Abstract
Insecticides were tested in the laboratory on obliquebanded leafroller (OBLR) larvae. Choristoneura rosaceana (Harris), from an established laboratory colony and on a colony recently collected in the field. The response of different colonies and various instars was compared with multiple probit regressions using a weight factor. In general, smaller fifth instars from both colonies were more tolerant to most of the insecticides per unit of body weight than larger larvae in the same instar. Larvae from the field colony were ca. 115-fold more tolerant to azinphosmethyl than those from the laboratory colony. These results suggest that field populations of OBLR have become resistant to this material, which is one of the most widely used organophosphorus insecticides in New York apple orchards. In contrast, field-collected larvae were only slightly more tolerant than those from the laboratory colony to chlorpyrifos, fenvalerate, and methomyl (currently recommended for control of OBLR larvae in New York orchards).
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