Joint Action of Pyrethroids with Organophosphorus and Carbamate Insecticides Applied to Western Spruce Budworm (Lepidoptera: Tortricidae)

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Abstract
A statistical method for testing mixtures of chemicals based on a model for independent joint action with no correlation in susceptibility is described. Antagonism (significantly less mortality than predicted by the model), synergism (significantly greater mortality than predicted), and independence (no significant difference in mortality) may be identified at one response level or over a range of levels by this method. Each of three pyrethroids (decamethrin, fenvalerate, and permethrin) was tested in a 1:10 mixture with each of four carbamates (aminocarb, carbaryl, methomyl, thiodicarb) and five organophosphorous insecticides (acephate, chlorpyrifos, fenitrothion, malathion, phosmet) by topical application to 6th-instar Choristoneura occidentalis Freeman. Co-toxicity ratios (LC50 predicted ÷ LC50 observed) ranged from 0.5 to 7.3 but were not reliable indicators of the type of joint action occurring because response lines of mixture components were not generally parallel. By the criterion of synergism in the upper (>50% mortality) concentration range, mixtures of fenvalerate with aminocarb, carbaryl, methomyl, thiodicarb, acephate, fenitrothion, or phosmet; decamethrin with acephate, carbaryl chlorpyrifos, or fenitrothion; and permethrin with carbaryl, methomyl, or thiodicarb appear to merit testing in more intensive bioassays involving spray applications to insects on foliage.