Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 13, 1996
Publication Date: N/A
Interpretive Summary: The gypsy moth is a major pest of trees, but, because it occurs in many populated and natural areas, so there is much concern for possible negative environmental effects of any materials used to control it. For this reason, the most commonly used material for gypsy moth control is a bacterium, Bacillus thuringiensis(Bt). Strains of Bt used against the gypsy moth only kill caterpillars; they are harmless to other insects and vertebrates. Previous studies have indicated, however, that Bt is much more effective when applied to some plants than when applied to others. We therefore surveyed 17 host trees of the gypsy moth for differences in the activity of Bt applied to them. At least 10 times as much Bt was required to kill larvae feeding on sweetgum or willow than to kill larvae feeding on oak. Other trees varied widely as well. Further tests showed that the differences between oak and sweetgum were not due to differences in the amount of Bt deposited on the foliage during treatment; they are probably related to the chemistry of the leaves. Because sweetgum is a very common tree in hardwood and mixed forests in the mid-Atlantic and southeastern U. S., and a favored food plant of the gypsy moth, these apparent negative effects on the activity of Bt could limit the efficacy of Bt as a control agent. These findings should aid in the development of management programs for the gypsy moth, and may also provide a basis for further investigation of the mechanisms involved in the interactions of Bt and plants.
The activity of Bacillus thuringiensis Berliner against larvae of the gypsy moth, Lymantria dispar (L.), was measured on foliage of 17 host trees collected in the field, plus a laboratory host, leaf lettuce, and dipped in suspensions of B. thuringiensis (Foray 48B, Novo Nordisk Bioindustrials). Large differences in mortality among hosts were found. For example, mortality on red or white oak treated with an intermediate rate was higher than that on sweetgum and black willow treated with a rate ten-fold higher. White oak and sweetgum were selected for further study. Mortality of larvae on foliage treated with B. thuringiensis suspensions of two concentrations and held in small cages was again higher on white oak than on sweetgum. The amount of B. thuringiensis deposited on the foliage, and the amount remaining after 3 d, was measured by extracting, culturing, and counting colonies produced by viable spores. There was a nonsignificant trend toward higher initial deposition of B. thuringiensis on oak than on sweetgum, but only at the high rate of B. thuringiensis. Otherwise, deposition of B. thuringiensis and survival of spores over 3 days did not differ between tree species, and was not closely related to differences in larval mortality. These results indicate that other factors, possibly secondary plant compounds or environmental factors, are involved.