Bacillus thuringiensis delta-endotoxin proteins show a correlation in toxicity and short circuit current inhibition against Helicoverpa zea

Pesticidal activity of Bacillus thuringiensis delta-endotoxins, Cry1Aa, Cry1Ab, Cry1Ac, and Cry2A, was determined by using the force-feeding bioassay method to 4(th) instar larvae of Helicoverpa zea. H. zea was susceptible to Bt toxins in the order Cry1Ac > Cry1Ab > Cry1Aa > Cry2A with 63.60, 89.04, 159.65, and 375.78 ng/larvae respectively. The abilities of selected Bacillus thuringiensis toxins to inhibit short circuit current (I(SC)) in midgut epithelia of H. zea were also investigated by voltage clamp assay. The voltage-clamp studies were conducted on isolated midguts, measuring the inhibition of short circuit current (I(SC)) by activated toxin. A Cry1Aa toxin dilution of 33.3 and 500 ng/ml resulted in inhibition of I(SC) of -2.29 microA/min (lag time 15 min) and -4.48 microA/min (lag time, 2 min) respectively. The Cry1Ab dilution of 25 ng/ml inhibited I(SC) to -1.39 microA/min, a lag time of 14 min, and 333.3 ng/ml dilution resulted in decay of I(SC) -2.49 microA/min, lag time 1 min respectively. The Cry1Ac lower dilution 16.7 ng/ml inhibited I(SC) to -1.39 microA/min, lag time 4 min, and a high dilution 333.3 ng/ml decay I(SC) to -2.44 microA/min, lag time 1 min. The inhibition of I(SC) (-1.10 microA/min, lag time 25) at lower dilution (33.3 ng/ml) and high dilution (500 ng/ml), decay (-2.38 microA/min, lag time 5 min), showed a correlation between toxin concentration and inhibitory response with Cry2A toxin. The lag time decreased with increasing concentration of toxin applied, which is additional evidence of dose response besides direct correlation of toxicity assays and I(SC).