Systemic insecticides

Treatment of elm trees with systemic insecticides that can be injected directly into the soil or trunk provides an attractive alternative to foliar sprays and their associated drift. Only a limited number of these compounds are available for insect control in forestry. One of the chemicals used as a systemic insecticide against elm bark beetles is dicrotophos (Table 18). Like all organophosphate insecticides, it is highly toxic to mammals; as a result, its use is restricted. Injection of small quantities of dicrotophos and the subsequent spread of the chemical through the system prevents bark beetle from feeding. The timing of application appears to be crucial, and needs to take place in the DED-susceptible period. However, dicrotophos injures the elm and is active only for a couple of days. Therefore, this insecticide is considered insufficient for bark beetle control {[230]}. Recently, new classes of systemic insecticides for elm insect control have been developed {[86]}. The chloronicotinyl compound imidacloprid and abamectin (an avermectin) are broad-spectrum insecticides that show activity against elm bark beetles (Table 18).

Table 18:        Chemical systemic insecticides for insect control in elm

In addition to chemical insecticides, agricultural pests have been controlled for more than two decades now by applying biological insecticides derived from bacteria like Bacillus thuringiensis. Purified polypeptide endotoxins of this bacterium have been shown to exhibit a detrimental effect on larvae of S. scolytus and S. multistriatus {[231]}. Colonization of elms with Phomopsis oblonga negatively affects the reproduction of bark beetles (see Enemies of the bark beetle). The two major metabolites of this fungus—phomopsolides A and B—appear to discourage these insects from boring and feeding. Noshita et al. {[835]} report on the in vitro synthesis of natural (+)-phomopsolide B. Possibly, BT toxins and/or phomopsolides will be interesting tools for DED control, but need further investigation {[231],[780],[835]}.