Development and use of pheromones for monitoring lepidopteran forest defoliators in North America

Forest Ecology and Management, 39 ( 1991 ) 153-162 Elsevier Science Publishers B.V., A m s t e r d a m

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Development and use of pheromones for monitoring lepidopteran forest defoliators in North America G.G. Grant Forest Pest Management Institute. Foresto, Canada, P.O. Box 490, Sauk Ste. Marie, Ontaru) P6A 5M7. Canada

ABSTRACT Grant, G.G., 1991. Development and use of pheromones for monitoring lepidopteran forest defoliators in North America. For. Ecol. Manage., 39: 153-162. Pheromone-baited traps are increasingly used in forestry to detect and monitor infestations of lepidopteran defoliators. Pheromone-based detection surveys, as exemplified by gypsy-moth trapping in Canada, are designed to indicate the presence and geographical distribution of pests and trigger other surveillance techniques, such as ground searches for egg masses. Pheromone monitoring systems are designed to quantitatively assess insect populations and provide early warning of outbreaks or damage. Key elements in developing monitoring systems are selection of trap design and lure potency that are effective over a wide range of population densities without trap saturation, and establishment of criteria for interpreting trap catches. Examples of recently developed pheromone monitoring systems for three serious forest pests, the Douglas-fir tussock moth, Orgyia pseudotsugata, the western spruce budworm, Choristoneura occidentalis, and the spruce budworm, (-~fumferana, are examined in detail.

INTRODUCTION

To make rational pest-management decisions, forest managers rely on various survey procedures to assess insect populations. Traditionally, larval, egg and defoliation sampling have been the principal methods of surveillance, but sex-pheromone-baited traps are now widely used because of their low cost, ease of use, and high sensitivity. In Canada, for example~ some 19 lepidopreran pests in a wide variety of forestry conditions were recently surveyed with pheromone traps to detect infestations and monitor populations (Table 1). Detection trapping is used qualitatively to determine the presence and geographical distribution of pests, particularly when they are at low levels. Interpretation of catches is simple and usually not a problem. In monitoring surveys, catches are used quantitatively to estimate insect abundance, track 0378-1127/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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G.G. GRANT

TABLE 1 Lepidopteran pests recently surveyed with sex-pheromone traps by Forestry, Canada Insect species

Location

Status a

Objective

Coniferous forest pests

Choristoneurafumiferana (spruce budworm )

Choristoneura pinus pinus (jack pine budworm )

Choristoneura occidentalis

Eastern North Semi-operational Monitoring (to forecast America outbreaks ) Eastern and central Developmental Detection survey Canada British Columbia Developmental Detection and monitoring

( western spruce budworm )

Coleophora laricella

British Columbia

Developmental

Detection survey

British Columbia

Operational

British Columbia

Developmental

Monitoring (to forecast outbreaks ) Detection survey

Ontario

Developmental

Monitoring

Ontario, Maritimes

Developmental

Eastern Canada, British Columbia Maritimes, Newfoundland Maritimes

Operational

Monitoring (replace egg sampling for forecasting defoliation ) Detection (triggers egg sampling leading to control measures) Detection (replaces annual eggmass surveys in Maritimes) Monitoring

British Columbia, (European pine shoot moth) Maritimes

Operational

Zeiraphera canadensis

Maritimes

Developmental

Detection survey (supports certification of shoot-moth-free nursery stock in B.C. ) Detection survey

Maritimes

Developmental

Detection Survey

British Columbia

Developmental

Detection survey

Maritimes

Developmental

Detection survey

Maritimes, Ontario

Developmental

Detection survey

Across Canada

Semi-operational

Detection and monitoring

British Columbia

Developmental

Detection survey

(larch case bearer)

Orgyia pseudotsugata ( Douglas-fir tussock moth )

Zeiraphera improbana (larch bud moth ) Deciduous forest pests

Choristoneura conflictana (large aspen tortrix )

Croesia semipurpurana (oak leaf shredder )

Lymantria dispar (Gypsy moth )

Malacosoma disstria (forest tent caterpillar)

Pseudexentera spoliana

Operational Developmental

(oak olethreutid leaf roller ) Plantation pests

Rh.vacionia buoliana

(spruce bud moth )

Zeiraphera unfortunanaa ( purplestriped shootworm ) Seed-orchard pests

Barbara colfaxiana ( Douglas-fir cone moth)

(),dia strobilella (spruce seed moth )

Dioryctria reniculelloides ( spruce coneworm ) Forest regeneration pests b

Actebiafennica (black army cutworm )

Peridroma saucia (variegated cutworm )

aTerms somewhat arbitrarily applied. "Operational' indicates that pheromones have been in use a long time and system works reasonable well for indicated purpose. 'Semi-operational' indicates room for improvement, but system works reasonably well. 'Developmental' indicates feasibility studies have been initiated. bThese insects attack coniferous seedlings in regeneration sites.

PHEROMONE MONITORING OF LEPIDOPTERAN DEFOLIATORS

155

population changes, or predict outbreaks and host damage. The trapping data may lead directly to control measures, but more often they trigger further sampling with other methods to provide more precise information about the infestation. However, few pheromone monitoring systems in forestry are operational. The difficulty lies in interpreting trap catches and making reliable forecasts based on them. This review focuses on several well-developed pheromone trapping systems for lepidopteran forest defoliators to examine current progress in solving this problem. PHEROMONE DETECTION SURVEYS

As Table 1 demonstrates, pheromone traps are most frequently used for detection purposes. An example is the pheromone trapping program in Canada for the gypsy moth, Lymantria dispar, which has helped contain the pest in the sporadically infested regions of Canada, particularly British Columbia and the Maritime provinces. Delta sticky traps, baited with 500/tg of the ( + ) enantiomer of disparlure, are deployed across Canada to locate new infestations and define areas where egg-mass searches should be concentrated (Kondo and Moody, 1987; A. Schmidt, personal communication, 1988 ). As in most pheromone detection surveys (Daterman, 1982), a strong lure is used to maximize trap sensitivity because low-level infestations are the targets and trap saturation is generally not a concern. In the Maritime provinces, where male gypsy moths immigrate on weather fronts from neighboring areas of the United States, interference from nonresident moths is minimized by carefully timing the deployment of traps to coincide with the flight period of indigenous moths, which usually occurs after the flight of the migrants (Magasi, 1988). Catches may be plotted on maps to show distribution patterns and pinpoint locations requiring egg-mass surveys. Control measures, including sanitation practices, mass trapping with pheromone-baited traps, and occasionally sprays of chemical insecticide or Bacillus thuringiensis (B.t.) are applied in areas with persistent, low-level infestations. CHARACTERISTICS OF P H E R O M O N E M O N I T O R I N G SYSTEMS

Many factors are considered in developing trapping systems to quantitatively assess insect populations (Daterman, 1978, 1982: Houseweart et al., 1981; Bradshaw et al., 1983; Card6 and Elkinton, 1984; Sanders, 1986a: Sanders and Meighen, 1987). One of the most important is the selection of a suitable combination of trap design and pheromone dosage (which affects pheromone release rate) that allows catches to be calibrated against population density or damage without trap saturation. Although numerous pheromone trap designs are available (Card6 and Elkinton, 1984: Sanders, 1986a ). there are essentially only two choices: sticky traps with a limited trapping

156

G.G. GRANT

surface, and large-capacity, nonsaturating traps. Sticky traps are relatively inexpensive and easily deployed, and the number of insects caught can be readily counted by visual inspection. A major disadvantage is their tendency to saturate with a relatively small number of moths (Daterman, 1978; Houseweart et al., 1981; Daterman et al., 1982 ). With saturation, the catch no longer has a quantitative relationship with population density or changes in density, and comparison of catches between trapping sites may not be meaningful. Use of nonsaturating traps allows a considerably greater catch, but these traps also have some important drawbacks. They are bulkier and more expensive than sticky traps, although the cost is reduced if they can be washed and reused. However, despite washing, pheromone contamination can persist (Grant, unpublished data, 1989 ) with unknown effects on subsequent catches. Handling and counting the large numbers of insects that are caught is inconvenient and time-consuming for large projects, and messy ifa liquid reservoir is used to kill the insects. Where an insecticide is used, the captured insects can become repellent, reducing trap efficiency (Sanders, 1986b; Elkinton, 1987 ). Thus, calibration of these traps with population density may be more complex than initially envisioned. Research on the problem of trap saturation is active, and several solutions have been proposed (Daterman, 1982). A particularly effective one, championed by Daterman and his colleagues (Daterman, 1978, 1982; Daterman et al., 1979; Sartwell et al., 1985; Shepherd et al., 1985), is the use of sticky traps with low-potency lures, usually with a release rate less than that of a female. This approach is described in two of the monitoring systems considered below. D E V E L O P M E N T O F P H E R O M O N E M O N I T O R I N O SYSTEMS

Douglas-fir tussock moth (Orgyia pseudotsugata) Females of the Douglas-fir tussock moth are flightless, and dispersal occurs through the silking behavior of young larvae. Consequently, populations are highly aggregated and difficult to find when they are below outbreak densities, and defoliation can appear with little warning. The easiest way to locate building infestations and follow population trends is with pheromone traps in permanent monitoring sites (Shepherd et al., 1985; Shepherd and Otvos, 1986). The pheromone monitoring system developed by Shepherd et al. (1985) to predict outbreaks of the tussock moth in British Columbia incorporated sticky traps because of operational convenience and previous experience in the western United States (Daterman, 1978; Daterman et al., 1979). An important problem was selection of a low-potency lure that would allow baited traps to track endemic populations and detect early increases in density with-

P H E R O M O N E M O N I T O R I N G OF L E P I D O P T E R A N DEFOLIATORS

157

out saturating at pre-outbreak densities. This was accomplished by comparing catches with various pheromone dosages over a wide range of larval densities during a tussock-moth population cycle (Fig. 1). The strongest lure tested, containing 1.0% pheromone ((Z)-6-heneicosen-11-one) in a polyvinyl chloride (PVC) formulation, led to frequent trap saturation, and no distinction could be made between low and high densities. Catches with the weakest lure, containing 0.0001% pheromone, were not effective at the lowest densities (Fig. 1 ), and they did not always follow the population trend (Fig. 2, Vernon). Only traps baited with lures containing 0.01% and 0.1% pheromone tracked populations over the entire range of larval densities encountered and were suitable for monitoring. The next problem was to identify population trends leading to outbreaks 100

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based on trap catches. Good correlations between catches and egg-mass densities or defoliation, the best indicators of future population trends, could not be obtained, but Shepherd et al. (1985) observed that an upward trend in catches over two or three successive years at the same sites (Fig. 2 ) provided a useful warning of outbreaks. However, in the absence of long-term data or when unexpectedly large increases in catches occur, this trend indicator is not reliable and the use of catch thresholds in conjunction with it was recommended. Two thresholds have been established. A catch of 8-10 moths per trap indicates that an outbreak is two or more years away, and additional traps should be deployed to define the area of infestation (Shepherd and Otvos, 1986). A catch of 25 or more moths per trap signals that an outbreak is probably underway and that defoliation will be evident. Reaching this threshold triggers a sequential egg-mass survey following the trapping season to verify the outbreak signal, and is necessary because trapping data tend to be highly variable and could be misleading if used alone (Shepherd et al., 1985). The egg-mass survey reliably provides at least one year of advance warning of an outbreak. The trend indicator as the main criterion for interpreting catches is a key element of this monitoring system. As Shepherd et al. (1985) stated, "The population-trend approach may work best with D F T M as population densities seem to progress through fairly uniform cycles and trends are more predictable with this insect than with most other c o m m o n forest defoliators". Western spruce budworm

(Choristoneura occidentalis)

This insect is a major defoliator of conifers in the West; a pheromone-monitoring system was developed to forecast defoliation damage (Sartwell et al., 1985 ). Its development was similar to that of the Douglas-fir tussock moth. Sticky traps were chosen for trapping because they were in expensive and easy to transport and deploy. Polyvinyl chloride lures, formulated with a 92: 8 blend of (E)-11 and (Z)-1 l-tetradecenal, were used as baits. A pheromone dosage to avoid trap saturation was established by comparing catches obtained with lures containing 0.01, 0.001 and 0.0001% pheromone in populations causing light, moderate, or severe defoliation. Only the very weak 0.0001% lure, which had a pheromone release rate equivalent to 1/ 100 that of a female, produced catches that could be related directly to defoliation (Fig. 3). The r 2 (coefficient of determination) values for regressions between moth catch with the low-potency lure and defoliation in the following year ranged from 76% to 98%, and the relationships were not substantially affected by the size of the area monitored or the number of traps deployed. Sartwell et al. (1985) concluded that defoliation in areas covering 1000 to 10 000 ha could be reliably predicted with as few as nine monitoring traps. The monitoring system is now operational, but is evolving towards the use of a trap-catch threshold to deter-

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mine if further sampling or control actions are required. (Sartwell, personal communication, 1989 ).

Spruce budworm (Choristoneura fumiferana) The spruce budworm, a major defoliator of coniferous forests, is probably one of the most sampled forest pests in North America. With the identification of its sex pheromone, it was realized that populations could be monitored with pheromone-baited traps which might provide advantages over conventional sampling methods. Two goals were envisioned (Sanders, 1988): to forecast the transition from an endemic to an epidemic population by tracking population trends over time; and to replace larval and egg sampling with a pheromone monitoring system useful for sparse to moderate densities. The latter requires a good correlation between trap catch and population density. There has been progress along these lines (Miller and McDougall, 1973; Ramaswamy et al., 1983; Allen et al., 1986; Sanders, 1988 ) but a high degree of variability in the results has prevented good correlations when the data cover heterogeneous forest types over large areas (Allen et al., 1986). Another problem apparent in these studies was the lack of a suitable combination of trap design (both sticky and nonsaturating types) and lure formulation. This problem has been studied in considerable detail (Houseweart et al., 1981; Ramaswamy and Card6, 1982; Jobin, 1985; Sanders, 1986a; Sanders and Meighen, 1987; Gimball, 1988) but has not been resolved for large-scale, operational conditions. Because budworm population cycles last some 30-40 years or more, and

160

G.G. GRANT

outbreaks cannot be economically predicted far in advance with current sampiing methods, the prospects of following population trends and predicting an outbreak much earlier with pheromone-baited traps is very appealing. A long-term population study of the budworm in northern Ontario (Sanders, 1988 ) has explored the feasibility of doing this. Despite periodic changes in trap models and lure formulations owing to improvements in technology during the course of the study, excellent relationships between trap catch and late-instar larvae in the same year (r2= 66%) and with larvae in the following year (r2=81%) were obtained for 20 years of trapping (Fig. 4). In a similar 12-year study carried out earlier in New Brunswick, Miller and McDougall (1973) used female-baited traps consisting of sticky boards and obtained r2=98% for the relationship between moth catch and larval density in the following year. Thus, in both studies moth captures reliably tracked budworm population over long periods of time. In considering the potential for pheromone traps to predict outbreaks, Sanders (1988) suggested that if either of two criteria for interpreting trap catches had been adopted, namely three consecutive years of increasing catches or a catch threshold of about 50 moths per sticky trap, the budworm outbreak at the study site in 1983 could have been predicted some six years in advance (arrow, Fig. 4). These criteria are similar to those used for the Douglas-fir tussock-moth monitoring system. As a consequence of this study, and the work by Allen et al. (1986), Jobin (1985), and others, a large-scale pheromone monitoring program has been initiated in eastern North America involving Canadian and American cooperators in a long-term project to forecast incipient outbreaks in susceptible spruce/balsam-fir stands (Sanders, 1988). The study currently incorporates '83

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PHEROMONE MONITORING OF LEPIDOPTERAN DEFOLIATORS

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Multi-pher ® nonsaturating traps (Jobin, 1985) baited with PVC lures containing 0.03% pheromone (a 95:5 blend of ( E ) - l l and (Z)-ll-tetradecenal), which is equivalent to a virgin female. The use of large-capacity traps contrasts with the sticky traps used for the Douglas-fir tussock moth and western spruce budworm, but the Multi-pher traps are effective and less likely to saturate with the 0.03% lure at moderate population densities (Sanders, 1986a). A further objective of the project is to establish a threshold catch that will trigger more precise sampling methods to verify the warning signal provided by the traps and accurately forecast the population trend.

CONCLUSION

There has been considerable progress in the use of pheromone traps to monitor lepidopteran forest pests and predict when they are reaching dangerous levels. The major challenge is the establishment of criteria by which trap catches can be translated into reliable forecasts. Depending on the target pest and monitoring objectives, several criteria for doing so are effective but they rely on traps that do not saturate before populations cause damage. Both sticky traps and large-capacity, nonsaturating traps are suitable so long as the lure potency is appropriate.

ACKNOWLEDGMENTS

I am grateful to P. De Groot for helpful comments on the manuscript, and to D. Frech for assistance in its preparation.

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Daterman, G.E., 1982. Monitoring insects with pheromones: Trapping objectives and bait formulations. In: A.F. Kydonieus and M. Beroza (Editors), Insect Suppression with Controlled Release Pheromone Systems, Vol. 1, CRC Press, Boca Raton, Florida, pp. 195-212. Daterman, G.E., Livingston, R.L., Wenz, J.M. and Sower, L.L., 1979. How to use traps to determine outbreak potential. USDA Douglas-fir Tussock Moth Agric. Handb., 546, 12 pp. Daterman, G.E., Sower, L.L. and Sartwell, C., 1982. Challenges in the use of pheromones for managing western forest Lepidoptera. In: B.A. Leonhardt and M. Beroza (Editors), Insect Pheromone Technology: Chemistry and Applications. American Chemical Society, Washington, DC, Symp. Ser. 190, pp. 243-254. Elkinton, J.S., 1987. Changes in efficiency of the pheromone-baited milk-carton trap as it fills with male gypsy moths (Lepidoptera: Lymantriidae). J. Econ. Entomol., 80: 754-757. Gimball, D.G., 1988. Pheromone lures to monitor sparse populations of spruce budworm, Choristoneurafumiferana ( Lepidoptera: Tortricidae). Great Lakes Entomol., 21 : 141-145. Houseweart, M.W., Jennings, D.T. and Sanders, C.J., 1981. Variables associated with pheromone traps for monitoring spruce budworm populations (Lepidoptera: Tortricidae). Can. Entomol., 113: 527-537. Jobin, L.J., 1985. Development of a large capacity pheromone trap for monitoring forest pest populations. In: C.J. Sanders, R.W. Stark, E.J. Mullins and J. Murphy (Editors), Recent Advances in Spruce Budworm Research. Proc. CANUSA Budworm Research Syrup., 16-20 September 1984, Bangor, Maine. Canadian Forestry Service, Ottawa, pp. 243-245. Kondo, E.S. and Moody, B.H. (Compilers), 1987. Forest and Insect Disease Conditions in Canada 1986. Canadian Forestry Service, Ottawa, 128 pp. Magasi, L.P., 1988. Forest pest conditions in the Maritimes 1987. Can. For. Serv. Maritimes, Inf. Rep. M-X-166, 109 pp. Miller, C.A. and McDougall, G.A., 1973. Spruce budworm moth trapping using virgin females. Can. J. Zool., 51: 853-858. Ramaswamy, S.B. and Card~, R.T., 1982. Nonsaturating traps and long-life attractant lures for monitoring spruce budworm males. J. Econ. Entomol., 75: 126-129. Ramaswamy, S.B., Card6, R.T. and Witter, J.A., 1983. Relationships between catch in pheromone-baited traps and larval density of the spruce budworm. Choristoneura fumiferana (Lepidoptera: Tortricidae). Can. Entomol., 115:1437-1443. Sanders, C.J., 1986a. Evaluation of high-capacity, nonsaturating sex-pheromone traps for monitoring population densities of spruce budworm (Lepidoptera: Tortricidae). Can. Entomol., 118:611-619. Sanders, C.J., 1986b. Accumulated dead insects and killing agents reduce catches of spruce budworm (Lepidoptera: Tortricidae) male moths in sex pheromone traps. J. Econ. Entomol., 79: 1351-1353. Sanders, C.J., 1988. Monitoring spruce budworm population density with sex pheromone traps. Can. Entomol., 120: 175-183. Sanders, C.J. and Meighen, E.A., 1987. Controlled-release sex pheromone lures for monitoring spruce budworm populations. Can. Entomol., 119:305-313. Sartweil, C., Daterman, G.E. and Twardus, D.B., 1985. Moth captures in pheromone-baited traps relative to subsequent defoliation of Douglas-fir by western spruce budworm. In: C.J. Sanders, R.W. Stark, E.J. Mullins and J. Murphy (Editors), Recent Advances in Spruce Budworm Research. Proc. CANUSA Spruce Budworm Research Symp., 16-20 September 1984, Bangor, Maine. Canadian Forestry Service, Ottawa, p. 240. Shepherd, R.F. and Otvos, I.S., 1986. Pest management of Douglas-fir tussock moth: procedures for insect monitoring, problem evaluation and control actions. Can. For. Serv., Inf. Rep. BC-X-270. Shepherd, R.F., Gray, T.G., Chorney, R.J. and Daterman, G.E., 1985. Pest management of Douglas-fir tussock moth, Orgyia pseudotsugata (Lepidoptera Lymantriidae): Monitoring endemic populations with pheromone traps to detect incipient outbreaks. Can. Entomol., 117: 839-848.