P.C. Nigam, S.E. Holmes, and E.G. Kettela
SERG Project #1995/18
The trend to replace chemical insecticides, including fenitrothion, with alternative products such as Bacillus thuringiensis var. kurstaki (B.t.k.) for use in pest management programs of the eastern spruce budworm (Choristoneura fumiferana (Clem.)) (SBW) spurred interest in understanding, in depth, the effectiveness of registered and pre-registered B.t.k. products. The rating of products containing B.t.k. by potency is based upon toxicity to the universally accepted laboratory animal, the cabbage looper, Trichoplusia ni, and not the target insect (the spruce budworm).
Thirteen registered and pre-registration B.t.k. formulations were tested in the laboratory for toxicity (hence mortality) to fourth and fifth instar larvae of SBW by the optimum (single) droplet method. Toxicity values, expressed as droplet diameter (µm) and International Units (IUs) contained in the discrete droplet doses, were closely related to the manufacturer’s batch potency ratings (BIU/L) for some products (Foray 48B®, Foray 76B®, Dipel 64AF®, Futura XLV-HP (NN)®) for both SBW instars, but were not closely related for others (several Futura formulations).
The more effective B.t.k. formulations approached fenitrothion in toxicity (droplet size) on SBW larvae.
The optimum droplet method is described in detail. The results of these experiments show that the technique is a useful tool to assess B.t.k. products in pre-registration stages and in consignment and post-purchase verification of batch toxicity to SBW.