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EFFECT OF SPRAY DROPLET SIZE AND DENSITY ON EFFICACY OF BACILLUS THURINGIENSIS BERLINER AGAINST THE SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA (CLEM.) (LEPIDOPTERA: TORTRICIDAE)

Published online by Cambridge University Press:  31 May 2012

Nicholas J. Payne  and
Kees van Frankenhuyzen
Nicholas J. Payne
Affiliation:
Natural Resources Canada, Forest Pest Management Institute, PO Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7
Kees van Frankenhuyzen
Affiliation:
Natural Resources Canada, Forest Pest Management Institute, PO Box 490, Sault Ste. Marie, Ontario, Canada P6A 5M7
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Abstract

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Because microbial insecticides based on Bacillus thuringiensis subsp. kurstaki cause temporary cessation of larval feeding at sublethal doses, we hypothesized that the delivery of a given dose in a single droplet is more efficacious than delivery of the same dose in multiple smaller droplets. We tested this hypothesis by exposing larvae of the spruce budworm, Choristoneura fumiferana (Clem.), to an LD50 of a commercial product (Dipel 8L) in combinations of droplet sizes and densities that are commonly observed on coniferous foliage after aerial application. A nominal dose of 5.2 International Units (IU) was presented to fifth-instar larvae on one, two, or four balsam fir, Abies balsamea L., needles in the form of one, two, or four droplets with diameters of 84, 66, or 52 μm, respectively. The combinations of droplet size and density were chosen to represent an increasing degree of dose dispersion. Overall mortality after a 24-h exposure was significantly reduced with increasing dose dispersion from an average of 66% when the dose was presented in one 84-μm droplet on one needle to 40% when presented in multiple droplets on several needles. Increased dose dispersion reduced the proportion of larvae that were able to ingest the full dose, presumably because of feeding inhibition caused by ingestion of sublethal droplets. In addition, mortality of fully dosed larvae declined significantly with increasing dispersion, implying a reduction in the effectiveness of the ingested dose. When compared with operational spray deposits, our results suggest that efficacy of spruce budworm sprays may be improved by increasing the proportion of needles receiving a lethal spray deposit by increasing product potency and possibly the active ingredient application rate.

Résumé

Administrés en doses sub-létales, les insecticides bactériens à base de Bacillus thuringiensis sous-esp. kurstaki entraînent l’arrêt temporaire de l’alimentation chez les larves; partant de ce principe, nous avons supposé que l’application d’une dose en une seule gouttelette est plus efficace que l’application de la même dose en une multitude de gouttelettes plus petites. Nous avons éprouvé cette hypothèse en exposant des larves de la Tordeuse des bourgeons de l’épinette, Choristoneura fumiferana (Clem.) à la dose LD50 d’un produit commercial (Dipel 8L) administré sous forme de gouttelettes de tailles et densités diverses, imitant les résidus que l’on retrouve sur le feuillage des conifères après un application aérienne. Une dose de 5,2 unités internationales (IU) a été utilisée chez des larves de cinquième stade sur une, deux ou quatre aiguilles du sapin baumier, Abies balsamea, sous forme de une, deux ou quatre gouttelettes de 84, 66 ou 52 μm. Les combinaisons taille-densité des gouttelettes ont été choisies de façon à simuler un gradient ascendant de dispersion de la dose. La mortalité globale après un exposition de 24 h était plus faible aux doses dispersées : elle était de 66% en moyenne lorsque la dose était administrée à une seule aiguille en une seule gouttelette de 84 μm et de 40% en moyenne lorsque la dose était adminstrée sous forme de gouttelettes multiples à plusieurs aiguilles. L’augmentation de la dispersion de la dose entraînait une réduction de la proportion de larves capables d’ingérer toute la dose, probablement à cause d’un phénomène d’inhibition de l’alimentation occasionné par l’ingestion de gouttelettes sub-létales. En outre, la mortalité des larves qui ont reçu une dose complète diminuait en fonction d’une augmentation de la dispersion de la dose, ce qui reflète une réduction de l’efficacité de la dose ingérée. Comparativement à la mesure des résidus laissés par vaporisation commerciale, nos résultats indiquent que l’efficacité des traitement des tordeuses par vaporisation peut être améliorée en augmentant la proportion des aiguilles qui reçoivent une dose létale complète par augmentation de la puissance du produit et possiblement par augmentation du taux d’application des ingrédients actifs.

[Traduit par la Rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 127 , Issue 1 , February 1995 , pp. 15 - 23
Copyright
Copyright © Entomological Society of Canada 1995

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