Abstract - Virtually complete mating disruption of Douglas-fir tussock moth, Orgyia pseudotsugata, has been demonstrated by applying 72 g/ha of synthetic sex pheromone (Z)-6-heneicosen-11-one in polyvinyl chloride beads. This work marks the first successful demonstration of mating disruption of a forest pest in Canada. A similar result was obtained when the dosage was reduced below 20 g/ha. Application of a nuclear polyhedrosis virus is also known to be highly effective. Circumstances occur where a combined application of pheromone and virus may be more effective that either treatment alone. Examples are given based on practical considerations of the spray application, and on the possible biological properties of the applied spray developed from modeling.
Key words - sex pheromone, mating disruption, virus treatment, Orgyia pseudotsugata, Lymantriidae, Lepidoptera
Pest bionomics
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In British Columbia the Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, typically damages trees in the same area about every ten years (Brooks et al. 1978). Defoliation of its main host, Douglas-fir, Pseudotsuga menzeisii Franco var. glauca Beissn. is sudden, and severe damage occurs in the first year of a pest outbreak. At this time defoliated trees are in distinct groups, but these groups usually merge in following years. Some branches, especially near the tree top are killed at first, but during the course of a normal three-year outbreak many trees are totally defoliated and die. The pest outbreak ends as abruptly as it started, and coincides with an extensive fatal viral infection of the larvae.
Tree damage results from larval feeding in May, June and July, when fresh needles and then older needles are consumed. Most spread of the pest occurs at this stage as small larvae disperse widely on silken threads blown by the wind, and larger larvae crawl to other trees. Following pupation in late July, adults emerge from their cocoons in early August. The female cannot fly, and emits a sex pheromone to attract the winged male to mate with her on her cocoon. She subsequently lays all of her fertilized eggs in groups on her cocoon. The eggs remain dormant throughout the winter, and hatch the following spring shortly after buds on the Douglas-fir trees have begun to burst.
Two methods of pest control demonstrated in British Columbia
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Apart from using chemical insecticides, two successful control treatments have been demonstrated in British Columbia, one based on mating disruption by applying synthetic sex pheromone, and the other based on a virus application.
Application of synthetic sex pheromone
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Virtually total disruption of mating of O. pseudotsugata was demonstrated recently, by applying a synthetic sex pheromone (Hulme & Gray 1994). It is the first successful demonstration of mating disruption of a Canadian forest pest.
The main component of the sex pheromone of O. pseudotsugata, (Z)-6-heneicosen-11-one, was synthesized, impregnated into polyvinyl chloride beads 250-400 µm in diameter, and applied using standard spray equipment. Application of 72 g/ha of the pheromone either from the ground or from the air completely prevented mating, measured by the number of male moths caught in sticky traps baited with feral females, or by the number of fertile eggs masses produced by wild females on trees in the sprayed areas.
When the pheromone was applied from the ground, substantial amounts of pheromone continued to be emitted one year after application (Gray & Hulme 1995). The ground application was conducted by simply applying the pheromone spray about 10 m up the trunk of each tree. These pheromone-impregnated beads were thus less exposed to weathering than aerially-applied beads which would accumulate on the outer foliage of the trees. One year after spraying, sufficient pheromone was still being emitted from the ground-sprayed beads to reduce the number of male moths caught in sticky traps baited with feral females by 80% compared with numbers caught in traps in unsprayed areas. In contrast, little of the aerially-applied pheromone continued to be emitted one year after application, since no significant reduction in numbers was found between trap catches in areas that had been treated aerially and those in unsprayed areas. More recent work has shown that the dosage of applied pheromone could be reduced below 20 g/ha without significantly altering the biological effects observed applying 72 g/ha of pheromone (Hulme & Gray 1996). Furthermore, an elusive minor component of the sex pheromone of O. pseudotsugata has just been identified as (Z,E)-6,8-heneicosadien-11-one (Gries et al. 1996). Trap catch remained the same when this newly identified dienic ketone was mixed with the monoenic ketone in the ratio 1:10 and the amount of pheromone in the lures was reduced 50-fold from that used with the monoene alone (Gries et al. 1996). Since excellent mating disruption has already been obtained with less than 20 g/ha of the monoene (Hulme & Gray 1996), prospects appear good for successfully disrupting mating using considerably less than 20 g/ha of the pheromone blend.
Application of virus
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A second pest control method demonstrated in British Columbia is based on application of the nuclear polyhedrosis virus that is widely found naturally in larval populations near the end of a pest epidemic (Shepherd et al. 1984). This virus has been propagated in white-marked tussock moth, Orgyia leucostigma Smith, and an aqueous suspension of milled insect parts comprising about 5% polyhedral inclusion bodies (PIB) is prepared from lyophilized virus-infected larvae. A viral epizootic is initiated by spraying this suspension from an aircraft or from the ground. When 22 billion PIB/ha were applied from aircraft or 2 billion PIB per tree from a ground sprayer, pest control was so successful that no fertile eggs could be found (Shepherd et al. 1994).
The virus should be applied soon after eggs hatch to allow the virus time to replicate and spread through the larval population prior to pupation. Foliage is not protected in the year of application because the larvae still destroy foliage before they sicken and die from the viral infection. Early treatment is essential both to increase the presence of the epizootic and to reduce foliage loss. During the year of application, control using the virus is about 60%. The infected larvae assist in spreading the virus throughout the treated area to infect the next generation of larvae.
The virus is now applied operationally in British Columbia following routine monitoring of male moth populations using sticky traps baited with (Z)-6-heneicosen-11-one (Shepherd et al. 1985). This sensitive monitoring using synthetic sex pheromone is crucial to prepare for application of the virus in the first year of a pest outbreak, because populations of O. pseudotsugata irrupt quickly.
Potential benefits of combining a pheromone and virus treatment There are both logistical and biological reasons why a combined treatment may be more successful than either treatment alone. One example from each category is presented for illustration. In effect, one treatment obviates potential deficiencies of the other, resulting in synergy between the two treatments.
Logistics are a major concern of the viral treatment. Precise timing of the spraying is crucial, because the application must immediately follow egg hatch for maximum larvicidal effect. Weather can interfere in two main ways. The spraying may be conducted in ideal weather conditions but at the wrong time, because the development rate of the insect is unexpectedly altered by unpredicted weather changes after the spraying date has been set. Alternatively, spraying may be conducted at the correct time but in the wrong weather conditions because persistent wet weather removes spray deposits from the foliage. Unfortunately, wet weather is more common at the time eggs of O. pseudotsugata hatch than when the larvae are fully grown. The end result in both cases is a lowering of the larvicidal effectiveness of the viral treatment. However, if pheromone is included with virus in the spray then unwanted adult moths produced from uninfected larvae would now be subjected to mating disruption. Synergy between the two treatments can obviate the logistical problems of using the virus alone.
Biological constraints may be a major concern for mating disruption. Depending on the mechanism of mating disruption, the method may not be effective when pest numbers are above a threshold value. Several disruption mechanisms have been proposed (Carde & Minks 1995), and a recent theoretical study indicates that for some mechanisms the effectiveness of mating disruption should depend on the density of the pest population being treated (Barclay & Judd 1995). For these mechanisms, this largely heuristic study shows that effective disruption is not possible above a threshold value of pest numbers, regardless of the amount of pheromone applied. This lack of effectiveness may be particularly marked where pest outbreaks begin from epicenters or where the pest aggregates. Both these behaviors are shown by O. pseudotsugata. If one of these density-dependent mechanisms were found to apply to the mating disruption of O. pseudotsugata, then high pest numbers may need lowering in some circumstances before applied pheromone could be effective. A viral application could accomplish this decrease in pest numbers. The synergistic effect of both treatments would thus overcome the biological restrictions that may result from applying the pheromone alone.
Conclusions
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Highly effective methods of controlling populations of O. pseudotsugata have been demonstrated in British Columbia based on the application of a viral insecticide to destroy the larvae, or the application of synthetic sex pheromone to disrupt adult mating. While either method can be effective alone, there are circumstances where a combination of the two treatments should be more effective.
References
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Barclay HJ, Judd GJR (1995) Models for mating disruption by means of pheromone for insect pest control. Res Populat Ecol 3, 239-247
Brooks MH, Stark RH, Campbell RW (1978) The Douglas-fir tussock moth: a synthesis. US For Serv Tech Bull 1585
Carde RT, Minks AK (1995) Control of moth pests by mating disruption: successes and constraints. Annu Rev Entomol 40, 559-585
Gray T, Hulme M (1995) Mating disruption of Douglas-fir tussock moth one and two years after the application of pheromone. J entomol Soc Brit Columbia 92, 101-105
Gries G, Slessor K, Gries R, Khaskin G, Wimalaratne P, Gray T, Grant G, Tracey A, Hulme M (1996) (Z)6,(E)8-heneicosadien-11-one: synergistic sex pheromone component of Douglas-fir tussock moth, Orgyia pseudotsugata (McDunnough) (Lepidoptera: Lymantriidae). J chem Ecol in press
Hulme M, Gray T (1994) Mating disruption of Douglas-fir tussock moth (Lepidoptera: Lymantriidae) using a sprayable bead formulation of Z-6-heneicosen-11-one. Environ Entomol 23, 1097-1100
Hulme M, Gray T (1996) Effect of pheromone dosage on the mating disruption of Douglas-fir tussock moth. J entomol Soc Brit Columbia in press
Shepherd RF, Otvos IS, Chorney RJ, Cunningham JC (1984) Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): prevention of an outbreak through early treatment with a nuclear polyhedrosis virus by ground and aerial application. Can Entomol 116, 1533-1542
Shepherd RF, Gray TG, Chorney RJ, Daterman GE (1985) Pest management of Douglas-fir tussock moth: monitoring endemic populations with pheromone traps to detect incipient outbreaks. Can Entomol 117, 839-848
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