Biochemical analysis has shown that populations of Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae) which have survived treatment with chemical insecticides (organophosphates and carbamates) have greater energy reserves and vigour than those from untreated areas: conversely, populations surviving treatment with Bacillus thuringiensis ( B.t.) are considerably less vigorous. Surviving pupae from untreated, organophosphate-treated and B.t.-treated areas gave, respectively, the following results on biochemical analysis: average weight, 69·5, 82·0 and 46·9 mg; Ca ++, 80·0, 114·5 and 54·0 mg/kg; total proteins, 27·6, 36·3 and 19·2 mg/kg; 3-hydroxybutyrate dehydrogenase, 1004, 1091 and 200 mU/g; alkaline phosphatase, 541, 580 and 251 mU/g. These analyses confirmed that B.t. treatments had a detrimental effect on survivors whereas chemical insecticides encouraged the resurgence of vigorous populations of C. fumiferana as a result of stimulation of these insects by sub-lethal dosages (hormoligosis).