Three trophic level interactions: allelochemicals, Manduca sexta (L.), and Bacillus thuringiensis var. kurstaki Berliner

The role of plant allelochemicals in reducing the ability of Bacillus thuringiensis var. kurstaki Berliner to kill larvae of Manduca sexta (L.) was studied by comparing the effects of the alkaloid nicotine and the flavonoid rutin on the toxicity of B. thuringiensis. Concentrations of nicotine and rutin reflecting levels found in tobacco and tomato plants (0, 0.2, 0.4, 0.6, 0.8, 1.0% wet weight) were incorporated into the synthetic diet containing 0.001% B. thuringiensis and B. thuringiensis-free diet. Survivorship of M. sexta larvae increased at higher levels of nicotine (0.6, 0.8, 1.0% wet weight), but higher concentrations of rutin did not increase larval survivorship. Experiments on colony growth of B. thuringiensis on nutrient agar containing nicotine or rutin (0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9% wet weight) demonstrated that nicotine reduced colony growth to a greater extent than rutin. Consequently, plant allelochemicals potentially are an important factor in reducing larval mortality caused by B. thuringiensis on certain crops. Specialist herbivores that can tolerate high concentrations of allelochemicals may gain protection from pathogens by feeding on plants or plant parts with higher levels of toxins