HOST-PLANT EFFECTS ON ACTIVITY OF BACILLUS-THURINGIENSIS AGAINST GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) LARVAE

The activity of Bacillus thuringiensis Berliner against larvae of the gypsy moth, Lymantria dispar (L.), was measured initially on field-col lected foliage of 17 host trees and a laboratory host (leaf lettuce) d ipped in suspensions of B. thuringiensis subsp. kurstaki. Large differ ences in larval mortality among hosts were found; the amount of B. thu ringiensis required to obtain comparable levels of mortality varied >1 0-foId among some hosts. White oak, Quercus alba L., and sweetgum, Liq uidambar styraciflua L., were selected for further study. Mortality of larvae on foliage treated with B. thuringiensis suspensions of 2 conc entrations and held in small cages was higher on white oak than on swe etgum. The amount of B. thuringiensis deposited on white oak and sweet gum foliage, and the amount remaining after 3 d, was measured by extra cting, culturing, and counting colonies produced by viable spores. The re was a nonsignificant trend toward higher initial deposition of B. t huringiensis 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 d did not differ between white oak and sweetgum, and was not closely related to differences in larval mortality. These resu lts indicate that other factors, possibly secondary plant compounds or environmental factors, are involved.