,c R.J. Holloway,a and W.S. Chattertona a Tasmanian Institute of Agricultural Research, 13 St. JohnÕs Avenue, New Town, Tasmania 7008, Australia Landcare Research New Zealand, Canterbury Agriculture and Science Centre, P.O. Box 69, Gerald St., Lincoln, New Zealand Department of Natural Resources and Environment, Keith Turnbull Research Institute, P.O. Box 48, Frankston, Vic. 3199, Australia b
c
Received 7 December 2001; accepted 25 September 2002
Abstract The gorse spider mite, Tetranychus lintearius, is widespread on Ulex europaeus in Europe and its foliage-feeding activities can cause severe damage to plants. It has been released as a biological control agent for U. europaeus in New Zealand, the United States, and Chile following tests on over 100 plant species or cultivars confirming its host specificity to Ulex species. The suitability of T. lintearius for release in Australia was tested on an additional 32 plant species or cultivars from the family Fabaceae that occur in Australia. The tests confirmed that T. lintearius was a genus-level specialist and would be unable to survive on any plant other than U. europaeus in Australia. The first Australian field releases were carried out in Tasmania and Victoria in December 1998. Since then, field releases from laboratory cultures at 116 sites in Tasmania and 90 sites in Victoria have resulted in T. lintearius initially becoming well established in both states. However, examination of T. lintearius colonies in Tasmania and Victoria has shown that predation by the introduced Chilean predatory mite, Phytoseiulus persimilis and the native coccinellid, Stethorus histrio, is already widespread. These predators have the potential to significantly suppress populations of T. lintearius and severely restrict its usefulness as a biological control agent in Australia. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Biological control; Gorse; Ulex europaeus; Tetranychus lintearius; Host specificity; Predation; Phytoseiulus persimilis; Stethorus spp.; Australia
1. Introduction Gorse, Ulex europaeus L. (Fabaceae), is a native of central and western Europe (Parsons and Cuthbertson, 2001) where it occurs in native heathland (Tubbs, 1974) and on disturbed or neglected farmland and forests (Zw€ olfer, 1962). It now occurs in most temperate regions of the world but has become a serious weed in Australia, Chile, Hawaii, New Zealand (Richardson and Hill, 1998), in the Pacific Coast States of the USA (Parsons and Cuthbertson, 2001), and in British Columbia, Canada. * Corresponding author. Fax: +613-6278-2716. E-mail address: [email protected] (J.E. Ireson).
European colonists introduced gorse into Australia during the early 1800s primarily as a hedge plant or ornamental but there was also interest in its potential use as a fodder. It was recorded as being grown in New South Wales in 1803 and was listed in a Tasmanian nursery catalogue in 1845 (Parsons and Cuthbertson, 2001). Its weed status was recognised in Victoria about 50 years later (Ewart, 1909). Gorse now occurs in all Australian states but it is particularly common in southeastern Australia. Its weed status appears to be related to latitude as the main problem regions are principally in Victoria and Tasmania (Parsons and Cuthbertson, 2001). Gorse is a spiny perennial shrub, which can grow to a height and diameter in excess of 3 m and may live to
1049-9644/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 1 0 4 9 - 9 6 4 4 ( 0 2 ) 0 0 1 2 8 - 7
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almost 30 years (Lee et al., 1986). Seeds may survive in the soil for a similar period (Moss, 1959). Flowers are bright yellow, approximately 20 mm long and borne all over the plant. Flowering occurs in spring/early summer and autumn/early winter, although flowers may be present at other times if climatic conditions suit. Brown to black seed, about 4 mm long, is produced in grey, hairy pods that hold ca. 3–5 seeds. Dispersal is mainly by the seeds either falling below the parent plant or by pod dehiscence, which results in the seed being ejected several metres away. Biological control of gorse in Australia began with the introduction of the gorse seed weevil, Exapion ulicis (Forster) (Coleoptera: Brentidae). Originally from Europe, this agent was introduced into Tasmania from New Zealand in 1939 (Evans, 1942). Although now widespread in Tasmania and parts of the mainland, its impact is restricted, probably because gorse flowers twice a year in these areas and larvae are not active when the autumn seed crop matures (Cowley, 1983; Hill et al., 1991a). A new program for the biological control of gorse was initiated in New Zealand during the late 1970s using information on European phytophagous invertebrate species associated with Ulex spp. (Hill, 1982) based on surveys by Zw€ olfer (1963, 1964). Of the 94 invertebrate species known to feed on gorse, the few considered sufficiently host specific to warrant further investigation as biological control agents included the gorse spider mite, Tetranychus lintearius Dufour (Acarina: Tetranychidae) (Hill, 1983). Zw€ olfer (1963) first identified T. lintearius as a potential control agent for gorse after observing large colonies causing severe damage on mature foliage. Subsequent investigations showed that T. lintearius was a distinct, recognizable species (Stone, 1986) that was reproductively isolated from other closely related species (Hill and OÕDonnell, 1991a). In addition, host specificity tests with about 70 species or cultivars of plants (Hill and OÕDonnell, 1991b) showed that it could survive only on gorse in the field. T. lintearius was therefore considered safe to release in New Zealand and first introduced there in 1989 (Hill et al., 1989). Additional tests on 51 plant species or cultivars confirmed these results and enabled the introduction of T. lintearius into Hawaii, Oregon, and California (USA) (Hill, 1990; Hill, 1991; Hill and Gourlay, 1993). T. lintearius is now widely established in New Zealand, Chile, Hawaii, and in the Pacific Coast states of the USA. However, several predatory species have now been recorded attacking T. lintearius populations in Chile, New Zealand, and the USA. In Chile, a beetle, Ologota sp. (Coleoptera: Staphylinidae) has been recorded attacking colonies of T. lintearius at every release site but has not prevented establishment and no species of predatory mite has been
recorded (Norambuena et al., 2000). In New Zealand, Hill et al. (1991b) noted that predation by the endemic beetle, Stethorus bifidus (Kapur) (Coleoptera: Coccinellidae), was common. Subsequent studies by Peterson et al. (1994) showed that S. bifidus had the ability to effectively regulate T. lintearius populations at some sites. Hill et al. (1991b) also recorded the Chilean predatory mite, Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae), a specialist predator of Tetranychus species (McMurtry and Croft, 1997), feeding on T. lintearius colonies, usually in the vicinity of horticultural developments. Although Hill et al. (1991b) noted that P. persimilis had eradicated laboratory cultures of T. lintearius in New Zealand, there was no evidence that it was playing a major role in suppressing field populations. However, subsequent exclusion experiments by Pratt et al. (in press) were used to demonstrate that P. persimilis and possibly other phytoseiid predators were able to significantly reduce T. lintearius field populations in Oregon, USA. Phytoseiulus persimilis and the endemic Stethorus histrio Chazeau and Stethorus vagans (Blackburn) are common predators of Tetranychus pest species in Tasmania and Victoria, so there was some concern as to their impact on the establishment of T. lintearius following its release in December 1998 (Ireson et al., 1999). This paper presents the results of additional hostspecificity tests on 32 Australian plant species, which enabled the release of T. lintearius in Australia. In addition, the results of post-release surveys to determine the success of establishment of T. lintearius and the occurrence of predators in Tasmania and Victoria are reported. The possible impact of the predators is discussed.
2. Materials and methods 2.1. Test plant species The safety of introducing T. lintearius into Australia was determined by examining its oviposition and survival on 32 plant species or cultivars including gorse. The list of plants used (Table 1) consisted of species or cultivars occurring in Australia that are closely related to gorse in the tribe Genisteae as well as other tribes in the family Fabaceae. The choice, approved by the Australian Quarantine and Inspection Service, was based on the centrifugal strategy detailed by Wapshere (1974) and by using an interpretation of the relationship between the tribes of the Fabaceae by Polhill (1981). In the tribe Genisteae, Cytisus palmensis (Christ.) Bisby and Nicholls and the cultivars of Lupinus angustifolius L. are important in agriculture; Cystisus sp. var. racemosus Hort. is grown as an ornamental. All test plants in the tribes Phaeseoleae and Trifoliae are of importance
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119
Table 1 Survival of T. lintearius females and their progeny on 32 species of Australian test plants Oviposition ratea
Larvae produced
3:3 0:6 0
4.1 0
f
f
0
0
0:3 0:6
0
0.5
f
0
1:7 1:5 0 0:3 0:6
0:3 0:6 0 0
0 0 0
0.4 0 0.1
0 0
0 0 0
Tribe Bossiaeeae Bossiaea buxifolia Cunn.c Goodia lotifolia Salisb.c Hovea acutifolia Cunn.c Platylobium formosum Sm.e
1:7 2:1 0 0 0
0 0 0 0
0 0 0 0
0.1 0 0 0
f
0 0 0
0 0 0 0
Tribe Crotalarieae Crotalaria cunninghamii R. Br.d
5.0
0
0
0
0
0
Tribe Indigofereae Indigofera australis Willd.c
0
0
0
0
0
0
Tribe Mirbelieae Aotus ericoides (Vent.) G. Done Daviesia mimosoides R. Br.c Dillwynia glaberrima Sm.e Eutaxia microphylla (R. Br.) J. Blacke Gompholobium huegelii Benth.e Kennedia prostrata R. Br.e Oxylobium ellipticum R. Br.c Pultenaea polifolia Cunn.c Sphaerolobium vimineum Sm.e
1:2 1:3 2:0 1:0 1:2 0:8 0:4 0:6 0:4 0:5 2:6 1:5 0:7 0:6 1:0 1:0 1:6 2:3
0 0 0 0 0 0 0 0 0:6 1:3
0 0 0 0 0 0 0 0 0:6 1:3
0 0 0 0 0 0 0 0 0.3
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Tribe Phaeseoleae Centrosema pubescens Benth.d Lablab purpureus (L.) Sweetd Macroptilium atropurpureum (DC.) Urband Phaeseolus vulgaris L. cv. Brokerc Phaeseolus vulgaris L. cv. Labradorc Phaesolus vulgaris L. cv. Floc Phaeseolus vulgaris L. cv. Rapierc Vigna radiata (L.) Wilczekd
2:3 2:1 1:3 0:6 3:3 0:6 0 2:0 1:0 2:0 1:0 1:3 1:5 1:3 1:2
0 0 0:7 0:6 0 1:0 1:0 1:7 1:5 1:3 1:5 0
0 0 0 0 0 1:7 1:5 1:3 1:5 0
0 0 0 0 0 1.8 0.9 0
0 0 0 0 0
0 0 0 0 0
f
f ;g
1:3 1:5
0
0
0 2:2 2:2
0 0
0 0
Species
Family Fabaceae Subfamily Faboideae Tribe Genisteae Subtribe Genistinae Ulex europaeus L.b Cytisus palmensis (Christ.) Bisby and Nichollsc Cytisus sp. var. racemosus Hort.c Subtribe Lupininae Lupinus angustifolius L. cv. Gungurru.c Lupinus angustifolius L. cv. Yandee.c Lupinus angustifolius L. cv. Merrit.c
Tribe Trifoliae Trifolium subterraneum L. var. ÔDenmarkÕc Subfamily Mimosoideae Tribe Acacieae Acacia dealbata Link.d Acacia mearnsii De wild.e a
Mean female survival (SE) Day 3
Day 10
Day 13 (remaining)
4:6 0:6 0:3 0:6
4:1 0:8 0
3:3 2:1
Eggs per replicate per day. Mean of results obtained from three series of tests. c;d;e Denotes whether plant species used in first, second, or third series of tests. f Affirmative. g Eggs produced by adults but juveniles died. b
f
f
Adults produced
f
f ;g
0
0
0
0
0
0 0
0 0
0 0
120
J.E. Ireson et al. / Biological Control 26 (2003) 117–127
to Australian agriculture and those in the tribes Bossiaeeae, Crotalarieae, Indigofereae, Mirbelieae, and Acacieae are Australian natives and most are grown as ornamentals. 2.2. Location of tests and source of plant material All testing was carried out in the insect quarantine facility at Landcare Research New Zealand, in Lincoln. Two series of oviposition and survival tests involving 23 species or cultivars plus gorse were carried out during 1995 (Table 1). A third series was carried out on an additional 8 species plus gorse during 1997 (Table 1). Plant material used in the tests was field-collected or glasshousegrown in Australia except for Goodia lotifolia Salisb., which was field-collected in New Zealand. Cut shoots of each species of Australian plant were covered at the base with moist paper towelling held in place with elastic bands. Each plant species was then placed separately in a sealed plastic bag. The bags were then placed together in a Polyfoam container and airfreighted overnight to New Zealand. All plant material was received in good condition and used immediately upon arrival. 2.3. Oviposition and survival tests An egg-laying colony of mixed-origin T. lintearius was used in the host specificity tests. This colony was originally collected near Dunedin on the South Island of New Zealand and maintained on potted gorse plants in the laboratory at ambient temperature. The colonies were a mixture of populations that had originated from the United Kingdom, North West Spain, and coastal Portugal (Hill et al., 1993). Well-fed mites continue to lay eggs for several days, even in petri dishes without plant material (Gourlay, unpublished data). To reduce the possibility of a spurious result, egg-laying adult females were removed from the laboratory cultures and kept in 8 6 cm plastic boxes for 2 days without food prior to the tests. For each test, one leaf of each test plant species or cultivar was placed onto saturated cotton wool in open 10 15 cm plastic containers. The wet cotton wool prevented T. lintearius from moving between the test species. Each box contained up to 8 separate species or cultivars. A gorse branch ca. 5 cm in length was included as a control. Three replicates were used for each species in the first two series of tests and five for the third series. Five randomly chosen gravid adult female T. lintearius were placed on each test species (including gorse). The boxes were held at 13 °C overnight, causing the mites to aggregate and settle, then held under a photoperiod of 16 h L:8 h D at 22 °C and 70% RH during the day and 16 °C and 80% RH at night. Any T. lintearius adults that fell into the wet cotton wool within the first 24 h were placed back on the plant
material. Emigration after this period was considered a rejection of the plant and the mites were counted as dead. After 3 days, 10 days, and then every 3 days, leaves were examined under a microscope and the survivorship of adults and immature stages was recorded. The number of eggs laid per replicate per day was calculated at the first examination on the third day. Test plant leaves were replaced if they deteriorated. Adult female T. lintearius were removed from the gorse after a minimum of 13 days because the eggs and juveniles became too numerous to count and had caused severe damage. Five new randomly chosen adult females from within the controls were then transferred onto a new gorse branch. 2.4. Release and establishment During October and November 1998, 6 strains of T. lintearius were imported to Australia from New Zealand. These strains had originally been imported into New Zealand from Cornwall in the United Kingdom (UK), from locations in Spain (Hio, O Grove, Cambados) and Portugal (S^ao Pedro and Viana do Castelo) (Hill et al., 1993). The strains from Spain and Portugal were introduced into New Zealand after the UK strain. This was because they were a closer climatic match and found better adapted to the warmer wetter areas of New Zealand, where the UK strain had originally failed to establish (Hill et al., 1993). The location of the New Zealand collection sites for each of the strains imported into Australia were: UK (43°490 S, 172°100 E); Hio (45°530 S, 170°230 E); O Grove (45°480 S, 170°200 E); Cambados (45°470 S, 170°320 E); S^ao Pedro (45°450 S, 170°360 E); and Viana do Castelo (45°490 S, 170°240 E). All six strains (UK, Hio, O Grove, Cambados, S^ ao Pedro, and Viana do Castelo) were collected from their original separate release sites in New Zealand for importation to Australia. No known mixing of the strains had occurred at these sites. Rearing procedures conducted in Tasmania and Victoria following the introduction of these strains were described by Ireson et al. (1999). Most of the T. lintearius colonies used for the field releases were reared by lodging starter colonies of at least 100 mites onto potted gorse plants about 30 cm high and 20 cm diameter. These cultures were held in controlled temperature glasshouses maintained at 18–20 °C. Mites for field release were removed once the colonies had multiplied to several thousand individuals. Approximately 200 mites, consisting mostly of female deutonymphs and adults, were then transferred by a sable hair brush to 15cm-long gorse cuttings. The cuttings had been inserted into water through a 4-mm-diameter hole in the centre of a screw top lid on a plastic vial 8 cm high and 3 cm in diameter. These colonies were then held in a controlled environment cabinet at 20–24 °C under a photoperiod of 16 h L:8 h D for 4–5 days, prior to field release. This enabled the colonies to be checked for the possible
J.E. Ireson et al. / Biological Control 26 (2003) 117–127
presence of predators, particularly P. persimilis, prior to field release. The infested cuttings were transferred to the release sites in the vials then removed and lodged securely into mature gorse plants. From 2 to 10 cuttings, usually distributed between 2 and 5 bushes in close proximity, were placed at each site. Releases were carried out at 116 sites in Tasmania (Fig. 1) and 90 sites in Victoria (Fig. 2) between December 1998 and April 2001 in all regions where major gorse infestations have been recorded. All six strains were released. Different strains were usually released at separate sites, although in a few instances, some or all of the strains were released at the same site on bushes about 10 m apart. Surveys for the establishment and
121
dispersal of T. lintearius were carried out at 35 of the Tasmanian sites between 12 and 26 months after release during autumn 2000 and the following late springsummer period in 2000/01. In Victoria, 66 sites were monitored between 2 and 30 months after the initial release of mites. Monitoring generally took place during late spring through to autumn each year, commencing in the summer of 1998/99. The dispersal of T. lintearius from the release point was determined by searching for webbing, although, in some cases, small, newly forming colonies were often located up to 20 m further out from the last webbed colony. Radial searches were carried out at each site up to 100 m from the last colony located, or as determined by the distribution of gorse.
Fig. 1. Locations where releases of the gorse spider mite, Tetranychus lintearius ( ), were carried out in Tasmania (December 1998 to April 2001) and locations where Phytoseiulus persimilis ðjÞ and Stethorus histrio ( ) have been identified, in relation to the main gorse infestations (stippling) and the 800 mm, 1600 mm, and 2400 mm annual isohyets.
122
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Fig. 2. Locations where releases of the gorse spider mite, Tetranychus lintearius ( ), were carried out in Victoria (December 1998 to April 2001) and locations where Phytoseiulus persimilis ðjÞ and Stethorus histrio ( ) have been identified.
Colonies were checked for the presence of P. persimilis and Stethorus spp. in the vicinity of the central release point using a dissecting microscope. Older webbed sections of gorse stems ca. 10 cm in length containing mostly eggs, larvae, or protonymphs (stages preferred by P. persimilis) were removed and examined in situ. Newly aggregating colonies of mites were avoided during these examinations because of their possible repellent effect on predators. If no predators where detected after examination of a maximum of 10 stem sections, a negative result was recorded, i.e., predators were not considered present at the site at that time. Voucher specimens of predators collected at each site were placed in tubes of 70% alcohol and returned to the laboratory for identification. The identity of P. persimilis was checked using the key of Schicha (1987). Stethorus specimens were identified using the key of Houston (1980). Relevant temperature and rainfall data for the Tasmanian release sites were obtained through the Australian Bureau of Meteorology.
tested (Table 1). F1 and F2 generations were produced on gorse ca. every 30 days. Adult female mites did not survive longer than 10 days or lay eggs on 24 of the remaining 31 test plants (Table 1). Adult female mites died within 13 days and all eggs and juveniles produced died within 28 days on Cytisus sp. var. racemosus, L. angustifolius cv. gungurru, L. angustifolius cv. Merrit, and Bossiaea buxifolia Cunn. Adult mites survived for 16 days on one replicate of Sphaerelobium vimineum Sm. and 34 eggs were laid on this plant, but only two eggs hatched and both mites died as larvae. Mite survival and oviposition on Phaseolus vulgaris L. varied between cultivars. On P. vulgaris cv. Broker, adult mites failed to survive 3 days and laid no eggs. On the other three cultivars, adult mites survived to a maximum of 10 days on cv. Labrador (3 adults), 19 days on cv. Flo (1 adult), and 28 days (1 adult) on cv. Rapier (Table 1). Only the eggs laid on cv. Flo and cv. Rapier hatched. Some immatures survived on these two cultivars to produce an F1 generation which also successfully produced eggs, but all died as juveniles.
3. Results 3.2. Field establishment 3.1. Host specificity No species or cultivar was an alternative host to gorse for T. lintearius. Oviposition rates were much higher on the gorse controls in comparison to the other species
In Tasmania, T. lintearius was recovered at all release sites. Colonies had increased in number, as indicated by the conspicuous webbing that often covered entire bushes. Individual colonies were often large, containing up
J.E. Ireson et al. / Biological Control 26 (2003) 117–127
to several hundred thousand mites. However, there was considerable variation in dispersal. Dispersal ranged from 5 m to 20 km at the 10 sites surveyed 22–27 months after release and from 3 m to 2 km at the 25 sites surveyed 12–17 months after release (Table 2). The sites at Zeehan (mean annual temperature 10.7 °C) and Hampshire (mean annual temperature 9.7 °C), where the lowest dispersal was recorded, are situated in the cool, wet areas of Western and North Western Tasmania, where the mean annual rainfall ranges from ca. 1600–2400 mm (Fig. 1). The sites where the highest dispersal was recorded are situated in warmer, drier areas where the mean annual rainfall is 600–800 mm (Fig. 1) and mean annual temperatures range from ca. 10.5 °C at Montana in the northern midlands to 13.6 °C at Bicheno on the East Coast (Fig. 1). The Cressy site (Fig. 1) where the highest dispersal was recorded, is not the warmest locality (mean annual temperature 11.2 °C), but is one of the driest
123
(mean annual rainfall 637 mm) and dispersal followed the direction of prevailing winds from the north and northwest. An assessment of the performance of the different strains was beyond the scope of this study but there was no indication from this survey that any strain will be more successful. Two contrasting sites were surveyed where all strains had been released together in close proximity (the relatively dry location at Perth and at Zeehan 3 where the rainfall is highest) (Fig. 1). All strains had formed many large viable colonies around the respective release points at the drier Perth site but none had dispersed more than 10 m. At the wetter Zeehan 3 site, all strains had dispersed no further than 5 m, although it was visually evident that colonies were fewer and smaller than at Perth. In Tasmania, S. histrio and/or P. persimilis were observed feeding on T. lintearius colonies at all but 8 sites (Table 2). Stethorus histrio was identified at 17 release sites, most of which were located inland (Fig. 1).
Table 2 Survey of 35 sites in Tasmania for the presence of T. lintearius and associated predators 12–27 months after release Locality
Mite strain releaseda
Date of survey
Months since release
Maximum estimated dispersal
Predator (s) foundb
Zeehan 1 Cressy Stonehenge Stanley Hwy Montana Bicheno Little Swanport Hampshire 1 Stanley Nut Campbell Town 1 Ross 1 Jericho Greens Beach Zeehan 2 Franklin Zeehan 3 Oatlands 1 Ross Oatlands 2 Woodbury Jericho Oatlands 3 Lucaston Hampshire 2 Hampshire 3 Campbell Town 2 Lymington Campbell Town 3 PetcheyÕs Bay Cranbrook Fingal Avoca Ormley Blackmans Bay Perth
U U U U U U U U U U U O O S V All C C S H U V S O, H, U S, C, V H H U O C U H S V, C All
14/3/01 1/2/01 16/2/01 15/3/01 24/1/01 8/2/01 8/2/01 15/3/01 13/12/00 16/11/00 22/2/01 1/2/01 1/3/00 14/3/01 10/1/01 14/3/01 30/11/00 24/1/01 30/11/00 30/11/00 30/11/00 30/11/00 18/1/01 15/3/01 15/3/01 30/11/00 28/11/00 30/11/00 28/11/00 1/12/00 7/12/00 7/12/00 7/12/00 7/11/01 1/12/00
27 26 26 26 25 24 24 24 23 22 17 17 16 16 16 16 15 15 15 15 15 15 15 15 15 14 14 14 14 13 13 13 13 13 12
5m 20 km 4 km 500 m 500 m 2 km 1 km 50 m 200 m 800 m 2 km 100 m 100 m 60 m 60 m 5m 100 m 100 m 80 m 75 m 50 m 30 m 30 m 5m 3m 180 m 50 m 30 m 30 m 280 m 100 m 80 m 50 m 50 m 10 m
N S S, S, S, P P S P S, N N N S P S N N S N S S P S S N P S P P P S S P S,
a
C, Cambados, H, Hio; O, O Grove; S, S^ao Pedro; V, Viana do Castelo; U, UK; All, all strains; released ca. 10 m apart. b N, No predators found; S, Stethorus histrio; P, Phytoseiulus persimilis.
P P P
P
P
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Table 3 Survey of 66 sites in Victoria for the presence of T. lintearius and associated predators 2–30 months after release Locality
Mite strain releaseda
Date of survey
Months since release
Maximum estimated dispersalb
Predator (s) foundc
Hastings 1 Barfold Digby 1 Digby 2 Digby 3 Digby 4 Digby 5 Taradale 1 PiperÕs Creek Spring Hill Hastings 2 Hastings 3 Tyabb 1 Baxter Taradale 2 Somerville Cranbourne 1 Cranbourne 2 Castlemaine 1 Castlemaine 2 Yendon 1 Darraweit Guim Queenscliff Yendon 2 Castlemaine 3 Lancefield Langley Grenville 1 Grenville 2 Cornish Hill Malmsbury Cranbourne 3 Cranbourne 4 CannonÕs Creek 1 CannonÕs Creek 2 CannonÕs Creek 3 Blind Blight Moorooduc 1 Moorooduc 2 Moorooduc 3 Portland 1 Portland 2 Portland 3 Creswick 1 Creswick 2 Corindhap 1 Corindhap 2 Snake Valley 1 Snake Valley 2 Somers Linton 1 Linton 2 Moorooduc 4 Bittern 1 Sandy Point Carrum Downs Tyabb 2 Bittern 2 Balnarring Castlemaine 4 Maldon Crib Point
O U O C V S H U O H H C O H, C U U V S V S S U U O V U U O O O S S U C H V O H S V V U C U U H H S S C C C C V S Mixed H U O O S H,V
04/6/01 16/1/01 23/4/01 23/4/01 23/4/01 23/4/01 23/4/01 16/1/01 16/1/01 16/1/01 18/12/00 18/12/00 11/1/01 11/1/01 16/1/01 10/1/01 10/1/01 10/1/01 22/11/00 22/11/00 23/11/00 7/5/01 3/5/01 23/11/00 24/7/01 16/1/01 16/1/01 31/5/01 31/5/01 16/1/01 28/12/00 10/1/00 10/1/01 10/1/01 10/1/01 10/1/01 10/1/01 11/1/01 11/1/01 11/1/01 24/4/01 24/4/01 24/4/01 29/11/00 29/11/00 27/11/00 27/11/00 23/11/00 23/11/00 10/1/01 23/11/00 23/11/00 11/1/01 10/1/01 10/1/01 10/1/01 10/1/01 11/1/01 11/1/01 24/7/01 24/7/01 10/1/01
30 25 23 23 23 23 23 23 22 20 20 20 22 21 21 21 20 20 20 20 18 17 17 16 15 13 13 12 12 9 8 8 8 8 8 8 8 8 8 8 8 8 8 7 7 7 7 7 7 7 7 6 6 6 6 5 5 5 5 5 5 4
1 km 850 m 50 m NE NE NE NE 200 m 1m 28 m 1m 4m 64 m 20 m 1 km 10 m 17m 40m 2m 20 m 40 m 5m 500 m 25 m 4 km 6m 1m 5m 200 m 1m 1m 20 m 8m 18 m 30 m 10 m 10 m 23 m 40 m 26 m 2m 20 m 10 m 2m 1m 100 m 1m 100 m 5m 2m 20 m 1m 75 m 7m NE 1m NE 1m 5m 15 m 10 m NE
P N N N N N N N P S N N S N N N P P P P S P N N N N P N N N P P N P N N N N N N N N N N N P P N N N N N N N N S N P P N N N
J.E. Ireson et al. / Biological Control 26 (2003) 117–127
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Table 3 (continued) Locality
Mite strain releaseda
Date of survey
Portarlington Chewton 1 Chewton 2 Phillip Island
O U S H
3/5/00 24/7/01 24/7/01 18/5/99
Months since release 4 3 3 2
Maximum estimated dispersalb
Predator (s) foundc
100 m 2m 1m 20 m
N N N N
a
C, Cambados; H, Hio; O, O Grove; S, S^ao Pedro; V, Viana do Castelo; U, UK; Mixed, interbred mix of all strains. NE, Not established. c N, No predators found; P, Phytoseiulus persimilis; S, Stethorus histrio. b
P. persimilis was identified at 15 release sites, most of which where located near coastal townships (10 sites) in the vicinity of a horticultural development or in similar situations further inland (Fig. 1). Although P. persimilis was not collected from the cooler, wetter sites in Western and North Western Tasmania (Fig. 1), adult mites were observed actively feeding on T. lintearius colonies at Montana and Stonehenge (Fig. 1) in frostprone areas during midwinter. In addition, at Stanley, Bicheno, PetcheyÕs Bay, and Cranbrook (Fig. 1) large numbers of P. persimilis were observed actively feeding on eggs and immature stages of T. lintearius. On some branches, it was suspected that P. persimilis was responsible for the destruction of established T. lintearius colonies as it was observed feeding amongst the webbing on the few eggs or immature stages that remained. In Victoria, T. lintearius colonies were detected at 59 of the 66 sites monitored (Table 3) in similar densities to that recorded in Tasmania. T. lintearius was not detected at the other 7 sites. Dispersal also varied considerably and ranged from 1 m to 4 km at 23 sites surveyed 13–30 months after release, from 1 to 200 m at 24 sites 7–12 months after release, and from 1 m to 100 m at 12 sites monitored 2–6 months after release. At the 59 sites in Victoria, where colonies of T. lintearius were active, P. persimilis was detected at 15 sites and S. histrio at 4 sites (Table 3, Fig. 2). As observed in Tasmania, P. persimilis and S. histrio were found at sites in coastal and inland areas with P. persimilis mainly occurring within the vicinity of horticultural enterprises.
4. Discussion Mite oviposition and survival on all test plants other than gorse was poor and there was no development to adult on two of the four cultivars of P. vulgaris. The results with P. vulgaris were similar to those obtained by Hill and OÕDonnell (1991b) using the same excised-leaf technique. They tested 22 cultivars of P. vulgaris and found that the performance of T. lintearius adults varied greatly depending on the cultivar. T. lintearius survived to maturity and produced fertile eggs on some cultivars,
but no cultivar was able to support mite development for more than one complete generation. They also found that T. lintearius was unable to successfully colonize P. vulgaris cultivars when transferred onto potted plants either in the glasshouse or the field. Although the Australian cultivars were not tested as potted plants, we would expect the same result. T. lintearius has never been recorded on beans or any other plant other than Ulex spp. in Europe (Hill and OÕDonnell, 1991b). There is also no evidence of host transfer in New Zealand where T. lintearius is now widespread since its release there in 1989. The results presented here confirm the previous studies (Hill and OÕDonnell, 1991b) which showed that T. lintearius has a narrow host range and is restricted to Ulex spp. under field conditions. T. lintearius was therefore considered safe to release in Australia. Hill et al. (1993) discussed the influence of rainfall and temperature in preventing the initial establishment of the UK strain of T. lintearius in the warmer, wetter areas of the North Island and the west coast of the South Island of New Zealand. High rainfall was attributed as being the major cause of establishment failure in association with different aggregation responses to temperature by different mite strains which enabled mites to retreat from the web and seek shelter at the onset of heavy rain. It is likely that the slow dispersal of all 6 T. lintearius strains (including the UK strain) at Zeehan and Hampshire in Western and North Western Tasmania is associated with the high annual rainfall of these localities. However, the continuing survival of all colonies in these locations does suggest some degree of rainfall tolerance. Although the high rainfall may have been a significant mortality factor, perhaps survival has been possible because the lower mean temperatures recorded in these areas did trigger some aggregation responses in the mite colonies, enabling some capacity to retreat into the webbing and seek shelter. The study has confirmed that biotic resistance to T. lintearius colonies by S. histrio and P. persimilis, as predicted by Ireson et al. (1999), has occurred at release sites in Tasmania and Victoria. Lawton and Brown (1986) state that if an invader is subject to attack from
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resident natural enemies, then the potential rate of increase of the invading populations must be greater than the rate at which enemies find and kill them if they are to survive. T. lintearius is multi-voltine and has good dispersal abilities. Although these features should enable its continuing survival, additional research is needed to assess the final geographic range and impact of S. histrio and P. persimilis once T. lintearius populations become more widespread in Australia. The work by Peterson et al. (1994) on the impact of S. vagans in New Zealand and by Pratt et al. (in press) on the impact of P. persimilis in Oregon, USA, shows that either of these predators could suppress T. lintearius populations and restrict its usefulness as a biological control agent in Australia.
Acknowledgments We thank Dr. Andrew Sheppard (CSIRO, Canberra) and Dr. John Hosking (NSW Agriculture, Tamworth) for their advice on the list of test plants. We also thank Peter Hodge (CSIRO, Canberra) for supplying some of the test plants, Dr. Owen Seeman (Tasmanian Department of Primary Industry and Fisheries Water and Environment) for assisting with the identification of predators and Dr. Richard Hill (Richard Hill and Associates) for his comments on the manuscript. Funding assistance for this project was provided by the Natural Heritage Trust, Canberra, Australia.
References Cowley, J.M., 1983. Lifecycle of Apion ulicis (Coleoptera: Apionidae), and gorse seed attack around Auckland, New Zealand. N. Z. J. Zool. 10, 83–86. Evans, J.W., 1942. The gorse weevil. Tasm. J. Agric. 13, 15–18. Ewart, A.J., 1909. The Weeds, Poison Plants and Naturalised Aliens of Victoria. Government printer, Melbourne. Hill, R.L., 1982. The phytophagous fauna of gorse and host plant quality. Ph.D. thesis, Imperial College, University of London, UK. Hill, R.L., 1983. Prospects for the biological control of gorse. In: Proceedings of the 36th New Zealand Weed Pest Control Conference. New Zealand Weed and Pest Control Society Inc., Palmerston North, New Zealand, pp. 56–58. Hill, R.L., 1990. Safety testing of Tetranychus lintearius Dufour, a potential control agent for gorse in Oregon. DSIR contract report (unpublished) 15 pp. Landcare Research New Zealand Ltd., Lincoln, New Zealand. Hill, R.L., 1991. Safety testing of Tetranychus lintearius Dufour, a potential control agent for gorse in Oregon. DSIR contract report (unpublished) 15 pp. Landcare Research New Zealand Ltd., Lincoln, New Zealand. Hill, R.L., Gourlay, A.H., 1993. Susceptibility of 23 Hawaiian plant species to the mite, Tetranychus lintearius, a biological control agent for gorse. Landcare Research contract report LC9293/43 (unpublished) 13 pp. Landcare Research New Zealand Ltd., Lincoln, New Zealand.
Hill, R.L., OÕDonnell, D.J., 1991a. Reproductive isolation between Tetranychus lintearius and two related mites, T. urticae and T. turkestani (Acarina:Tetranychidae). Exp. Appl. Acarol. 11, 241– 251. Hill, R.L., OÕDonnell, D.J., 1991b. The host range of Tetranychus lintearius (Acarina: Tetranychidae). Exp. Appl. Acarol. 11, 253– 269. Hill, R.L., Gourlay, A.H., Martin, L., 1991a. Seasonal and geographic variation in the predation of gorse seed, Ulex europaeus L., by the seed weevil Apion ulicis Forst. N. Z. J. Zool. 18, 37–43. Hill, R.L., Gourlay, A.H., Wigley, P.J., 1989. The introduction of gorse spider mite, Tetranychus lintearius, for biological control of gorse. In: Proceedings of the 42nd New Zealand Weed Pest Control Conference. New Zealand Weed and Pest Control Society Inc., Palmerston North, New Zealand, pp. 137–139. Hill, R.L., Gourlay, A.H., Winks, C.J., 1993. Choosing gorse spider mite strains to improve establishment in different climates. In: Prestidge, R.A. (Ed.), Proceedings of the 6th Australasian Grassland Invertebrates Ecology Conference. AgResearch, Hamilton, New Zealand, pp. 377–384. Hill, R.L., Grindell, J.M., Winks, C.J., Sheat, J.J., Hayes, L.M., 1991b. Establishment of gorse spider mite as a control agent for gorse. In: Proceedings of the 44th New Zealand Weed Pest Control Conference. New Zealand Weed and Pest Control Society Inc., Palmerston North, New Zealand, pp. 31–34. Houston, K.J., 1980. A revision of the Australian species of Stethorus Weise (Coleoptera: Coccinellidae). J. Aust. Entomol. Soc. 19, 81– 91. Ireson, J.E., Gourlay, A.H., Kwong, R.M., Holloway, R.J., Chatterton, W.S., 1999. Progress on the rearing, release and establishment of the gorse spider mite, Tetranychus lintearius Dufour, for the biological control of gorse in Australia. In: Bishop, A.C., Boersma, M., Barnes, C.D. (Eds.), Proceedings of the 12th Australian Weeds Conference, 12–16 September 1999, Hobart, Tasmania. Tasmanian Weed Society, Devonport, pp. 320–324. Lawton, J.H., Brown, K.C., 1986. The population and community ecology of invading insects. Philos. Trans. R. Soc. Lond. 314, 733– 742. Lee, W.G., Allen, R.B., Johnson, P.N., 1986. Succession and dynamics of gorse (Ulex europaeus L.) communities in the Dunedin ecological district, South Island, New Zealand. N. Z. J. Bot. 24, 279– 292. McMurtry, J.A., Croft, B.A., 1997. Life-styles of phytoseiid mites and their roles in biological control. Annu. Rev. Entomol. 42, 291– 321. Moss, G.R., 1959. The gorse seed problem. In: Proceedings of the 12th New Zealand Weed Pest Control Conference. New Zealand Weed and Pest Control Conference Inc., Wellington, New Zealand, pp. 59–64. Norambuena, H., Escobar, S., Rodriguez, F., 2000. The biocontrol of gorse, Ulex europaeus, in Chile: A Progress report. In: Spencer, Neal R. (Ed.), Proceedings of the X International Symposium Biological Control Weeds, 4–14 July 1999. Montana State University, Bozeman, Montana, pp. 955–961. Parsons, W.T., Cuthbertson, E.G., 2001. Noxious Weeds of Australia, second ed. CSIRO Publishing, Victoria, Australia. Peterson, P.G., McGregor, P.G., Springett, B.P., 1994. Development of Stethorus bifidus in relation to temperature: implications for regulation of gorse mite populations. In: Proceedings of the 47th New Zealand Plant Protection Conference. New Zealand Weed and Pest Control Society Inc., Palmerston North, New Zealand, pp. 103–106. Polhill, R.M., 1981. Papilionoideae. In: Polhill, R.M., Raven, P.H. (Eds.), Advances in Legume Systematics Part 1. Royal Botanic Gardens, Kew, pp. 995–1027. Pratt, P.D., Coombs, E.M., Croft, B.A., Predation by phytoseiid mites on Tetranychus lintearius (Acari: Tetranychidae), an established
J.E. Ireson et al. / Biological Control 26 (2003) 117–127 weed biological control agent on gorse (Ulex europaeus). Biol. Control. (in press). Richardson, R.G., Hill, R.L., 1998. The biology of Australian Weeds 34. Ulex europaeus L. Plant Prot. Quart. 13, 46–58. Schicha, E., 1987. Phytoseiidae of Australia and Neighbouring Areas. Indira Publishing House, Oak Park, MI, USA. Stone, C., 1986. An investigation into the morphology and biology of Tetranychus lintearius Dufour (Acari: Tetranychidae). Exp. Appl. Acarol. 2, 173–186. Tubbs, C.R., 1974. Heathland management in the New Forest, Hampshire, England. Biol. Conserv. 6, 303–306.
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Wapshere, A.J., 1974. A strategy for evaluating the safety of organisms for biological weed control. Ann. Appl. Biol. 77, 201–211. Zw€ olfer, H., 1962. Ulex europaeus project: European investigations for New Zealand. Unpublished report no. 1. Commonwealth Institute of Biological Control, Delemont, Switzerland. Zw€ olfer, H., 1963. Ulex europaeus project: European investigations for New Zealand. Unpublished report no. 2. Commonwealth Institute of Biological Control, Delemont, Switzerland. Zw€ olfer, H., 1964. Ulex europaeus project: European investigations for New Zealand. Unpublished report no. 3. Commonwealth Institute of Biological Control, Delemont, Switzerland.
Host specificity, release, and establishment of the gorse spider mite, Tetranychus lintearius Dufour (Acarina: Tetranychidae), for the biological control of gorse, Ulex europaeus L. (Fabaceae), in Australia J.E. Ireson,a,* A.H. Gourlay,b R.M. Kwong,c R.J. Holloway,a and W.S. Chattertona a Tasmanian Institute of Agricultural Research, 13 St. JohnÕs Avenue, New Town, Tasmania 7008, Australia Landcare Research New Zealand, Canterbury Agriculture and Science Centre, P.O. Box 69, Gerald St., Lincoln, New Zealand Department of Natural Resources and Environment, Keith Turnbull Research Institute, P.O. Box 48, Frankston, Vic. 3199, Australia b
c
Received 7 December 2001; accepted 25 September 2002
Abstract The gorse spider mite, Tetranychus lintearius, is widespread on Ulex europaeus in Europe and its foliage-feeding activities can cause severe damage to plants. It has been released as a biological control agent for U. europaeus in New Zealand, the United States, and Chile following tests on over 100 plant species or cultivars confirming its host specificity to Ulex species. The suitability of T. lintearius for release in Australia was tested on an additional 32 plant species or cultivars from the family Fabaceae that occur in Australia. The tests confirmed that T. lintearius was a genus-level specialist and would be unable to survive on any plant other than U. europaeus in Australia. The first Australian field releases were carried out in Tasmania and Victoria in December 1998. Since then, field releases from laboratory cultures at 116 sites in Tasmania and 90 sites in Victoria have resulted in T. lintearius initially becoming well established in both states. However, examination of T. lintearius colonies in Tasmania and Victoria has shown that predation by the introduced Chilean predatory mite, Phytoseiulus persimilis and the native coccinellid, Stethorus histrio, is already widespread. These predators have the potential to significantly suppress populations of T. lintearius and severely restrict its usefulness as a biological control agent in Australia. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Biological control; Gorse; Ulex europaeus; Tetranychus lintearius; Host specificity; Predation; Phytoseiulus persimilis; Stethorus spp.; Australia
1. Introduction Gorse, Ulex europaeus L. (Fabaceae), is a native of central and western Europe (Parsons and Cuthbertson, 2001) where it occurs in native heathland (Tubbs, 1974) and on disturbed or neglected farmland and forests (Zw€ olfer, 1962). It now occurs in most temperate regions of the world but has become a serious weed in Australia, Chile, Hawaii, New Zealand (Richardson and Hill, 1998), in the Pacific Coast States of the USA (Parsons and Cuthbertson, 2001), and in British Columbia, Canada. * Corresponding author. Fax: +613-6278-2716. E-mail address: [email protected] (J.E. Ireson).
European colonists introduced gorse into Australia during the early 1800s primarily as a hedge plant or ornamental but there was also interest in its potential use as a fodder. It was recorded as being grown in New South Wales in 1803 and was listed in a Tasmanian nursery catalogue in 1845 (Parsons and Cuthbertson, 2001). Its weed status was recognised in Victoria about 50 years later (Ewart, 1909). Gorse now occurs in all Australian states but it is particularly common in southeastern Australia. Its weed status appears to be related to latitude as the main problem regions are principally in Victoria and Tasmania (Parsons and Cuthbertson, 2001). Gorse is a spiny perennial shrub, which can grow to a height and diameter in excess of 3 m and may live to
1049-9644/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 1 0 4 9 - 9 6 4 4 ( 0 2 ) 0 0 1 2 8 - 7
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almost 30 years (Lee et al., 1986). Seeds may survive in the soil for a similar period (Moss, 1959). Flowers are bright yellow, approximately 20 mm long and borne all over the plant. Flowering occurs in spring/early summer and autumn/early winter, although flowers may be present at other times if climatic conditions suit. Brown to black seed, about 4 mm long, is produced in grey, hairy pods that hold ca. 3–5 seeds. Dispersal is mainly by the seeds either falling below the parent plant or by pod dehiscence, which results in the seed being ejected several metres away. Biological control of gorse in Australia began with the introduction of the gorse seed weevil, Exapion ulicis (Forster) (Coleoptera: Brentidae). Originally from Europe, this agent was introduced into Tasmania from New Zealand in 1939 (Evans, 1942). Although now widespread in Tasmania and parts of the mainland, its impact is restricted, probably because gorse flowers twice a year in these areas and larvae are not active when the autumn seed crop matures (Cowley, 1983; Hill et al., 1991a). A new program for the biological control of gorse was initiated in New Zealand during the late 1970s using information on European phytophagous invertebrate species associated with Ulex spp. (Hill, 1982) based on surveys by Zw€ olfer (1963, 1964). Of the 94 invertebrate species known to feed on gorse, the few considered sufficiently host specific to warrant further investigation as biological control agents included the gorse spider mite, Tetranychus lintearius Dufour (Acarina: Tetranychidae) (Hill, 1983). Zw€ olfer (1963) first identified T. lintearius as a potential control agent for gorse after observing large colonies causing severe damage on mature foliage. Subsequent investigations showed that T. lintearius was a distinct, recognizable species (Stone, 1986) that was reproductively isolated from other closely related species (Hill and OÕDonnell, 1991a). In addition, host specificity tests with about 70 species or cultivars of plants (Hill and OÕDonnell, 1991b) showed that it could survive only on gorse in the field. T. lintearius was therefore considered safe to release in New Zealand and first introduced there in 1989 (Hill et al., 1989). Additional tests on 51 plant species or cultivars confirmed these results and enabled the introduction of T. lintearius into Hawaii, Oregon, and California (USA) (Hill, 1990; Hill, 1991; Hill and Gourlay, 1993). T. lintearius is now widely established in New Zealand, Chile, Hawaii, and in the Pacific Coast states of the USA. However, several predatory species have now been recorded attacking T. lintearius populations in Chile, New Zealand, and the USA. In Chile, a beetle, Ologota sp. (Coleoptera: Staphylinidae) has been recorded attacking colonies of T. lintearius at every release site but has not prevented establishment and no species of predatory mite has been
recorded (Norambuena et al., 2000). In New Zealand, Hill et al. (1991b) noted that predation by the endemic beetle, Stethorus bifidus (Kapur) (Coleoptera: Coccinellidae), was common. Subsequent studies by Peterson et al. (1994) showed that S. bifidus had the ability to effectively regulate T. lintearius populations at some sites. Hill et al. (1991b) also recorded the Chilean predatory mite, Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae), a specialist predator of Tetranychus species (McMurtry and Croft, 1997), feeding on T. lintearius colonies, usually in the vicinity of horticultural developments. Although Hill et al. (1991b) noted that P. persimilis had eradicated laboratory cultures of T. lintearius in New Zealand, there was no evidence that it was playing a major role in suppressing field populations. However, subsequent exclusion experiments by Pratt et al. (in press) were used to demonstrate that P. persimilis and possibly other phytoseiid predators were able to significantly reduce T. lintearius field populations in Oregon, USA. Phytoseiulus persimilis and the endemic Stethorus histrio Chazeau and Stethorus vagans (Blackburn) are common predators of Tetranychus pest species in Tasmania and Victoria, so there was some concern as to their impact on the establishment of T. lintearius following its release in December 1998 (Ireson et al., 1999). This paper presents the results of additional hostspecificity tests on 32 Australian plant species, which enabled the release of T. lintearius in Australia. In addition, the results of post-release surveys to determine the success of establishment of T. lintearius and the occurrence of predators in Tasmania and Victoria are reported. The possible impact of the predators is discussed.
2. Materials and methods 2.1. Test plant species The safety of introducing T. lintearius into Australia was determined by examining its oviposition and survival on 32 plant species or cultivars including gorse. The list of plants used (Table 1) consisted of species or cultivars occurring in Australia that are closely related to gorse in the tribe Genisteae as well as other tribes in the family Fabaceae. The choice, approved by the Australian Quarantine and Inspection Service, was based on the centrifugal strategy detailed by Wapshere (1974) and by using an interpretation of the relationship between the tribes of the Fabaceae by Polhill (1981). In the tribe Genisteae, Cytisus palmensis (Christ.) Bisby and Nicholls and the cultivars of Lupinus angustifolius L. are important in agriculture; Cystisus sp. var. racemosus Hort. is grown as an ornamental. All test plants in the tribes Phaeseoleae and Trifoliae are of importance
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119
Table 1 Survival of T. lintearius females and their progeny on 32 species of Australian test plants Oviposition ratea
Larvae produced
3:3 0:6 0
4.1 0
f
f
0
0
0:3 0:6
0
0.5
f
0
1:7 1:5 0 0:3 0:6
0:3 0:6 0 0
0 0 0
0.4 0 0.1
0 0
0 0 0
Tribe Bossiaeeae Bossiaea buxifolia Cunn.c Goodia lotifolia Salisb.c Hovea acutifolia Cunn.c Platylobium formosum Sm.e
1:7 2:1 0 0 0
0 0 0 0
0 0 0 0
0.1 0 0 0
f
0 0 0
0 0 0 0
Tribe Crotalarieae Crotalaria cunninghamii R. Br.d
5.0
0
0
0
0
0
Tribe Indigofereae Indigofera australis Willd.c
0
0
0
0
0
0
Tribe Mirbelieae Aotus ericoides (Vent.) G. Done Daviesia mimosoides R. Br.c Dillwynia glaberrima Sm.e Eutaxia microphylla (R. Br.) J. Blacke Gompholobium huegelii Benth.e Kennedia prostrata R. Br.e Oxylobium ellipticum R. Br.c Pultenaea polifolia Cunn.c Sphaerolobium vimineum Sm.e
1:2 1:3 2:0 1:0 1:2 0:8 0:4 0:6 0:4 0:5 2:6 1:5 0:7 0:6 1:0 1:0 1:6 2:3
0 0 0 0 0 0 0 0 0:6 1:3
0 0 0 0 0 0 0 0 0:6 1:3
0 0 0 0 0 0 0 0 0.3
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
Tribe Phaeseoleae Centrosema pubescens Benth.d Lablab purpureus (L.) Sweetd Macroptilium atropurpureum (DC.) Urband Phaeseolus vulgaris L. cv. Brokerc Phaeseolus vulgaris L. cv. Labradorc Phaesolus vulgaris L. cv. Floc Phaeseolus vulgaris L. cv. Rapierc Vigna radiata (L.) Wilczekd
2:3 2:1 1:3 0:6 3:3 0:6 0 2:0 1:0 2:0 1:0 1:3 1:5 1:3 1:2
0 0 0:7 0:6 0 1:0 1:0 1:7 1:5 1:3 1:5 0
0 0 0 0 0 1:7 1:5 1:3 1:5 0
0 0 0 0 0 1.8 0.9 0
0 0 0 0 0
0 0 0 0 0
f
f ;g
1:3 1:5
0
0
0 2:2 2:2
0 0
0 0
Species
Family Fabaceae Subfamily Faboideae Tribe Genisteae Subtribe Genistinae Ulex europaeus L.b Cytisus palmensis (Christ.) Bisby and Nichollsc Cytisus sp. var. racemosus Hort.c Subtribe Lupininae Lupinus angustifolius L. cv. Gungurru.c Lupinus angustifolius L. cv. Yandee.c Lupinus angustifolius L. cv. Merrit.c
Tribe Trifoliae Trifolium subterraneum L. var. ÔDenmarkÕc Subfamily Mimosoideae Tribe Acacieae Acacia dealbata Link.d Acacia mearnsii De wild.e a
Mean female survival (SE) Day 3
Day 10
Day 13 (remaining)
4:6 0:6 0:3 0:6
4:1 0:8 0
3:3 2:1
Eggs per replicate per day. Mean of results obtained from three series of tests. c;d;e Denotes whether plant species used in first, second, or third series of tests. f Affirmative. g Eggs produced by adults but juveniles died. b
f
f
Adults produced
f
f ;g
0
0
0
0
0
0 0
0 0
0 0
120
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to Australian agriculture and those in the tribes Bossiaeeae, Crotalarieae, Indigofereae, Mirbelieae, and Acacieae are Australian natives and most are grown as ornamentals. 2.2. Location of tests and source of plant material All testing was carried out in the insect quarantine facility at Landcare Research New Zealand, in Lincoln. Two series of oviposition and survival tests involving 23 species or cultivars plus gorse were carried out during 1995 (Table 1). A third series was carried out on an additional 8 species plus gorse during 1997 (Table 1). Plant material used in the tests was field-collected or glasshousegrown in Australia except for Goodia lotifolia Salisb., which was field-collected in New Zealand. Cut shoots of each species of Australian plant were covered at the base with moist paper towelling held in place with elastic bands. Each plant species was then placed separately in a sealed plastic bag. The bags were then placed together in a Polyfoam container and airfreighted overnight to New Zealand. All plant material was received in good condition and used immediately upon arrival. 2.3. Oviposition and survival tests An egg-laying colony of mixed-origin T. lintearius was used in the host specificity tests. This colony was originally collected near Dunedin on the South Island of New Zealand and maintained on potted gorse plants in the laboratory at ambient temperature. The colonies were a mixture of populations that had originated from the United Kingdom, North West Spain, and coastal Portugal (Hill et al., 1993). Well-fed mites continue to lay eggs for several days, even in petri dishes without plant material (Gourlay, unpublished data). To reduce the possibility of a spurious result, egg-laying adult females were removed from the laboratory cultures and kept in 8 6 cm plastic boxes for 2 days without food prior to the tests. For each test, one leaf of each test plant species or cultivar was placed onto saturated cotton wool in open 10 15 cm plastic containers. The wet cotton wool prevented T. lintearius from moving between the test species. Each box contained up to 8 separate species or cultivars. A gorse branch ca. 5 cm in length was included as a control. Three replicates were used for each species in the first two series of tests and five for the third series. Five randomly chosen gravid adult female T. lintearius were placed on each test species (including gorse). The boxes were held at 13 °C overnight, causing the mites to aggregate and settle, then held under a photoperiod of 16 h L:8 h D at 22 °C and 70% RH during the day and 16 °C and 80% RH at night. Any T. lintearius adults that fell into the wet cotton wool within the first 24 h were placed back on the plant
material. Emigration after this period was considered a rejection of the plant and the mites were counted as dead. After 3 days, 10 days, and then every 3 days, leaves were examined under a microscope and the survivorship of adults and immature stages was recorded. The number of eggs laid per replicate per day was calculated at the first examination on the third day. Test plant leaves were replaced if they deteriorated. Adult female T. lintearius were removed from the gorse after a minimum of 13 days because the eggs and juveniles became too numerous to count and had caused severe damage. Five new randomly chosen adult females from within the controls were then transferred onto a new gorse branch. 2.4. Release and establishment During October and November 1998, 6 strains of T. lintearius were imported to Australia from New Zealand. These strains had originally been imported into New Zealand from Cornwall in the United Kingdom (UK), from locations in Spain (Hio, O Grove, Cambados) and Portugal (S^ao Pedro and Viana do Castelo) (Hill et al., 1993). The strains from Spain and Portugal were introduced into New Zealand after the UK strain. This was because they were a closer climatic match and found better adapted to the warmer wetter areas of New Zealand, where the UK strain had originally failed to establish (Hill et al., 1993). The location of the New Zealand collection sites for each of the strains imported into Australia were: UK (43°490 S, 172°100 E); Hio (45°530 S, 170°230 E); O Grove (45°480 S, 170°200 E); Cambados (45°470 S, 170°320 E); S^ao Pedro (45°450 S, 170°360 E); and Viana do Castelo (45°490 S, 170°240 E). All six strains (UK, Hio, O Grove, Cambados, S^ ao Pedro, and Viana do Castelo) were collected from their original separate release sites in New Zealand for importation to Australia. No known mixing of the strains had occurred at these sites. Rearing procedures conducted in Tasmania and Victoria following the introduction of these strains were described by Ireson et al. (1999). Most of the T. lintearius colonies used for the field releases were reared by lodging starter colonies of at least 100 mites onto potted gorse plants about 30 cm high and 20 cm diameter. These cultures were held in controlled temperature glasshouses maintained at 18–20 °C. Mites for field release were removed once the colonies had multiplied to several thousand individuals. Approximately 200 mites, consisting mostly of female deutonymphs and adults, were then transferred by a sable hair brush to 15cm-long gorse cuttings. The cuttings had been inserted into water through a 4-mm-diameter hole in the centre of a screw top lid on a plastic vial 8 cm high and 3 cm in diameter. These colonies were then held in a controlled environment cabinet at 20–24 °C under a photoperiod of 16 h L:8 h D for 4–5 days, prior to field release. This enabled the colonies to be checked for the possible
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presence of predators, particularly P. persimilis, prior to field release. The infested cuttings were transferred to the release sites in the vials then removed and lodged securely into mature gorse plants. From 2 to 10 cuttings, usually distributed between 2 and 5 bushes in close proximity, were placed at each site. Releases were carried out at 116 sites in Tasmania (Fig. 1) and 90 sites in Victoria (Fig. 2) between December 1998 and April 2001 in all regions where major gorse infestations have been recorded. All six strains were released. Different strains were usually released at separate sites, although in a few instances, some or all of the strains were released at the same site on bushes about 10 m apart. Surveys for the establishment and
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dispersal of T. lintearius were carried out at 35 of the Tasmanian sites between 12 and 26 months after release during autumn 2000 and the following late springsummer period in 2000/01. In Victoria, 66 sites were monitored between 2 and 30 months after the initial release of mites. Monitoring generally took place during late spring through to autumn each year, commencing in the summer of 1998/99. The dispersal of T. lintearius from the release point was determined by searching for webbing, although, in some cases, small, newly forming colonies were often located up to 20 m further out from the last webbed colony. Radial searches were carried out at each site up to 100 m from the last colony located, or as determined by the distribution of gorse.
Fig. 1. Locations where releases of the gorse spider mite, Tetranychus lintearius ( ), were carried out in Tasmania (December 1998 to April 2001) and locations where Phytoseiulus persimilis ðjÞ and Stethorus histrio ( ) have been identified, in relation to the main gorse infestations (stippling) and the 800 mm, 1600 mm, and 2400 mm annual isohyets.
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Fig. 2. Locations where releases of the gorse spider mite, Tetranychus lintearius ( ), were carried out in Victoria (December 1998 to April 2001) and locations where Phytoseiulus persimilis ðjÞ and Stethorus histrio ( ) have been identified.
Colonies were checked for the presence of P. persimilis and Stethorus spp. in the vicinity of the central release point using a dissecting microscope. Older webbed sections of gorse stems ca. 10 cm in length containing mostly eggs, larvae, or protonymphs (stages preferred by P. persimilis) were removed and examined in situ. Newly aggregating colonies of mites were avoided during these examinations because of their possible repellent effect on predators. If no predators where detected after examination of a maximum of 10 stem sections, a negative result was recorded, i.e., predators were not considered present at the site at that time. Voucher specimens of predators collected at each site were placed in tubes of 70% alcohol and returned to the laboratory for identification. The identity of P. persimilis was checked using the key of Schicha (1987). Stethorus specimens were identified using the key of Houston (1980). Relevant temperature and rainfall data for the Tasmanian release sites were obtained through the Australian Bureau of Meteorology.
tested (Table 1). F1 and F2 generations were produced on gorse ca. every 30 days. Adult female mites did not survive longer than 10 days or lay eggs on 24 of the remaining 31 test plants (Table 1). Adult female mites died within 13 days and all eggs and juveniles produced died within 28 days on Cytisus sp. var. racemosus, L. angustifolius cv. gungurru, L. angustifolius cv. Merrit, and Bossiaea buxifolia Cunn. Adult mites survived for 16 days on one replicate of Sphaerelobium vimineum Sm. and 34 eggs were laid on this plant, but only two eggs hatched and both mites died as larvae. Mite survival and oviposition on Phaseolus vulgaris L. varied between cultivars. On P. vulgaris cv. Broker, adult mites failed to survive 3 days and laid no eggs. On the other three cultivars, adult mites survived to a maximum of 10 days on cv. Labrador (3 adults), 19 days on cv. Flo (1 adult), and 28 days (1 adult) on cv. Rapier (Table 1). Only the eggs laid on cv. Flo and cv. Rapier hatched. Some immatures survived on these two cultivars to produce an F1 generation which also successfully produced eggs, but all died as juveniles.
3. Results 3.2. Field establishment 3.1. Host specificity No species or cultivar was an alternative host to gorse for T. lintearius. Oviposition rates were much higher on the gorse controls in comparison to the other species
In Tasmania, T. lintearius was recovered at all release sites. Colonies had increased in number, as indicated by the conspicuous webbing that often covered entire bushes. Individual colonies were often large, containing up
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to several hundred thousand mites. However, there was considerable variation in dispersal. Dispersal ranged from 5 m to 20 km at the 10 sites surveyed 22–27 months after release and from 3 m to 2 km at the 25 sites surveyed 12–17 months after release (Table 2). The sites at Zeehan (mean annual temperature 10.7 °C) and Hampshire (mean annual temperature 9.7 °C), where the lowest dispersal was recorded, are situated in the cool, wet areas of Western and North Western Tasmania, where the mean annual rainfall ranges from ca. 1600–2400 mm (Fig. 1). The sites where the highest dispersal was recorded are situated in warmer, drier areas where the mean annual rainfall is 600–800 mm (Fig. 1) and mean annual temperatures range from ca. 10.5 °C at Montana in the northern midlands to 13.6 °C at Bicheno on the East Coast (Fig. 1). The Cressy site (Fig. 1) where the highest dispersal was recorded, is not the warmest locality (mean annual temperature 11.2 °C), but is one of the driest
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(mean annual rainfall 637 mm) and dispersal followed the direction of prevailing winds from the north and northwest. An assessment of the performance of the different strains was beyond the scope of this study but there was no indication from this survey that any strain will be more successful. Two contrasting sites were surveyed where all strains had been released together in close proximity (the relatively dry location at Perth and at Zeehan 3 where the rainfall is highest) (Fig. 1). All strains had formed many large viable colonies around the respective release points at the drier Perth site but none had dispersed more than 10 m. At the wetter Zeehan 3 site, all strains had dispersed no further than 5 m, although it was visually evident that colonies were fewer and smaller than at Perth. In Tasmania, S. histrio and/or P. persimilis were observed feeding on T. lintearius colonies at all but 8 sites (Table 2). Stethorus histrio was identified at 17 release sites, most of which were located inland (Fig. 1).
Table 2 Survey of 35 sites in Tasmania for the presence of T. lintearius and associated predators 12–27 months after release Locality
Mite strain releaseda
Date of survey
Months since release
Maximum estimated dispersal
Predator (s) foundb
Zeehan 1 Cressy Stonehenge Stanley Hwy Montana Bicheno Little Swanport Hampshire 1 Stanley Nut Campbell Town 1 Ross 1 Jericho Greens Beach Zeehan 2 Franklin Zeehan 3 Oatlands 1 Ross Oatlands 2 Woodbury Jericho Oatlands 3 Lucaston Hampshire 2 Hampshire 3 Campbell Town 2 Lymington Campbell Town 3 PetcheyÕs Bay Cranbrook Fingal Avoca Ormley Blackmans Bay Perth
U U U U U U U U U U U O O S V All C C S H U V S O, H, U S, C, V H H U O C U H S V, C All
14/3/01 1/2/01 16/2/01 15/3/01 24/1/01 8/2/01 8/2/01 15/3/01 13/12/00 16/11/00 22/2/01 1/2/01 1/3/00 14/3/01 10/1/01 14/3/01 30/11/00 24/1/01 30/11/00 30/11/00 30/11/00 30/11/00 18/1/01 15/3/01 15/3/01 30/11/00 28/11/00 30/11/00 28/11/00 1/12/00 7/12/00 7/12/00 7/12/00 7/11/01 1/12/00
27 26 26 26 25 24 24 24 23 22 17 17 16 16 16 16 15 15 15 15 15 15 15 15 15 14 14 14 14 13 13 13 13 13 12
5m 20 km 4 km 500 m 500 m 2 km 1 km 50 m 200 m 800 m 2 km 100 m 100 m 60 m 60 m 5m 100 m 100 m 80 m 75 m 50 m 30 m 30 m 5m 3m 180 m 50 m 30 m 30 m 280 m 100 m 80 m 50 m 50 m 10 m
N S S, S, S, P P S P S, N N N S P S N N S N S S P S S N P S P P P S S P S,
a
C, Cambados, H, Hio; O, O Grove; S, S^ao Pedro; V, Viana do Castelo; U, UK; All, all strains; released ca. 10 m apart. b N, No predators found; S, Stethorus histrio; P, Phytoseiulus persimilis.
P P P
P
P
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Table 3 Survey of 66 sites in Victoria for the presence of T. lintearius and associated predators 2–30 months after release Locality
Mite strain releaseda
Date of survey
Months since release
Maximum estimated dispersalb
Predator (s) foundc
Hastings 1 Barfold Digby 1 Digby 2 Digby 3 Digby 4 Digby 5 Taradale 1 PiperÕs Creek Spring Hill Hastings 2 Hastings 3 Tyabb 1 Baxter Taradale 2 Somerville Cranbourne 1 Cranbourne 2 Castlemaine 1 Castlemaine 2 Yendon 1 Darraweit Guim Queenscliff Yendon 2 Castlemaine 3 Lancefield Langley Grenville 1 Grenville 2 Cornish Hill Malmsbury Cranbourne 3 Cranbourne 4 CannonÕs Creek 1 CannonÕs Creek 2 CannonÕs Creek 3 Blind Blight Moorooduc 1 Moorooduc 2 Moorooduc 3 Portland 1 Portland 2 Portland 3 Creswick 1 Creswick 2 Corindhap 1 Corindhap 2 Snake Valley 1 Snake Valley 2 Somers Linton 1 Linton 2 Moorooduc 4 Bittern 1 Sandy Point Carrum Downs Tyabb 2 Bittern 2 Balnarring Castlemaine 4 Maldon Crib Point
O U O C V S H U O H H C O H, C U U V S V S S U U O V U U O O O S S U C H V O H S V V U C U U H H S S C C C C V S Mixed H U O O S H,V
04/6/01 16/1/01 23/4/01 23/4/01 23/4/01 23/4/01 23/4/01 16/1/01 16/1/01 16/1/01 18/12/00 18/12/00 11/1/01 11/1/01 16/1/01 10/1/01 10/1/01 10/1/01 22/11/00 22/11/00 23/11/00 7/5/01 3/5/01 23/11/00 24/7/01 16/1/01 16/1/01 31/5/01 31/5/01 16/1/01 28/12/00 10/1/00 10/1/01 10/1/01 10/1/01 10/1/01 10/1/01 11/1/01 11/1/01 11/1/01 24/4/01 24/4/01 24/4/01 29/11/00 29/11/00 27/11/00 27/11/00 23/11/00 23/11/00 10/1/01 23/11/00 23/11/00 11/1/01 10/1/01 10/1/01 10/1/01 10/1/01 11/1/01 11/1/01 24/7/01 24/7/01 10/1/01
30 25 23 23 23 23 23 23 22 20 20 20 22 21 21 21 20 20 20 20 18 17 17 16 15 13 13 12 12 9 8 8 8 8 8 8 8 8 8 8 8 8 8 7 7 7 7 7 7 7 7 6 6 6 6 5 5 5 5 5 5 4
1 km 850 m 50 m NE NE NE NE 200 m 1m 28 m 1m 4m 64 m 20 m 1 km 10 m 17m 40m 2m 20 m 40 m 5m 500 m 25 m 4 km 6m 1m 5m 200 m 1m 1m 20 m 8m 18 m 30 m 10 m 10 m 23 m 40 m 26 m 2m 20 m 10 m 2m 1m 100 m 1m 100 m 5m 2m 20 m 1m 75 m 7m NE 1m NE 1m 5m 15 m 10 m NE
P N N N N N N N P S N N S N N N P P P P S P N N N N P N N N P P N P N N N N N N N N N N N P P N N N N N N N N S N P P N N N
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Table 3 (continued) Locality
Mite strain releaseda
Date of survey
Portarlington Chewton 1 Chewton 2 Phillip Island
O U S H
3/5/00 24/7/01 24/7/01 18/5/99
Months since release 4 3 3 2
Maximum estimated dispersalb
Predator (s) foundc
100 m 2m 1m 20 m
N N N N
a
C, Cambados; H, Hio; O, O Grove; S, S^ao Pedro; V, Viana do Castelo; U, UK; Mixed, interbred mix of all strains. NE, Not established. c N, No predators found; P, Phytoseiulus persimilis; S, Stethorus histrio. b
P. persimilis was identified at 15 release sites, most of which where located near coastal townships (10 sites) in the vicinity of a horticultural development or in similar situations further inland (Fig. 1). Although P. persimilis was not collected from the cooler, wetter sites in Western and North Western Tasmania (Fig. 1), adult mites were observed actively feeding on T. lintearius colonies at Montana and Stonehenge (Fig. 1) in frostprone areas during midwinter. In addition, at Stanley, Bicheno, PetcheyÕs Bay, and Cranbrook (Fig. 1) large numbers of P. persimilis were observed actively feeding on eggs and immature stages of T. lintearius. On some branches, it was suspected that P. persimilis was responsible for the destruction of established T. lintearius colonies as it was observed feeding amongst the webbing on the few eggs or immature stages that remained. In Victoria, T. lintearius colonies were detected at 59 of the 66 sites monitored (Table 3) in similar densities to that recorded in Tasmania. T. lintearius was not detected at the other 7 sites. Dispersal also varied considerably and ranged from 1 m to 4 km at 23 sites surveyed 13–30 months after release, from 1 to 200 m at 24 sites 7–12 months after release, and from 1 m to 100 m at 12 sites monitored 2–6 months after release. At the 59 sites in Victoria, where colonies of T. lintearius were active, P. persimilis was detected at 15 sites and S. histrio at 4 sites (Table 3, Fig. 2). As observed in Tasmania, P. persimilis and S. histrio were found at sites in coastal and inland areas with P. persimilis mainly occurring within the vicinity of horticultural enterprises.
4. Discussion Mite oviposition and survival on all test plants other than gorse was poor and there was no development to adult on two of the four cultivars of P. vulgaris. The results with P. vulgaris were similar to those obtained by Hill and OÕDonnell (1991b) using the same excised-leaf technique. They tested 22 cultivars of P. vulgaris and found that the performance of T. lintearius adults varied greatly depending on the cultivar. T. lintearius survived to maturity and produced fertile eggs on some cultivars,
but no cultivar was able to support mite development for more than one complete generation. They also found that T. lintearius was unable to successfully colonize P. vulgaris cultivars when transferred onto potted plants either in the glasshouse or the field. Although the Australian cultivars were not tested as potted plants, we would expect the same result. T. lintearius has never been recorded on beans or any other plant other than Ulex spp. in Europe (Hill and OÕDonnell, 1991b). There is also no evidence of host transfer in New Zealand where T. lintearius is now widespread since its release there in 1989. The results presented here confirm the previous studies (Hill and OÕDonnell, 1991b) which showed that T. lintearius has a narrow host range and is restricted to Ulex spp. under field conditions. T. lintearius was therefore considered safe to release in Australia. Hill et al. (1993) discussed the influence of rainfall and temperature in preventing the initial establishment of the UK strain of T. lintearius in the warmer, wetter areas of the North Island and the west coast of the South Island of New Zealand. High rainfall was attributed as being the major cause of establishment failure in association with different aggregation responses to temperature by different mite strains which enabled mites to retreat from the web and seek shelter at the onset of heavy rain. It is likely that the slow dispersal of all 6 T. lintearius strains (including the UK strain) at Zeehan and Hampshire in Western and North Western Tasmania is associated with the high annual rainfall of these localities. However, the continuing survival of all colonies in these locations does suggest some degree of rainfall tolerance. Although the high rainfall may have been a significant mortality factor, perhaps survival has been possible because the lower mean temperatures recorded in these areas did trigger some aggregation responses in the mite colonies, enabling some capacity to retreat into the webbing and seek shelter. The study has confirmed that biotic resistance to T. lintearius colonies by S. histrio and P. persimilis, as predicted by Ireson et al. (1999), has occurred at release sites in Tasmania and Victoria. Lawton and Brown (1986) state that if an invader is subject to attack from
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resident natural enemies, then the potential rate of increase of the invading populations must be greater than the rate at which enemies find and kill them if they are to survive. T. lintearius is multi-voltine and has good dispersal abilities. Although these features should enable its continuing survival, additional research is needed to assess the final geographic range and impact of S. histrio and P. persimilis once T. lintearius populations become more widespread in Australia. The work by Peterson et al. (1994) on the impact of S. vagans in New Zealand and by Pratt et al. (in press) on the impact of P. persimilis in Oregon, USA, shows that either of these predators could suppress T. lintearius populations and restrict its usefulness as a biological control agent in Australia.
Acknowledgments We thank Dr. Andrew Sheppard (CSIRO, Canberra) and Dr. John Hosking (NSW Agriculture, Tamworth) for their advice on the list of test plants. We also thank Peter Hodge (CSIRO, Canberra) for supplying some of the test plants, Dr. Owen Seeman (Tasmanian Department of Primary Industry and Fisheries Water and Environment) for assisting with the identification of predators and Dr. Richard Hill (Richard Hill and Associates) for his comments on the manuscript. Funding assistance for this project was provided by the Natural Heritage Trust, Canberra, Australia.
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