Toxicity of Activated Cryl Proteins from Bacillus thuringiensis to Six Forest Lepidoptera and Bombyx mori

Abstract

The insecticidal activity of the CryIB, CryIC, CryID, and CryIE δ-endotoxin proteins from Bacillus thuringiensis was investigated in force-feeding experiments with larvae of Choristoneura fumiferana, Orgyia leucostigma, Lymantria dispar, Malacosoma disstria, Lambdina fiscellaria fiscellaria, Actebia fennica, and Bombyx mori. The toxins were prepared by proteolytic activation of protoxins expressed in Escherichia coli and quantified by scanning densitometry after separation on sodium dodecyl sulfate-polyacrylamide gels. All toxins retained full insecticidal activity during 7 months of refrigerator storage at pH 10.5. The most sensitive response was recorded for B. mori, the only species that exhibited sufficient mortality at generally <10 ng of toxin per larva to permit estimation of lethal dose (LD 50) requirements. In all forest insect species, low and variable mortality necessitated the use of a feeding inhibition response (frass failure dose, FFD 50). C. fumiferana and M. disstria were susceptible to all four toxins. O. leucostigma and L. dispar were not susceptible to CryIB and CryIE. L. fiscellaria and B. mori showed no response to CryIB but were highly susceptible to CryIE. None of the toxins elicited a response in A. fennica . Activity of the toxins was compared to previously published activity of the CryIA toxins by calculating potencies relative to the activated toxin mixture derived from native HD-1 crystals. The four proteins were in most cases less active than the individual CryIA proteins and were at best equally but usually less active than the HD-1 toxin mixture. A notable exception was CryID, which was about as potent as the most active CryIA toxin against M. disstria and B. mori. Published binassay data for seven CryI proteins against 16 target insects are summarized.