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Evaluation of the biological control agent, Lixus cardui, on Onopordum thistles: Establishment and initial field impact
Biological Control 47 (2008) 108–114
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Evaluation of the biological control agent, Lixus cardui, on Onopordum thistles: Establishment and initial field impact A.E. Swirepik a, P.J. Turner b, D.T. Briese a,* a b
CSIRO Entomology and CRC for Australian Weed Management, GPO Box 1700, Canberra 2601, Australia Applied Ecology Research Group and CRC for Australian Weed Management, University of Canberra, ACT 2601, Australia
a r t i c l e
i n f o
Article history: Received 26 April 2008 Accepted 13 June 2008 Available online 27 June 2008 Keywords: Lixus cardui Larinus latus Onopordum thistles Biological control Field establishment Impact assessment
a b s t r a c t Between 1993 and 1999 a biological control program focused on the establishment and redistribution of the stem boring weevil Lixus cardui on its host Onopordum thistles in south-eastern Australia. Three release strategies (caged, free-release and stem release) were assessed as a part of a redistribution process that involved engagement with community members, with an overall establishment rate of 97%. Long-term monitoring of seven sites between 1998 and 2004 showed that L. cardui densities fluctuated by up to 30-fold among sites and 12-fold among seasons, driven largely by climatic effects on host plant size and with no measurable effect of plant density. During 2002 a comparison was made of plant performance at seven release sites with L. cardui present and six matched control sites without L. cardui. Plant size and L. cardui density varied between release sites and, on average, L. cardui reduced plant height by 33% and seed production by 65%. A second agent, the seed weevil Larinus latus, directly destroyed 56% of the seed produced at the sites, leading to a mean overall reduction of 84% in seed added to the soil during 2002 (ranging from 37% to 99% across sites). The significant reductions in seed rain that are being accrued by these two agents augur well for the long-term success of this project. Crown copyright Ó 2008 Published by Elsevier Inc. All rights reserved.
1. Introduction Release strategies and the evaluation of biological control agents are key aspects of biological control that would benefit from further research (McClay, 1995; McFadyen, 1998; Briese et al., 2003; Briese, 2004; Day et al., 2004). Only about 60% of biological control agents released against target weeds establish (McFadyen, 1998). There are a number of clear ecological reasons for this, such as agent-target or agent-climate incompatibility, but some failures are attributable to release protocols that fail to maximise the chances of establishment due to problems such as Allee effects (Hopper and Roush, 1993). Less than half the established biological control agents have had their impact on the target weeds scientifically evaluated (Thomas and Reid, 2007), leading to a severe lack of data to measure the effectiveness of weed biological control (McFadyen, 1998). Where it occurs, such evaluation often involves only laboratory studies or assessment over very limited spatial scales (Thomas and Reid, 2007). Post release field evaluation of the impacts of biological control agents may take the form of exclusion studies (using pesticides or cages), studies of reference sites before and after agent release, or comparisons of release and control sites. In this study, a range of release strategies and both temporal and spatial evaluation are * Corresponding author. Fax: +61 2 6246 4000. E-mail address: [email protected] (D.T. Briese).
presented for the stem-boring weevil, Lixus cardui Olivier, released in Australia in 1993 (Briese et al., 2002a), as part of a strategy for the biological control of exotic Onopordum thistles (Asteraceae: Cardueae). The main aims of this program were to reduce soil seed reserves to levels where emergent plant densities would not be economically damaging and to minimise the deleterious effects of remaining thistles during this period of seed bank decline (Briese et al., 2002b). These objectives were to be achieved by introducing a complementary suite of control agents targeting different stages of the Onopordum life-cycle. Studies in its native European range (Briese, 1996) had shown that L. cardui feeding damage could contribute towards both objectives because their damage reduces both the growth of flowering stems and seed production of Onopordum by up to 50% and 80%, respectively, and leads to early senescence of mature plants. L. cardui has now been redistributed throughout the range of Onopordum thistles in south-eastern Australia (Swirepik and Briese, 2000). This study is an extension of the work by Briese et al. (2004), where the life-cycle and impact of L. cardui were determined under caged conditions. Briese et al. (2004) measured the longevity, fecundity and survival of L. cardui in Australia following release and concluded that the weevil had the capacity for rapid population increase. In addition, through an experiment in which L. cardui were caged at fixed known densities on plants of varying size in a natural population of Onopordum, they were able to describe the
1049-9644/$ - see front matter Crown copyright Ó 2008 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2008.06.006
A.E. Swirepik et al. / Biological Control 47 (2008) 108–114
relationship between plant growth and reproduction on different sized plants and at different levels of attack by the weevil. However, they cautioned that these relationships were likely to be dampened once the behavioural responses of L. cardui, namely a preference for larger plants and emigration when food quality deteriorated, were considered. This paper reports on research undertaken from post-release field studies where L. cardui behaviour was unconstrained. It firstly describes the initial release and establishment phase for L. cardui in Australia, including population changes during the first few years following release, and then evaluates the impact of unconstrained natural populations of the weevil at a number of sites where populations had reached relatively high densities. The combination of detailed impact data from a few selected sites combined with broader monitoring of establishment and population changes of the agent should enable a regional view of biological control success to be developed (Swirepik and Smyth, 2003). To evaluate the impact of unconstrained L. cardui populations, plant production was compared between sites where the agent was present and ‘‘control” sites where the agent was absent. The interaction of L. cardui with another introduced biological control agent, the seedweevil, Larinus latus Herbst, was also assessed to monitor the combined effect of the two agents. 2. Materials and methods 2.1. Release strategies Three different release methods were used to deploy L. cardui during a seven-year period commencing on 14 November 1993. The release process was initially modelled on the ‘‘multiplying cages” procedure described by Briese et al. (1996). Only 240 weevils were available for these initial releases, so release cages provided a safeguard against Allee effects. For this method a mesh release cage (3.6 m 1.8 m 1.8 m) was hung between steel posts over a dense patch of thistles. Twenty to 50 adults were then placed inside the cage and allowed to complete a generation on the enclosed thistles. Landholders belonging to a local Landcare Group (see Campbell, 1994) were invited to a ‘‘field day” at the site the following spring, where freshly emerged F1 adults were harvested and placed into small mesh bags with cut leaves. These were then given to the Landcare members, together with a cage and a release kit, enabling them to make new releases on their own properties. Once establishment of the agent was confirmed, a second ‘‘free-release” method was used which relied upon releasing adults but without the expense or effort of erecting cages. The third release method, ‘‘stem release”, was introduced to facilitate mass distribution. This simplified the release process and created a greater window of opportunity for redistributing L. cardui. ‘‘Stem releases” involved harvesting dead stems containing overwintering adults. Stems could then be transported for release at any time from the end of summer until the beginning of spring, as the adults remain dormant in situ during this period. Relocated stems were placed on the ground at the release site and the adults emerged during the following spring to continue the life cycle. 2.2. Evaluating L. cardui population growth and impacts The monitoring of release and population performance for L. cardui was based on the three-tiered system described by Swirepik and Smyth (2003), where tier three provides the least detailed information on establishment and spread, tier two measures both agent and target population dynamics and tier one defines agent impact in the greatest detail.
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In the tier-three evaluation, all L. cardui release sites were monitored for establishment in the spring following release, when they were revisited for agent collection and redistribution. Lixus cardui was considered to have established at a site if sufficient adults could be found to collect for redistribution at this time. Due to the scale of the release process and the available project resources, effort placed into monitoring for establishment and agent spread was reduced as the project grew. At the three initial 1994 release sites, data on the number of overwintering adults were also collected over a three year period from 1996 to 1998 by dissecting 40 randomly collected Onopordum stems following senescence. In the tier-two evaluation, seven release sites located throughout the Southern Tablelands of New South Wales were monitored to measure plant density and agent population size from 1997 to 2003. Lixus cardui had been released at these sites in either 1993 or 1994. Estimates of the population densities of Onopordum thistles and L. cardui were obtained using the T-Square method of sampling (see Krebs, 1989). At each site, after the plants had senesced, a 30 m transect tape was laid out and the distance to the nearest thistle within a 360° radius of the transect tape measured at 2 m intervals. Once the distance to the first plant was recorded the distance to a second thistle within 180° to the outside of the first thistle was measured and recorded. This provided a sample of 30 plants which were then dug up from the soil. The diameter of the taproot at the base of the crown of each plant was measured, and the roots were bagged individually and transported to the laboratory for further processing. Taproot diameter is a good predictor of plant height and seed production prior to the elongation of Onopordum flowering stems (Briese et al., 2004). Between 1999 and 2003, plant height was also recorded before the plants were harvested. Stems collected from the field were dissected in the laboratory, and the total number of L. cardui per stem was recorded. The counts were converted to field population densities of weevils per m2, based on the respective plant densities measured at each site in each year. Plants affected by sheep or cattle grazing were discarded to ensure that only the effects of L. cardui were measured. Sample size was never less than 14 plants at any given site in any given year. In addition, the capitula were counted and measured to determine seed production per plant, calculated using a previously determined regression equation relating seed number to capitulum diameter (see Pettit et al., 1996). In the tier-one evaluation, chemical exclusion trials are often used to measure the impact of insect herbivores on their host plants (see Dhileepan, 2003). In the case of L. cardui, this was considered impractical as the adult weevils are highly mobile and, at the scale of feasible insecticide applications, considerable feeding damage could still be expected from immigrant L. cardui. Consequently, the known relationship between plant size prior to bolting and the subsequent reproductive output of Onopordum thistles (Briese et al., 2004) was used to predict potential plant growth and seed production and to statistically determine the changes in these parameters due to L. cardui feeding. In 2002, the data collected from the seven tier-two sample sites were compared to data collected from plants harvested in the same manner from six control sites where the agent was not yet present, but also located across the Southern Tablelands of New South Wales. Three criteria were used to select control sites: (a) practical proximity to release sites while remaining L. cardui free; (b) climatic and physical similarity to release sites; and (c) phenotypic similarity to the hybrid variety of Onopordum acanthium illyricum (O’Hanlon et al., 1999) at the monitored release sites. At each control site, 50 rosettes were selected in September 2001 to include the full range of plant sizes, based on average rosette diameter (see Briese et al., 2004). These plants were marked and the area fenced. Plants were then left to complete flowering
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and senesce over the summer before collection in February 2002. These plants were then harvested and processed in the laboratory together with plants collected in the tier-two populations. The rosettes that did not flower or that had been grazed were discarded, leaving a total of 235 control plants across the six sites compared to 171 plants across the seven release sites. Release sites also contained a second agent, the seed-weevil Larinus latus. The number of L. latus larvae in each capitulum of harvested plants was recorded. The impact of the seed-weevil was calculated using the equation described in Briese (2000), which relates potential seed production to the number of seed weevil larvae present per capitulum and capitulum diameter. 2.3. Statistical analyses Tier two data were analysed per site using repeated measures regression analysis for plant size against rainfall. Multiple repeated measures regression of mean Log(n + 1) L. cardui number per stem was also calculated against mean Log plant taproot diameter and Log plant density for each site and year. Log transformations were applied to normalise the data. Tier one data were used to analyse the impact of L. cardui feeding by comparing the regressions of Log taproot diameter (plant size prior to bolting) against subsequent Log stem height and Log potential seed production (Briese et al., 2004) between release and control sites. Further seed reduction due to L. latus was analysed using ANOVA of Log seed production for treatments with and without L. Latus. Log transformations were necessary to normalise the data.
Following releases of 40 caged adult weevils in 1994 at Lanyon, Bungendore and Galong, there was a rapid increase in L. cardui population density and in the percentage of Onopordum plants attacked at each site (Table 2). Within four years, all plants sampled were attacked and weevil population densities (Table 2) had attained levels higher than generally observed in the weevil’s native European range (D.T. Briese, pers. obs.). The overall establishment rate of the three strategies utilized for the release and establishment of L. cardui was 97%. The release cage, though necessary because of low agent numbers, was the least effective of the three strategies used with a success rate of 93.5%. Once release numbers could be increased, the ‘‘free-release” method led to an establishment rate of 98.4% and replaced the more labour intensive cage method from 1997 (Table 1). This reduced the resources needed for the release of L. cardui. Weevil populations continued to expand to a point where, in spring 1998, heavily attacked stems could readily be found in the near neighbourhood of a release site. This enabled the collection of attacked stems containing large numbers of dormant weevils in spring for redistribution and release. Such ‘‘stem-releases” were used for further redistribution of L. cardui from 1999 onward and the 100% establishment rates made it the most successful strategy adopted during the release process. A contributing factor to this high rate was the use of nursery sites by landholders to protect initial weevil colonies from trampling or overgrazing by livestock. These sites were either small temporarily-fenced enclosures or already-fenced areas that had been set aside for tree-planting and/or native revegetation. 3.2. L. cardui population change at tier-two evaluation sites
3. Results 3.1. Release strategies Of the 195 releases made between 1993 and 1996 (Fig. 1), 183 were monitored; exceptions occurring where landholders could not be contacted after release. As the release network for L. cardui expanded and information was gathered on the high establishment rate of the species, monitoring was restricted to a sub-sample of releases made between 1997 and 1999 (Table 1).
Across the seven tier two release sites, L. cardui numbers increased up until 2001 and then declined, following lower than average rainfall and an ensuing drought (Fig. 2). In 2003, the tier two sites were heavily damaged by stock during drought conditions and at two sites no plants remained, leading to a recorded zero L. cardui population size. The following year the weevil was found again at these two sites at densities of 1.4 and 5.4 adults m 2, showing that L. cardui either survives periods of drought and over-grazing or recolonises areas from elsewhere. Average plant density across all the sites did not vary significantly throughout the study period, despite variable rainfall (Fig. 2). Plant height, but not taproot diameter, was positively correlated with rainfall in the year of flowering (Fig. 3). This is not surprising as an Onopordum population comprises mixed age cohorts (Pettit et al., 1996) whose growth would have depended on earlier rainfall events. Multiple regression of mean L. cardui per stem against mean taproot diameter and mean plant density per site for each of the seven sites over seven seasons from 1997 to 2003 showed that weevil numbers were significantly positively related to plant size (taproot diameter) (R2 = 0.465, F1,40 = 102.0, P < 0.001) while the effects of plant density were not significant (F1,38 = 0.01). The attraction of L. cardui to larger Onopordum plants can also be seen when attack rates and numbers of F1 generation weevils developing in the stems are separated into different plant size categories (Table 3). The smallest category of plants is rarely attacked, while larger size categories support increasingly more progeny per flowering stem. 3.3. Tier one; quantifying the relationship between plant size, reproduction and agent density
Fig. 1. The distribution of Onopordum acanthium and O. illyricum in Australia (map from Parsons and Cuthbertson, 1992), overlaid with the Lixus cardui release sites. Releases were concentrated in the most severely infested part of the range.
There was a significant Log–Log relationship between taproot diameter and plant height at the six control sites in 2002, explaining 73% of the variance in plant height. However, this relationship
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A.E. Swirepik et al. / Biological Control 47 (2008) 108–114 Table 1 Summary of Lixus cardui release and establishment details Caged release (%)
1993 1994 1995 1996 1997 1998 1999
6 14 62 113 56 175 120
40 23 31 32 42 56 150
100 100 97 90 7
Total
546
Free release (%)
3 10 93 99 10
Table 2 Establishment and development of Lixus cardui populations over 3 years on three Onopordum thistle infestations following release in 1994 (N = 40 plants) 1. Site
1996
1997
1998
Percentage plants attacked (%) Galong 80 Bungendore 84 Lanyon 68
98 93 95
100 100 100
Mean (±SE) no. Lixus per stem Galong 6.8 ± 1.2 Bungendore 9.7 ± 1.2 Lanyon 5.1 ± 1.1
20.1 ± 3.2 17.4 ± 3.0 15.5 ± 2.1
20.3 ± 3.2 34.1 ± 3.0 11.6 ± 1.5
Maximum no. Lixus per stem Galong 39 Bungendore 51 Lanyon 32
103 77 64
102 185 37
disappeared in the presence of L. cardui at the release sites (Fig. 4a). Over all sites, multiple regression showed a significant effect of both taproot diameter (F1,11 = 4.88, P < 0.05) and presence of L. cardui (F1,11 = 8.50, P < 0.01) on stem height; taproot diameter and L. cardui explaining 31% and 36% of the variation in stem height, respectively. The relationship between taproot diameter and stem height at control sites was used to predict the expected plant height of Onopordum thistles at the seven release sites had L. cardui not been present (Fig. 4b). Mean (±SE) observed and predicted stem heights per plant at release sites were 59.4 ± 1.7 cm and 88.9 ± 1.8 cm respectively; an overall reduction of 33%. There was also a significant relationship between taproot diameter and seed production at both control and release sites, explaining 91% and 63% of variation, respectively (Fig. 5a). Over all sites, multiple regression showed a significant effect of both taproot diameter (F1,11 = 36.1 P < 0.001) and presence of L. cardui (F1,11 = 16.1, P < 0.01) on seed production; taproot diameter and L. cardui explaining 66% and 29% of the variation in seed production, respectively. The relationship between taproot diameter and seed production at control sites was used to predict the expected seed production of Onopordum thistles at the seven release sites had L. cardui not been present (Fig. 5b). Mean (±SE) observed and predicted potential seed production per plant at release and control sites were 585 ± 50 and 1690 ± 129, respectively, representing an overall reduction of 65%. When L. cardui density for individual plants was considered, rather than the mean overall density per unit area at a site, virtually none of the seed reduction could be attributed to agent density (R2 = 0.0007, F1,169 = 0.13, NS). This apparent contradiction can be explained by the fact that the density measurement on an individual plant included only progeny feeding and developing within each stem, whereas the mean site density measurement provided an indication of total feeding pressure, including defoliation by highly mobile adults prior to and during oviposition. This interpretation conforms to inferences from cage studies (Briese et al., 2004)
Stem release (%)
Fenced (%)
Sites monitored (%)
Sites established (%)
1 90
33 60 44 31 90 31 51
100 100 94 93 68 22 17
83 100 87 97 96 98 100
49
51
97
800 Annual rainfall (mm)
Average no. of individuals released
600 400 200 0
Flowering plants m-2
No. of release sites
1997
1998
1999
2000
2001
2002
2003
1997
1998
1999
2000
2001
2002
2003
1998
1999
2000
2001
2002
2003
2004
6 5 4 3 2 1 0
100 80 L. carduim-2
Year
60 40 20 0
Fig. 2. Mean (±SE) annual rainfall, Lixus cardui adult density at the end of summer and plant density at seven sites in south-eastern New South Wales over a 7-year period.
in which larval feeding within stems did not cause a decline in seed production. The seed weevil, L. latus, caused a further significant decline in seeds per plant (Fig. 5b). ANOVA of the difference in seed production with and without L. latus across the seven release sites was significant (F1,6 = 8.1, P < 0.05) explaining 30% of the variation in potential seed production. Mean (±SE) viable seed production per plant was reduced to 258 ± 30 seeds in the presence of L. latus; a destruction of 56% of seed produced which, overall, reduced seeds per plant by 84%. At the site with the highest density of L. latus, ‘‘Wallah”, potential Onopordum seed production was reduced by 99% (ca. 3100 seeds m 2).
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140
a 100
Stem height cm (log scale)
Stem height (cm)
120 100 80 60 40 20
30 10
0 500
600
700
800
900
1000
Rainfall (mm)
b
Fig. 3. Repeated measures regression analysis of mean stem height (circles, diamonds and squares—different symbols are different sites) and mean taproot diameter (grey triangles) per site against annual rainfall per site per year in the year preceding measurement for seven sites in south-eastern New South Wales. Regression equation of stem height against rainfall y = 0.21x 64.21, R2 = 0.52 (F1,21 = 22.6, P < 0.001).
Table 3 Levels of larval attack by Lixus cardui on different size categories of Onopordum thistle (data pooled from the seven tier 2 release sites) Root basal diameter (mm)
No. of plants in size category
Percentage plants attacked (%)
Mean (±SE) no. Lixus per stem
<15 15–20 20–25 25–30 30–35 35–40 >40
20 28 37 27 20 17 22
5 39 59 44 60 41 59
0.4 ± 0.4 3.6 ± 1.3 8.5 ± 1.5 10.7 ± 2.9 18.5 ± 5.9 25.1 ± 8.1 36.9 ± 10.6
4. Discussion 4.1. Release strategies Ten years after first being released in Australia, the stem-boring weevil, L. cardui, was successfully redistributed throughout the range of its host Onopordum thistles. The simple release methods developed during this process proved so successful that, for the last five years of redistribution, this work was taken up by landholder groups. This allowed additional project resources to be focussed on the evaluation of agent impact in the Onopordum thistle biological control program. The primary aims of the Onopordum biological control program were to reduce soil seed reserves to levels at which emergent plant densities were not economically damaging and to minimise the impact of remaining plants during this period of seed bank decline (Briese et al., 2002b). Briese et al. (2004) showed that L. cardui could reduce both plant growth and reproduction, but the results presented in this paper are the first demonstration of an impact by naturally occurring populations under unconstrained field conditions. Averaged over a range of sites representative of Onopordum-infested pastures in south-eastern Australia, L. cardui reduced plant height by 33% and seed production by 65%. In addition, a complementary agent present at the sites, the seed weevil L. latus, further directly destroyed 56% of the seed produced, leading
Ave plant height (cm)
400
50 Taproot diameter mm (log scale)
120
Predicted
Observed
100 80 60 40 20 0
Site name Fig. 4. Impact of Lixus cardui on Onopordum stem height. (a) Relationship between mean taproot diameter (±SE) and mean stem height (±SE) at 6 controls (filled circles) and 7 release (open circles) sites. This relationship was only significant at control sites (Log stem height = 0.73*Log taproot diameter + 0.92, R2 = 0.73, F1,4=10.9, P < 0.03). (b) Mean (±SE) predicted height of plants compared to heights observed in the presence of L. cardui across the 7 release sites.
to an overall reduction of 84% in the seed added to the soil seed reserve in 2002. Given the widespread success in establishing L. cardui and L. latus throughout the range of Onopordum thistles in Australia, and the measured decline in seed production, indications are that, almost 10 years after release, the two agents are making an important contribution to the control of the thistles. Nonetheless, the desired outcome will take time given the high initial densities of soil seed reserves, reaching over 7500 seeds m 2 at one of the release sites (Pettit et al., 1996), and seed longevity of more than 20 years (Toole and Brown, 1946). Artificial seed removal experiments showed that a 90% reduction in annual seed input could reduce Onopordum soil seed reserves by a similar amount in 10 years (Briese and Walker, unpublished data). If the levels of agent impact measured here are maintained, this should be achievable. However, Briese and Walker (unpublished data) also found a 5-year lag between seedbank and plant density decline, due in part to seed longevity and in part to the facultative perennial nature of Onopordum rosettes. This explains why, during the study period, the above ground density of the thistles did not change significantly. Additionally, both this study and that of Pettit et al. (1996) found that Onopordum growth and seed production vary considerably between years, to a large extent dependent on climate. The positive relationship between L. cardui density and mean plant size at our study sites confirms Briese et al. (2004) prediction that such
A.E. Swirepik et al. / Biological Control 47 (2008) 108–114
Seeds per plant (log scale)
a
113
L. cardui to seed rain reduction reported here may change as a result of the guild’s interactions. The impact of L. cardui has now been clearly demonstrated in the field. Ultimately though, seed bank numbers will need to be determined (Cavers et al., 1995) to confirm how the reduction in seed output described here translates to soil seed reserves. There are, however, already some direct benefits of L. cardui, for reduced plant size should lead to less damage to stock and less contamination of wool. The reduced vigour of attacked thistles should also shift the competitive balance to more desirable pasture species. This will help achieve the secondary aim of the biological control strategy, namely to minimise weed impact during the period of seed-bank decline.
300
30
Acknowledgments 3 10
50 Taproot diameter mm (log scale)
b Number of seeds per plant
4000
Predicted
Observed
With two agents
3000
The authors would like to acknowledge the many people that helped with field collection and processing of samples, including John Lester, Amar Singh and Matthew Smyth. We would also like to thank Andy Sheppard for assistance with statistical analyses and valuable comments on an earlier draft of this manuscript. Funding for this work was provided by the CRC for Australian Weed Management, Meat and Livestock Australia and Australian Wool Innovation. References
2000
1000
0
Site name Fig. 5. Impact of Lixus cardui on Onopordum potential seed production. (a) Relationship between mean taproot diameter (±SE) and mean seed production per plant (±SE) at 6 controls (filled circles) and 7 release (open circles) sites. This relationship was significant at both control (Log seeds per plant = 2.28*Log taproot diameter + 0.16, R2 = 0.91, F1,4 = 41.2, P < 0.003) and release (Log seeds per plant = 1.45*Log taproot diameter + 0.55, R2 = 0.63, F1,5 = 8.4, P < 0.03) sites. (b) Mean (±SE) predicted seeds per plant compared to seeds observed in the presence of L. cardui only and two agents (L. cardui and L. latus) across the 7 release sites.
density dependent effects would modify impact of the weevil in the field. Given that plant size determines the attractiveness of individual plants to L. cardui and can affect the weevil’s reproductive success, the level of seed reduction caused by the control agents is also expected to vary. Ideally, measurements of impact over several seasons would be needed to accurately determine to what extent this occurs, particularly with regard to the effects of drought and consequent fluctuations in agent density. Two further agents, the crown weevil Trichosirocalus briesei and the petiole moth Eublemma amoena, which attack the rosette or vegetative stage of Onopordum spp. thistles, have since been released in Australia (Briese et al., 2002b). The aim is to increase plant mortality and, or, reduce plant size prior to stem elongation. Early impact assessments on both insect species indicate that they are at least capable of reducing plant size (A. Swirepik and M. Smyth, personal observation). In the event that either or both of these agents reach large population densities across the range of Onopordum spp. thistles in south-eastern Australia, it is possible that the combined damage may cause a decline in the population densities of L. cardui. Careful evaluation of these interactions will highlight the value of correctly prioritising the release schedule for agents and also shed new light on the interactions between members of herbivore guild, as the outright contribution of
Briese, D.T., 1996. Potential impact of the stem boring weevil Lixus cardui on the growth and reproductive capacity of Onopordum thistles. Biocontrol Science and Technology 6, 251–261. Briese, D.T., 2000. Impact of the Onopordum capitulum weevil Larinus latus on seed production by its host. Journal of Applied Ecology 37, 238–246. Briese, D.T., 2004. Weed biological control: applying science to solve seemingly intractable problems. Australian Journal of Entomology 43, 304–317. Briese, D.T., Pettit, W.J., Walker, A., 1996. Multiplying cages: a strategy for the rapid redistribution of agents with slow rates of increase. In: Moran, V.C., Hoffman, J.H. (Eds.), Proceedings of the IX International Symposium on Biological Control of Weeds. University of Cape Town, Rondebosch, pp. 243–247. Briese, D.T., Walker, A., Pettit, W., Sagliocco, J-L., 2002a. Host-specificity of candidate agents for Onopordum spp. thistles in Australia: an assessment of testing procedures. Biocontrol Science and Technology 12, 149–163. Briese, D.T., Pettit, W., Swirepik, A., Walker, A., 2002b. A strategy for the biological control of Onopordum spp. thistles in south-eastern Australia. Biocontrol Science and Technology 12, 121–137. Briese, D.T., Heard, T.A., McFadyen, R.E., Sheppard, A.W., Spafford Jacobs, H., 2003. The selection, testing and evaluation of weed biological control agents: Is there still room for improvement? Improving the Selection, Testing and Evaluation of Weed Biological Control Agents. CRC Technical Series No. 7, pp. 1–3. Briese, D.T., Pettit, W.J., Walker, A., 2004. Evaluation of the biological control agent, Lixus cardui, on Onopordum thistles: experimental studies on agent demography and impact. Biological Control 31, 165–171. Campbell, A., 1994. Landcare: Communities Shaping Their Future. Allen and Unwin, Sydney. Cavers, P.B., Groves, R.H., Kaye, P.E., 1995. Seed population dynamics of Onopordum over 1 year in southern New South Wales. Journal of Applied Ecology 32, 425– 433. Day, M.D., Briese, D.T., Grace, B.S., Holtkamp, R.H., Ireson, J.E., Sheppard, A.W., Spafford Jacob, H., 2004. Improving release strategies to increase the establishment rate of weed biocontrol agents. In: Sindel, B. (Ed.), Proceedings of the 13th Australian Weeds Conference. NSW Weeds Society, Sydney, pp. 369–373. Dhileepan, K., 2003. Evaluating the effectiveness of weed biological control at the local scale. In: Spafford Jacob, H., Briese, D.T. (Eds.), Improving the Selection, Testing and Evaluation of Weed Biological Control Agents, CRC for Australian Weed Management Technical Series #7. CRC for Australian Weed Management, Adelaide, pp. 51–60. Hopper, K.R., Roush, R.T., 1993. Mate finding, dispersal, number released and the success of biological control introductions. Ecological Entomology 18, 321–331. Krebs, C.J., 1989. Ecological Methodology. Springer, Berlin. pp. 141. McClay, A., 1995. Beyond before and after: experimental design and evaluation in classical weed biological control. In: Delfosse, E.S., Scott, R.R. (Eds.), Proceedings of the VIIIth International Symposium on Biological Control of Weeds, CSIRO, Melbourne, pp. 213–219. McFadyen, R.E., 1998. Biological control of weeds. Annual Review of Entomology 43, 369–393. O’Hanlon, P.C., Peakall, R., Briese, D.T., 1999. AFLP reveals introgression in weedy Onopordum thistles: hybridisation and invasion. Molecular Ecology 8, 1239– 1246.
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Parsons, W.T., Cuthbertson, E.G., 1992. Noxious Weeds of Australia. Inkata Press, Melbourne. Pettit, W.J., Briese, D.T., Walker, A., 1996. Aspects of thistle population dynamics with reference to Onopordum. Plant Protection Quarterly 11 (Suppl. 2), 232– 235. Swirepik, A.E., Briese, D.T., 2000. Agent release techniques: what is appropriate for redistribution projects that involve community groups? In: Proceedings of the Xth International Symposium on Biological Control of Weeds. Montana State University, pp. 783–788.
Swirepik, A.E., Smyth, M.J., 2003. Evaluating biological control at the regional scale. In: Spafford Jacob, H., Briese, D.T. (Eds.), Improving the Selection, Testing and Evaluation of Weed Biological Control Agents, CRC for Australian Weed Management Technical Series #7. CRC for Australian Weed Management, Adelaide, pp. 61–67. Thomas, M.B., Reid, A.M., 2007. Are exotic natural enemies an effective way of controlling invasive plants? TREE 22, 447–453. Toole, E.H., Brown, E., 1946. Final results of the Duvel buried seed experiment. Journal of Agricultural Research 72, 201–210.
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Biological Control journal homepage: www.elsevier.com/locate/ybcon
Evaluation of the biological control agent, Lixus cardui, on Onopordum thistles: Establishment and initial field impact A.E. Swirepik a, P.J. Turner b, D.T. Briese a,* a b
CSIRO Entomology and CRC for Australian Weed Management, GPO Box 1700, Canberra 2601, Australia Applied Ecology Research Group and CRC for Australian Weed Management, University of Canberra, ACT 2601, Australia
a r t i c l e
i n f o
Article history: Received 26 April 2008 Accepted 13 June 2008 Available online 27 June 2008 Keywords: Lixus cardui Larinus latus Onopordum thistles Biological control Field establishment Impact assessment
a b s t r a c t Between 1993 and 1999 a biological control program focused on the establishment and redistribution of the stem boring weevil Lixus cardui on its host Onopordum thistles in south-eastern Australia. Three release strategies (caged, free-release and stem release) were assessed as a part of a redistribution process that involved engagement with community members, with an overall establishment rate of 97%. Long-term monitoring of seven sites between 1998 and 2004 showed that L. cardui densities fluctuated by up to 30-fold among sites and 12-fold among seasons, driven largely by climatic effects on host plant size and with no measurable effect of plant density. During 2002 a comparison was made of plant performance at seven release sites with L. cardui present and six matched control sites without L. cardui. Plant size and L. cardui density varied between release sites and, on average, L. cardui reduced plant height by 33% and seed production by 65%. A second agent, the seed weevil Larinus latus, directly destroyed 56% of the seed produced at the sites, leading to a mean overall reduction of 84% in seed added to the soil during 2002 (ranging from 37% to 99% across sites). The significant reductions in seed rain that are being accrued by these two agents augur well for the long-term success of this project. Crown copyright Ó 2008 Published by Elsevier Inc. All rights reserved.
1. Introduction Release strategies and the evaluation of biological control agents are key aspects of biological control that would benefit from further research (McClay, 1995; McFadyen, 1998; Briese et al., 2003; Briese, 2004; Day et al., 2004). Only about 60% of biological control agents released against target weeds establish (McFadyen, 1998). There are a number of clear ecological reasons for this, such as agent-target or agent-climate incompatibility, but some failures are attributable to release protocols that fail to maximise the chances of establishment due to problems such as Allee effects (Hopper and Roush, 1993). Less than half the established biological control agents have had their impact on the target weeds scientifically evaluated (Thomas and Reid, 2007), leading to a severe lack of data to measure the effectiveness of weed biological control (McFadyen, 1998). Where it occurs, such evaluation often involves only laboratory studies or assessment over very limited spatial scales (Thomas and Reid, 2007). Post release field evaluation of the impacts of biological control agents may take the form of exclusion studies (using pesticides or cages), studies of reference sites before and after agent release, or comparisons of release and control sites. In this study, a range of release strategies and both temporal and spatial evaluation are * Corresponding author. Fax: +61 2 6246 4000. E-mail address: [email protected] (D.T. Briese).
presented for the stem-boring weevil, Lixus cardui Olivier, released in Australia in 1993 (Briese et al., 2002a), as part of a strategy for the biological control of exotic Onopordum thistles (Asteraceae: Cardueae). The main aims of this program were to reduce soil seed reserves to levels where emergent plant densities would not be economically damaging and to minimise the deleterious effects of remaining thistles during this period of seed bank decline (Briese et al., 2002b). These objectives were to be achieved by introducing a complementary suite of control agents targeting different stages of the Onopordum life-cycle. Studies in its native European range (Briese, 1996) had shown that L. cardui feeding damage could contribute towards both objectives because their damage reduces both the growth of flowering stems and seed production of Onopordum by up to 50% and 80%, respectively, and leads to early senescence of mature plants. L. cardui has now been redistributed throughout the range of Onopordum thistles in south-eastern Australia (Swirepik and Briese, 2000). This study is an extension of the work by Briese et al. (2004), where the life-cycle and impact of L. cardui were determined under caged conditions. Briese et al. (2004) measured the longevity, fecundity and survival of L. cardui in Australia following release and concluded that the weevil had the capacity for rapid population increase. In addition, through an experiment in which L. cardui were caged at fixed known densities on plants of varying size in a natural population of Onopordum, they were able to describe the
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relationship between plant growth and reproduction on different sized plants and at different levels of attack by the weevil. However, they cautioned that these relationships were likely to be dampened once the behavioural responses of L. cardui, namely a preference for larger plants and emigration when food quality deteriorated, were considered. This paper reports on research undertaken from post-release field studies where L. cardui behaviour was unconstrained. It firstly describes the initial release and establishment phase for L. cardui in Australia, including population changes during the first few years following release, and then evaluates the impact of unconstrained natural populations of the weevil at a number of sites where populations had reached relatively high densities. The combination of detailed impact data from a few selected sites combined with broader monitoring of establishment and population changes of the agent should enable a regional view of biological control success to be developed (Swirepik and Smyth, 2003). To evaluate the impact of unconstrained L. cardui populations, plant production was compared between sites where the agent was present and ‘‘control” sites where the agent was absent. The interaction of L. cardui with another introduced biological control agent, the seedweevil, Larinus latus Herbst, was also assessed to monitor the combined effect of the two agents. 2. Materials and methods 2.1. Release strategies Three different release methods were used to deploy L. cardui during a seven-year period commencing on 14 November 1993. The release process was initially modelled on the ‘‘multiplying cages” procedure described by Briese et al. (1996). Only 240 weevils were available for these initial releases, so release cages provided a safeguard against Allee effects. For this method a mesh release cage (3.6 m 1.8 m 1.8 m) was hung between steel posts over a dense patch of thistles. Twenty to 50 adults were then placed inside the cage and allowed to complete a generation on the enclosed thistles. Landholders belonging to a local Landcare Group (see Campbell, 1994) were invited to a ‘‘field day” at the site the following spring, where freshly emerged F1 adults were harvested and placed into small mesh bags with cut leaves. These were then given to the Landcare members, together with a cage and a release kit, enabling them to make new releases on their own properties. Once establishment of the agent was confirmed, a second ‘‘free-release” method was used which relied upon releasing adults but without the expense or effort of erecting cages. The third release method, ‘‘stem release”, was introduced to facilitate mass distribution. This simplified the release process and created a greater window of opportunity for redistributing L. cardui. ‘‘Stem releases” involved harvesting dead stems containing overwintering adults. Stems could then be transported for release at any time from the end of summer until the beginning of spring, as the adults remain dormant in situ during this period. Relocated stems were placed on the ground at the release site and the adults emerged during the following spring to continue the life cycle. 2.2. Evaluating L. cardui population growth and impacts The monitoring of release and population performance for L. cardui was based on the three-tiered system described by Swirepik and Smyth (2003), where tier three provides the least detailed information on establishment and spread, tier two measures both agent and target population dynamics and tier one defines agent impact in the greatest detail.
109
In the tier-three evaluation, all L. cardui release sites were monitored for establishment in the spring following release, when they were revisited for agent collection and redistribution. Lixus cardui was considered to have established at a site if sufficient adults could be found to collect for redistribution at this time. Due to the scale of the release process and the available project resources, effort placed into monitoring for establishment and agent spread was reduced as the project grew. At the three initial 1994 release sites, data on the number of overwintering adults were also collected over a three year period from 1996 to 1998 by dissecting 40 randomly collected Onopordum stems following senescence. In the tier-two evaluation, seven release sites located throughout the Southern Tablelands of New South Wales were monitored to measure plant density and agent population size from 1997 to 2003. Lixus cardui had been released at these sites in either 1993 or 1994. Estimates of the population densities of Onopordum thistles and L. cardui were obtained using the T-Square method of sampling (see Krebs, 1989). At each site, after the plants had senesced, a 30 m transect tape was laid out and the distance to the nearest thistle within a 360° radius of the transect tape measured at 2 m intervals. Once the distance to the first plant was recorded the distance to a second thistle within 180° to the outside of the first thistle was measured and recorded. This provided a sample of 30 plants which were then dug up from the soil. The diameter of the taproot at the base of the crown of each plant was measured, and the roots were bagged individually and transported to the laboratory for further processing. Taproot diameter is a good predictor of plant height and seed production prior to the elongation of Onopordum flowering stems (Briese et al., 2004). Between 1999 and 2003, plant height was also recorded before the plants were harvested. Stems collected from the field were dissected in the laboratory, and the total number of L. cardui per stem was recorded. The counts were converted to field population densities of weevils per m2, based on the respective plant densities measured at each site in each year. Plants affected by sheep or cattle grazing were discarded to ensure that only the effects of L. cardui were measured. Sample size was never less than 14 plants at any given site in any given year. In addition, the capitula were counted and measured to determine seed production per plant, calculated using a previously determined regression equation relating seed number to capitulum diameter (see Pettit et al., 1996). In the tier-one evaluation, chemical exclusion trials are often used to measure the impact of insect herbivores on their host plants (see Dhileepan, 2003). In the case of L. cardui, this was considered impractical as the adult weevils are highly mobile and, at the scale of feasible insecticide applications, considerable feeding damage could still be expected from immigrant L. cardui. Consequently, the known relationship between plant size prior to bolting and the subsequent reproductive output of Onopordum thistles (Briese et al., 2004) was used to predict potential plant growth and seed production and to statistically determine the changes in these parameters due to L. cardui feeding. In 2002, the data collected from the seven tier-two sample sites were compared to data collected from plants harvested in the same manner from six control sites where the agent was not yet present, but also located across the Southern Tablelands of New South Wales. Three criteria were used to select control sites: (a) practical proximity to release sites while remaining L. cardui free; (b) climatic and physical similarity to release sites; and (c) phenotypic similarity to the hybrid variety of Onopordum acanthium illyricum (O’Hanlon et al., 1999) at the monitored release sites. At each control site, 50 rosettes were selected in September 2001 to include the full range of plant sizes, based on average rosette diameter (see Briese et al., 2004). These plants were marked and the area fenced. Plants were then left to complete flowering
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and senesce over the summer before collection in February 2002. These plants were then harvested and processed in the laboratory together with plants collected in the tier-two populations. The rosettes that did not flower or that had been grazed were discarded, leaving a total of 235 control plants across the six sites compared to 171 plants across the seven release sites. Release sites also contained a second agent, the seed-weevil Larinus latus. The number of L. latus larvae in each capitulum of harvested plants was recorded. The impact of the seed-weevil was calculated using the equation described in Briese (2000), which relates potential seed production to the number of seed weevil larvae present per capitulum and capitulum diameter. 2.3. Statistical analyses Tier two data were analysed per site using repeated measures regression analysis for plant size against rainfall. Multiple repeated measures regression of mean Log(n + 1) L. cardui number per stem was also calculated against mean Log plant taproot diameter and Log plant density for each site and year. Log transformations were applied to normalise the data. Tier one data were used to analyse the impact of L. cardui feeding by comparing the regressions of Log taproot diameter (plant size prior to bolting) against subsequent Log stem height and Log potential seed production (Briese et al., 2004) between release and control sites. Further seed reduction due to L. latus was analysed using ANOVA of Log seed production for treatments with and without L. Latus. Log transformations were necessary to normalise the data.
Following releases of 40 caged adult weevils in 1994 at Lanyon, Bungendore and Galong, there was a rapid increase in L. cardui population density and in the percentage of Onopordum plants attacked at each site (Table 2). Within four years, all plants sampled were attacked and weevil population densities (Table 2) had attained levels higher than generally observed in the weevil’s native European range (D.T. Briese, pers. obs.). The overall establishment rate of the three strategies utilized for the release and establishment of L. cardui was 97%. The release cage, though necessary because of low agent numbers, was the least effective of the three strategies used with a success rate of 93.5%. Once release numbers could be increased, the ‘‘free-release” method led to an establishment rate of 98.4% and replaced the more labour intensive cage method from 1997 (Table 1). This reduced the resources needed for the release of L. cardui. Weevil populations continued to expand to a point where, in spring 1998, heavily attacked stems could readily be found in the near neighbourhood of a release site. This enabled the collection of attacked stems containing large numbers of dormant weevils in spring for redistribution and release. Such ‘‘stem-releases” were used for further redistribution of L. cardui from 1999 onward and the 100% establishment rates made it the most successful strategy adopted during the release process. A contributing factor to this high rate was the use of nursery sites by landholders to protect initial weevil colonies from trampling or overgrazing by livestock. These sites were either small temporarily-fenced enclosures or already-fenced areas that had been set aside for tree-planting and/or native revegetation. 3.2. L. cardui population change at tier-two evaluation sites
3. Results 3.1. Release strategies Of the 195 releases made between 1993 and 1996 (Fig. 1), 183 were monitored; exceptions occurring where landholders could not be contacted after release. As the release network for L. cardui expanded and information was gathered on the high establishment rate of the species, monitoring was restricted to a sub-sample of releases made between 1997 and 1999 (Table 1).
Across the seven tier two release sites, L. cardui numbers increased up until 2001 and then declined, following lower than average rainfall and an ensuing drought (Fig. 2). In 2003, the tier two sites were heavily damaged by stock during drought conditions and at two sites no plants remained, leading to a recorded zero L. cardui population size. The following year the weevil was found again at these two sites at densities of 1.4 and 5.4 adults m 2, showing that L. cardui either survives periods of drought and over-grazing or recolonises areas from elsewhere. Average plant density across all the sites did not vary significantly throughout the study period, despite variable rainfall (Fig. 2). Plant height, but not taproot diameter, was positively correlated with rainfall in the year of flowering (Fig. 3). This is not surprising as an Onopordum population comprises mixed age cohorts (Pettit et al., 1996) whose growth would have depended on earlier rainfall events. Multiple regression of mean L. cardui per stem against mean taproot diameter and mean plant density per site for each of the seven sites over seven seasons from 1997 to 2003 showed that weevil numbers were significantly positively related to plant size (taproot diameter) (R2 = 0.465, F1,40 = 102.0, P < 0.001) while the effects of plant density were not significant (F1,38 = 0.01). The attraction of L. cardui to larger Onopordum plants can also be seen when attack rates and numbers of F1 generation weevils developing in the stems are separated into different plant size categories (Table 3). The smallest category of plants is rarely attacked, while larger size categories support increasingly more progeny per flowering stem. 3.3. Tier one; quantifying the relationship between plant size, reproduction and agent density
Fig. 1. The distribution of Onopordum acanthium and O. illyricum in Australia (map from Parsons and Cuthbertson, 1992), overlaid with the Lixus cardui release sites. Releases were concentrated in the most severely infested part of the range.
There was a significant Log–Log relationship between taproot diameter and plant height at the six control sites in 2002, explaining 73% of the variance in plant height. However, this relationship
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A.E. Swirepik et al. / Biological Control 47 (2008) 108–114 Table 1 Summary of Lixus cardui release and establishment details Caged release (%)
1993 1994 1995 1996 1997 1998 1999
6 14 62 113 56 175 120
40 23 31 32 42 56 150
100 100 97 90 7
Total
546
Free release (%)
3 10 93 99 10
Table 2 Establishment and development of Lixus cardui populations over 3 years on three Onopordum thistle infestations following release in 1994 (N = 40 plants) 1. Site
1996
1997
1998
Percentage plants attacked (%) Galong 80 Bungendore 84 Lanyon 68
98 93 95
100 100 100
Mean (±SE) no. Lixus per stem Galong 6.8 ± 1.2 Bungendore 9.7 ± 1.2 Lanyon 5.1 ± 1.1
20.1 ± 3.2 17.4 ± 3.0 15.5 ± 2.1
20.3 ± 3.2 34.1 ± 3.0 11.6 ± 1.5
Maximum no. Lixus per stem Galong 39 Bungendore 51 Lanyon 32
103 77 64
102 185 37
disappeared in the presence of L. cardui at the release sites (Fig. 4a). Over all sites, multiple regression showed a significant effect of both taproot diameter (F1,11 = 4.88, P < 0.05) and presence of L. cardui (F1,11 = 8.50, P < 0.01) on stem height; taproot diameter and L. cardui explaining 31% and 36% of the variation in stem height, respectively. The relationship between taproot diameter and stem height at control sites was used to predict the expected plant height of Onopordum thistles at the seven release sites had L. cardui not been present (Fig. 4b). Mean (±SE) observed and predicted stem heights per plant at release sites were 59.4 ± 1.7 cm and 88.9 ± 1.8 cm respectively; an overall reduction of 33%. There was also a significant relationship between taproot diameter and seed production at both control and release sites, explaining 91% and 63% of variation, respectively (Fig. 5a). Over all sites, multiple regression showed a significant effect of both taproot diameter (F1,11 = 36.1 P < 0.001) and presence of L. cardui (F1,11 = 16.1, P < 0.01) on seed production; taproot diameter and L. cardui explaining 66% and 29% of the variation in seed production, respectively. The relationship between taproot diameter and seed production at control sites was used to predict the expected seed production of Onopordum thistles at the seven release sites had L. cardui not been present (Fig. 5b). Mean (±SE) observed and predicted potential seed production per plant at release and control sites were 585 ± 50 and 1690 ± 129, respectively, representing an overall reduction of 65%. When L. cardui density for individual plants was considered, rather than the mean overall density per unit area at a site, virtually none of the seed reduction could be attributed to agent density (R2 = 0.0007, F1,169 = 0.13, NS). This apparent contradiction can be explained by the fact that the density measurement on an individual plant included only progeny feeding and developing within each stem, whereas the mean site density measurement provided an indication of total feeding pressure, including defoliation by highly mobile adults prior to and during oviposition. This interpretation conforms to inferences from cage studies (Briese et al., 2004)
Stem release (%)
Fenced (%)
Sites monitored (%)
Sites established (%)
1 90
33 60 44 31 90 31 51
100 100 94 93 68 22 17
83 100 87 97 96 98 100
49
51
97
800 Annual rainfall (mm)
Average no. of individuals released
600 400 200 0
Flowering plants m-2
No. of release sites
1997
1998
1999
2000
2001
2002
2003
1997
1998
1999
2000
2001
2002
2003
1998
1999
2000
2001
2002
2003
2004
6 5 4 3 2 1 0
100 80 L. carduim-2
Year
60 40 20 0
Fig. 2. Mean (±SE) annual rainfall, Lixus cardui adult density at the end of summer and plant density at seven sites in south-eastern New South Wales over a 7-year period.
in which larval feeding within stems did not cause a decline in seed production. The seed weevil, L. latus, caused a further significant decline in seeds per plant (Fig. 5b). ANOVA of the difference in seed production with and without L. latus across the seven release sites was significant (F1,6 = 8.1, P < 0.05) explaining 30% of the variation in potential seed production. Mean (±SE) viable seed production per plant was reduced to 258 ± 30 seeds in the presence of L. latus; a destruction of 56% of seed produced which, overall, reduced seeds per plant by 84%. At the site with the highest density of L. latus, ‘‘Wallah”, potential Onopordum seed production was reduced by 99% (ca. 3100 seeds m 2).
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140
a 100
Stem height cm (log scale)
Stem height (cm)
120 100 80 60 40 20
30 10
0 500
600
700
800
900
1000
Rainfall (mm)
b
Fig. 3. Repeated measures regression analysis of mean stem height (circles, diamonds and squares—different symbols are different sites) and mean taproot diameter (grey triangles) per site against annual rainfall per site per year in the year preceding measurement for seven sites in south-eastern New South Wales. Regression equation of stem height against rainfall y = 0.21x 64.21, R2 = 0.52 (F1,21 = 22.6, P < 0.001).
Table 3 Levels of larval attack by Lixus cardui on different size categories of Onopordum thistle (data pooled from the seven tier 2 release sites) Root basal diameter (mm)
No. of plants in size category
Percentage plants attacked (%)
Mean (±SE) no. Lixus per stem
<15 15–20 20–25 25–30 30–35 35–40 >40
20 28 37 27 20 17 22
5 39 59 44 60 41 59
0.4 ± 0.4 3.6 ± 1.3 8.5 ± 1.5 10.7 ± 2.9 18.5 ± 5.9 25.1 ± 8.1 36.9 ± 10.6
4. Discussion 4.1. Release strategies Ten years after first being released in Australia, the stem-boring weevil, L. cardui, was successfully redistributed throughout the range of its host Onopordum thistles. The simple release methods developed during this process proved so successful that, for the last five years of redistribution, this work was taken up by landholder groups. This allowed additional project resources to be focussed on the evaluation of agent impact in the Onopordum thistle biological control program. The primary aims of the Onopordum biological control program were to reduce soil seed reserves to levels at which emergent plant densities were not economically damaging and to minimise the impact of remaining plants during this period of seed bank decline (Briese et al., 2002b). Briese et al. (2004) showed that L. cardui could reduce both plant growth and reproduction, but the results presented in this paper are the first demonstration of an impact by naturally occurring populations under unconstrained field conditions. Averaged over a range of sites representative of Onopordum-infested pastures in south-eastern Australia, L. cardui reduced plant height by 33% and seed production by 65%. In addition, a complementary agent present at the sites, the seed weevil L. latus, further directly destroyed 56% of the seed produced, leading
Ave plant height (cm)
400
50 Taproot diameter mm (log scale)
120
Predicted
Observed
100 80 60 40 20 0
Site name Fig. 4. Impact of Lixus cardui on Onopordum stem height. (a) Relationship between mean taproot diameter (±SE) and mean stem height (±SE) at 6 controls (filled circles) and 7 release (open circles) sites. This relationship was only significant at control sites (Log stem height = 0.73*Log taproot diameter + 0.92, R2 = 0.73, F1,4=10.9, P < 0.03). (b) Mean (±SE) predicted height of plants compared to heights observed in the presence of L. cardui across the 7 release sites.
to an overall reduction of 84% in the seed added to the soil seed reserve in 2002. Given the widespread success in establishing L. cardui and L. latus throughout the range of Onopordum thistles in Australia, and the measured decline in seed production, indications are that, almost 10 years after release, the two agents are making an important contribution to the control of the thistles. Nonetheless, the desired outcome will take time given the high initial densities of soil seed reserves, reaching over 7500 seeds m 2 at one of the release sites (Pettit et al., 1996), and seed longevity of more than 20 years (Toole and Brown, 1946). Artificial seed removal experiments showed that a 90% reduction in annual seed input could reduce Onopordum soil seed reserves by a similar amount in 10 years (Briese and Walker, unpublished data). If the levels of agent impact measured here are maintained, this should be achievable. However, Briese and Walker (unpublished data) also found a 5-year lag between seedbank and plant density decline, due in part to seed longevity and in part to the facultative perennial nature of Onopordum rosettes. This explains why, during the study period, the above ground density of the thistles did not change significantly. Additionally, both this study and that of Pettit et al. (1996) found that Onopordum growth and seed production vary considerably between years, to a large extent dependent on climate. The positive relationship between L. cardui density and mean plant size at our study sites confirms Briese et al. (2004) prediction that such
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Seeds per plant (log scale)
a
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L. cardui to seed rain reduction reported here may change as a result of the guild’s interactions. The impact of L. cardui has now been clearly demonstrated in the field. Ultimately though, seed bank numbers will need to be determined (Cavers et al., 1995) to confirm how the reduction in seed output described here translates to soil seed reserves. There are, however, already some direct benefits of L. cardui, for reduced plant size should lead to less damage to stock and less contamination of wool. The reduced vigour of attacked thistles should also shift the competitive balance to more desirable pasture species. This will help achieve the secondary aim of the biological control strategy, namely to minimise weed impact during the period of seed-bank decline.
300
30
Acknowledgments 3 10
50 Taproot diameter mm (log scale)
b Number of seeds per plant
4000
Predicted
Observed
With two agents
3000
The authors would like to acknowledge the many people that helped with field collection and processing of samples, including John Lester, Amar Singh and Matthew Smyth. We would also like to thank Andy Sheppard for assistance with statistical analyses and valuable comments on an earlier draft of this manuscript. Funding for this work was provided by the CRC for Australian Weed Management, Meat and Livestock Australia and Australian Wool Innovation. References
2000
1000
0
Site name Fig. 5. Impact of Lixus cardui on Onopordum potential seed production. (a) Relationship between mean taproot diameter (±SE) and mean seed production per plant (±SE) at 6 controls (filled circles) and 7 release (open circles) sites. This relationship was significant at both control (Log seeds per plant = 2.28*Log taproot diameter + 0.16, R2 = 0.91, F1,4 = 41.2, P < 0.003) and release (Log seeds per plant = 1.45*Log taproot diameter + 0.55, R2 = 0.63, F1,5 = 8.4, P < 0.03) sites. (b) Mean (±SE) predicted seeds per plant compared to seeds observed in the presence of L. cardui only and two agents (L. cardui and L. latus) across the 7 release sites.
density dependent effects would modify impact of the weevil in the field. Given that plant size determines the attractiveness of individual plants to L. cardui and can affect the weevil’s reproductive success, the level of seed reduction caused by the control agents is also expected to vary. Ideally, measurements of impact over several seasons would be needed to accurately determine to what extent this occurs, particularly with regard to the effects of drought and consequent fluctuations in agent density. Two further agents, the crown weevil Trichosirocalus briesei and the petiole moth Eublemma amoena, which attack the rosette or vegetative stage of Onopordum spp. thistles, have since been released in Australia (Briese et al., 2002b). The aim is to increase plant mortality and, or, reduce plant size prior to stem elongation. Early impact assessments on both insect species indicate that they are at least capable of reducing plant size (A. Swirepik and M. Smyth, personal observation). In the event that either or both of these agents reach large population densities across the range of Onopordum spp. thistles in south-eastern Australia, it is possible that the combined damage may cause a decline in the population densities of L. cardui. Careful evaluation of these interactions will highlight the value of correctly prioritising the release schedule for agents and also shed new light on the interactions between members of herbivore guild, as the outright contribution of
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