SYNERGISTIC EFFECT OF THE BACILLUS-THURINGIENSIS TOXINS CRYIAA AND CRYIAC ON THE GYPSY-MOTH, LYMANTRIA-DISPAR

The insecticidal activity of toxins CryIAa, CryIAb, and CryIAc against Lymantria dispar (gypsy moth) and Bombyx mori (silkworm) was examined by force-feeding bioassays. Toxin CryIAa exhibited higher toxicity th an toxins CryIAb and CryIAc for L. dispar and B. mori. To evaluate pos sible synergism among these toxins, bioassays were performed with mixt ures of CryIAa and CryIAb, CryIAb and CryIAc, and CryIAa and CryIAc, E xpected toxicity was calculated from the activity of each individual t oxin and its proportion in thr mixture by using the equation described by Tabashnik (B. E. Tabashnik, Appl. Environ. Microbiol. 58:3343-3316 , 1992). Observed 50% growth-inhibitory doses were calculated from mix ing experiments by probit analysis, In L. dispar bioassays, synergism was observed with a mixture of CryIAa and CryIAc while a mixture of Cr yIAa and CryIAb exhibited an antagonistic effect. No synergistic effec t on B. mori was observed with any toxin combination, Voltage clamping assays of isolated L. dispar midguts also demonstrated that the mixtu re of CryIAa and CryIAc induced a greater slope of inhibition of short circuit current than did other toxin combinations.