Insecticides for Control of Bark Beetles that Spread Dutch Elm Disease

Abstract

Chlorpyrifos, carbaryl and methoxychlor were compared for the following uses against elm bark beetles: 1, elimination of broods in infested elm wood; 2, exclusion of colonization of uninfested elm logs; 3, toxicity to beetles "landing" on treated elm bark; 4, prophylaxis against twig feeding. Chlorpyrifos killed virtually 100% of the beetle broods, completely excluded colonization of uninfested logs and was toxic to 90% or more of the beetles exposed for 10 seconds to bark sprayed 6 weeks earlier. Methoxychlor ranked second in inflicting mortality on beetle brood and it excluded colonization of green logs, but it was marginally toxic to "landing" beetles. Although less toxic than chlorpyrifos, methoxychlor was superior for suppressing twig-feeding by European elm bark beetles. Carbaryl ranked third in effectiveness in all of these tests. We conclude that chlorpyrifos applied at 0.5% can be an alternative to burning or burying elm wood and that the effects of trap trees can be considerably enhanced by this treatment. Healthy elms can be made prophylactic to branch-feeding or overwintering native elm bark beetles by treatment with chlorpyrifos. However, owing to its repellancy, methoxychlor remains the insecticide of choice to protect healthy elms in regions where the European elm bark beetle is the dominant Dutch elm disease vector.