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THEORETICAL OPTIMIZATION OF BACILLUS THURINGIENSIS BERLINER FOR CONTROL OF THE EASTERN SPRUCE BUDWORM, CHORISTONEURA FUMIFERANA CLEM. (LEPIDOPTERA: TORTRICIDAE): ESTIMATES OF LETHAL AND SUBLETHAL DOSE REQUIREMENTS, PRODUCT POTENCY, AND EFFECTIVE DROPLET SIZES

Published online by Cambridge University Press:  31 May 2012

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

We present calculations of droplet sizes and product potencies that are theoretically required to deliver a lethal dose of Bacillus thuringiensis in one droplet to larvae of the eastern spruce budworm in aerial spray applications. The calculations are based on estimates of the dose required for 50% and 95% mortality [4.3 and 26.9 International Units (IU), respectively] and of the lowest dose that caused discernable feeding inhibition (0.5 IU) in force-feeding assays with sixth-instar larvae. For products containing 12.7–16.9 billion IU (BIU) per litre, the most widely used potency range, a 150- to 160-μm droplet is needed to deliver an LD95 and an 80- to 90-μm droplet to deliver an LD50, whereas droplets down to 40 μm are expected to cause extensive feeding inhibition. Our calculations suggest that current application prescriptions result in the delivery of a marginally effective dose to the target foliage and that budworm larvae have to ingest multiple droplets to obtain a lethal dose, a process that is thought to contribute to inconsistent spray efficacy. We predict that dose acquisition can be maximized by increasing product potency to 95 BIU/L, which would enable larvae to acquire a lethal dose by ingestion of only one or two droplets in the size range that is most commonly encountered on coniferous foliage (≤ 80 µm) after aerial application.

Résumé

Nous avons calculé la taille des gouttelettes et la concentration de produit qui sont théoriquement requises pour que la dose de Bacillus thuringiensis contenue dans une gouttelette soit létale à des larves de Tordeuses des bourgeons de l’épinette au cours de vaporisations aériennes. Les calculs tiennent compte de l’estimation des doses requises pour causer 50% et 95% de mortalité [4,3 et 26,9 unités internationales (IU)] et de l’estimation de la dose la plus faible qui puisse produire une inhibition décelable de l’alimentation (0,5 IU) au cours d’expériences de gavage chez des larves de sixième stade. Dans le cas de produits contenant 12,7–16,9 milliards d’IU (BIU) par litre, l’étendue des concentrations les plus utilisées, il faut une gouttelette de 150–160 μm pour causer une mortalité de LD95 et une gouttelette de 80–90 μm pour causer une mortalité de LD50 et des gouttelettes de 40 μm doivent théoriquement entraîner une inhibition importante de l’alimentation. Nos calculs révèlent que les conditions de vaporisation de feuillage utilisées couramment entraînent l’application de doses tout juste efficaces et indiquent donc que chaque larve doit ingérer plusieurs gouttelettes du produit pour mourir, un processus qui explique sans doute les variations dans l’efficacité des vaporisations. Nous croyons que la dose administrée peut être optimisée en augmentant la concentration du produit à 95 BIU/L, ce qui rendrait létale l’ingestion de une ou deux gouttelettes dans la gamme des tailles ordinairement rencontrées (≤ 80 µm) après la vaporisation aérienne de conifères.

[Traduit par la rédaction]

Type
Articles
Information
The Canadian Entomologist , Volume 125 , Issue 3 , June 1993 , pp. 473 - 478
Copyright
Copyright © Entomological Society of Canada 1993

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