Occurrence of resistance to insecticides in populations of the obliquebanded leafroller from orchards

Abstract

AbstractThe occurrence and distribution of resistance to insecticides in populations of the obliquebanded leafroller, Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae), from apple, Malus × domestica Borkhauser (Rosaceae), and pear, Pyrus communis L. (Rosaceae), orchards in the various production areas of southern Ontario were assessed in the laboratory and field from 1993 to 1999. Laboratory bioassays were conducted with neonate larvae from the first laboratory generation. Responses of populations from commercial orchards where control failures had occurred or where populations were large or damaging were compared with populations from unsprayed wild hosts. Resistance to azinphosmethyl and to pyrethroids and methomyl was identified in populations from all areas. Resistance levels ranged from 4- to 27-fold for azinphosmethyl, 4- to 8-fold for cypermethrin (a representative pyrethroid), and 3- to 5-fold for methomyl. In the field, deltamethrin was more effective than azinphosmethyl against a population resistant to both insecticides. Resistance to azinphosmethyl was unstable and rapidly declined in a population newly established in the laboratory and not selected with azinphosmethyl. After selection for nine generations, resistance declined only slowly when selection pressure was removed for four generations. This instability may be exploited in the management of resistance, but the possible cross-resistance between azinphosmethyl and pyrethroids needs clarification.