DROPLET SIZE AND DENSITY EFFECTS OF BACILLUS-THURINGIENSIS KURSTAKI ON GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) LARVAE

Differences in droplet density and spectrum of pesticide deposited on foliage after spray were examined to determine differences in gypsy moth, Lymantria dispar L., larval mortality. The susceptibility of 2nd 3rd, and 4th instars of the gypsy moth to undiluted Bacillus thuringiensis kurstaki was examined under laboratory conditions. Droplets of known size (100, 200, and 300 mu m) and density (1, 5, and 10 drops/cm(2)) of B, thuringiensis kurstaki were sprayed onto leaf disks of oak foliage and fed to gypsy moth larvae. Parameters tested were time required for mortality leaf area consumed, and changes in larval weight. The time to mortality increased as droplet density and size decreased and as larval size increased. Droplet densities of 5 and 10 drops/cm(2) were effective (> 90% mortality) against 2nd and 3rd instars. Fourth instars had high mortality rates at 5 and 10 drops/cm(2) at the 200 mu m and 300 mu m droplet sizes. Low droplet densities (1 drop/cm(2)) at 100 mu m were ineffective in controlling 3rd and 4th instars. Surviving treated larvae had lower post treatment weights than control larvae and exhibited weight loss during the experiment. As droplet density increased, larvae consumed less foliage. Foliage consumed by surviving treated larvae was 2-6 times less than foliage consumed by untreated larvae.