Host Plant Influence on Activity of Bacillus thuringiensisBerliner Against Lepidopterous Pests of Crops

Mortality of second-instar Helicoverpa zea (Boddie), Spodoptera exigua (Hübner) and Pseudoplusia includens (Walker) fed Bacillus thuringiensis Berliner (Bt), Dipel ES®-sprayed leaves of field grown cotton, soybean and tomato were compared in laboratory bioassays. The median lethal concentration (LC₅₀) for larvae of all species fed Bt-treated leaf tissue was higher for cotton than for soybean or tomato. The LC₅₀ for larvae fed Bt-treated soybean and tomato leaves did not differ significantly for any species of insects. When the mean number of days until death was plotted against percent survival at that rate, the mean number of days until death increased with an increase in percent survival. Survival time of the three insect species exposed to Bt increased most rapidly when fed cotton leaves. Results show that foliarly-applied Bt on field-grown cotton leaves is less effective against larvae of these pest species in terms of mortality and speed of kill.