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SUSCEPTIBILITY AND VULNERABILITY OF THIRD-INSTAR LARVAE OF THE SPRUCE BUDWORM (LEPIDOPTERA: TORTRICIDAE) TO BACILLUS THURINGIENSIS SUBSP. KURSTAKI

Published online by Cambridge University Press:  31 May 2012

Aurélie Massé ,
Kees van Frankenhuyzen  and
John Dedes
Aurélie Massé
Affiliation:
École nationale des ingénieurs des travaux de l'horticulture et du paysage, 49045 Angers Cédex 01, France
Kees van Frankenhuyzen*
Affiliation:
Great Lakes Forestry Centre, Canadian Forest Service, Natural Resources Canada, P.O. Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7
John Dedes
Affiliation:
Great Lakes Forestry Centre, Canadian Forest Service, Natural Resources Canada, P.O. Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7
*
1 Author to whom all corresponding should be addressed (E-mail: [email protected]).
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Abstract

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A droplet-imbibing assay was used to assess the susceptibility of third-instar larvae of the spruce budworm, Choristoneura fumiferana Clemens, to Foray 48B, a commercial formulation of Bacillus thuringiensis subsp. kurstaki containing 12.7 billion international units (IU) per litre. We observed an LD50 of 1.17 IU/larva for third instars, as compared with 3.96 IU/larva for fifth instars. Comparison with previously published data on susceptibility of later instars revealed that third instars were two-to three-fold more susceptible to Foray 48B than fourth and fifth instars and about eightfold more susceptible than sixth instars. Vulnerability of third instars to simulated aerial spray deposits was investigated by using potted balsam firs, Abies balsamea L. Potted trees were infested in the greenhouse when the buds were starting to swell, using a density of about one newly emerged second-instar larva per bud. When 90% of the larvae had reached the third instar and 52% of the buds were breaking (4 d after infesting), infested twigs were harvested and sprayed with undiluted Foray 48B in a spray chamber. Spray droplets on the buds measured between 25 and 125 μm in diameter, with 80% having a diameter of 80 μm or less. A density of 4.2 ± 1.0 droplets per bud resulted in spruce budworm mortality of 83.4 ± 4.0% and a corresponding reduction in larval density of 86.5 ± 3.9% (means ± SD, n = 6) after 5 d at 25 °C. Results of the spray chamber test suggest that third-instar spruce budworms were able to acquire a lethal dose, despite their concealed feeding habits.

Résumé

Une expérience d’absorption de gouttelettes a été utilisée pour évaluer la susceptibilité des larves de 3e stade de la Tordeuse des bourgeons de l’épinette (Choristoneura fumiferana Clemens), au mélange Foray 48B, une préparation commerciale de Bacillus thuringiensnsis subsp. kurstaki contenant 12,7 milliards d’unités internationales (IU)/litres. Nous avons observé un LD50 de 1,17 IU/larve dans le cas des larves de 3e stade, comparativement à 3,96 IU/larve dans le cas des larves de 5e stade. La comparaison avec des données publiées antérieurement sur la sensibilité des stades avancés a révélé que les larves de 3e stade sont de deux à trois fois plus sensibles à la préparation Foray 48B que les larves de 4e ou 5e stades et environ huit fois plus sensibles que les larves de 6e stade. La vulnérabilité des larves de 3e stade à des simulacres de résidus d’arrosage aérien a été examinée sur des sapins baumiers Abies balsamea L. en pot. Les arbres en pot ont été infestés dans la serre au moment où les bourgeons se sont mis à gonfler, à raison d’environ une larve de 2e stade fraîchement émergée par bourgeon. Lorsque 90% des larves ont atteint le 3e stade et que 52% des bourgeons se sont ouverts (4 jours après le début de l’infection), les rameaux infectés ont été recueillis et arrosés de Foray 48B non dilué dans une enceinte de vaporisation. Les gouttelettes du produit sur les bourgeons mesuraient de 25 à 125 μm et 80% mesuraient moins de 80 μm de diamètre. Une densité de 4,2 ± 1,0 gouttelettes par bourgeon a entraîné une mortalité de 83,4 ± 4,0% des tordeuses, ce qui correspond à une réduction de la densité des larves de 86,5 ± 3,9% (moyenne ± écart type, n = 6) après 5 jours à 25 °C. Les résultats du test dans l’enceinte de vaporisation indiquent que les larves de tordeuse de 3e stade sont en mesure d’absorber des doses létales, en dépit du fait qu’il ne soit pas possible d’observer directement leur alimentation.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 132 , Issue 5 , October 2000 , pp. 573 - 580
Copyright
Copyright © Entomological Society of Canada 2000

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