Zwittermicin A acts synergistically with the insecticidal activity of Bacillus thuringiensis subsp. kurstaki Berliner against gypsy moth (Lymantria dispar (L.)) larvae. The objective of this study was to assess the influence of insect source and diet on this synergy. Zwittermicin A increased the mortality caused by B. thuringiensis subsp. kurstaki in gypsy moths collected from four population sources feeding on artificial diet, and on larvae feeding on four tree species, in a dose-dependent manner. Zwittermicin A did not cause mortality of L. dispar when applied alone. The ability of zwittermicin A to act synergistically with B. thuringiensis subsp. kurstaki did not differ greatly among these four populations, although mortality was slightly lower in a field-collected population from Michigan. Zwittermicin A increased the activity of B. thuringiensis subsp. kurstaki on L. dispar feeding on white oak, aspen, larch, and willow. Larval mortality was directly proportional to the concentration of zwittermicin A applied to foliage, although the synergistic effect of zwittermicin A differed among host species. These results suggest strategies for employing synergists in the application and resistance management of microbial pesticides.