Insecticide resistance in spotted tentiform leafminer (Lepidoptera: Gracillariidae): mechanisms and management

Resistance to organophosphorous insecticides, pyrethroids, and methomyl occurred in populations of spotted tentiform leafminer, Phyllonorycter blancardella (F.), from southern Ontario. Resistance to organophosphorous insecticides occurred in all populations from commercial orchards. Resistance appeared to be due to an insensitive target acetylcholinesterase (AChE). Addition of several types of synergists to azinphosmethyl solutions did not affect toxicity. Resistance to methomyl appeared to be partially due to enhanced metabolism by aliesterases and partially to reduced inhibition of AChE. Selection for methomyl resistance was separate from resistance to organophosphorous insecticides. Increased activity of glutathione S-transferases was not implicated in resistance to either organophosphorous insecticides or methomyl. With current pest control practices, management of resistance to organophosphorous insecticides is not feasible in Ontario apple orchards. Resistant populations are regularly subjected to selection pressure by organophosphorous insecticides applied for control of other orchard pests. Pyrethroid resistance has been managed by modifications in the timing of applications to coincide with a predominance of the most sensitive life stages. Use of this strategy over three seasons has not resulted in detectably higher levels of resistance. Resistance to methomyl was relatively rare compared with organophosphorous or pyrethroid resistance. Methomyl resistance could be managed by strategies similar to those used for pyrethroids, i.e., modification of spray timing to coincide with the presence of early larval stages, or by restricted use. Where use was restricted for 3 yr after resistance was detected, resistance to methomyl declined from 9-fold to approximately 4.5-fold