Insecticides

Chemical insecticides are designed to prevent DED transmission by killing beetles when they arrive to feed or breed in elm trees {[776]}. In areas exhibiting high beetle populations, reduction of the DED inoculum potential using these compounds is still preferred {[641]}. Various groups of chemical insecticides are known, e.g., contact insecticides (sprayed directly onto the tree), and systemic insecticides (injected into the tree).

Contact insecticides

Over the years, a number of contact insecticides have been shown to protect healthy elm trees against twig-crotch feeding and bark colonization by elm bark beetles (Table 17, {[230],[235],[ 437]}). The compounds presented in Table 17 differ in their level of toxicity to elm bark beetles, e.g., compared to methoxychlor, such pyrethroid insecticides as cypermethrin and esfenvalerate appear to be 222-548 times more toxic to Scolytus multistriatus {[131]}.

Contact insecticides appear to be most effective when treatment of the host tree occurs while insects are dormant (i.e. before spring flight). The insecticides must remain operative throughout the entire flight period of the insects, or be periodically reapplied. Foliar application of these chemicals may serve as a supplement to increase host tree protection {[776]}. Especially when more bark beetle species are present in the same area, a thorough spray coverage of all tree surfaces is necessary for successful insect control. To protect trees from scolytid vectors, all twig crotches one to four years old should be covered. Protection against Hylurgopines rufipes requires coverage of the trunk and all large branches of the elm {[776]}. Contact insecticides may also be used to poison elm wood that has been cut {[776]}.

The method used to apply the insecticides is important for the effectivity of the insecticide {[641]}. Spraying insecticides from a helicopter results in top coverage of the trees. This treatment will protect the elm against beetles feeding in the upper crown, e.g., S. multistriatus. Hydraulic sprayers provide coverage throughout the crown. Therefore, application of contact insecticides using these sprayers is effective against both scolytid beetles and H. rufipes. Mist blowers should be used only for small trees or when H. rufipes is the principal vector of DED in a certain area {[230]}.

An important disadvantage of the broad spectrum contact insecticides listed in Table 17 is that they are toxic to the environment. For this reason, despite their effectivity with respect to bark beetle control, such chlorinated hydrocarbons as DDT have been banned. The drift of contact insecticides during application is an additional disadvantage restricting their use {[86]}. Nowadays, the organophosphorus chemical chlorpyrifos—volume-wise, the world's leading contact insecticide—may be used to control elm bark beetles. Although less toxic and less persistent than DDT, this compound is classified by the World Health Organization as a moderately hazardous pesticide (class II, {[37],[131],[246],[442],[777]}). Lanier {[778]} showed that spraying uninfested elm logs with chlorpyrifos completely prevented elm bark beetle colonization. When beetles landed on elm bark that had been sprayed with 0.5% chlorpyrifos 6 weeks earlier, approximately 90% were killed after a 10 s exposure. In addition, the chemical completely eliminated broods in infested elm wood. Cypermethrin is another insecticide used in forestry applications for bark beetle control. Jin et al. {[343]} reported that this pyrethroid insecticide remains 100% effective against H. rufipes for approximately 60 days after spraying on elm bark. Dissipation half-lives of cypermethrin and chlorpyrifos in soil strongly depend on soil type {[32],[37],[342]}.

Table 17  Chemical contact insecticides capable of preventing feeding and bark colonization of elm bark beetles {[230],[774],[775],[778]}