Susceptibility of different species of lepidoptera insects to strain Bacillus thuringiensis ssp. aizawai

Abstract

The results of assessing the effect of Bacillus thuringiensis ssp. aizawai strain on fi ve species of lepidopteran pests (Lepidoptera) belonging to different families are presented. The mortality of larvae infected with B. thuringiensis strain varied signifi cantly depending on the species of insect. In four species: the cabbage white butterfl y (Pieris brassicae L.), the cabbage moth (Mamestra brassicae L.), the greater wax moth (Galleria melonella L.) and the black-veined white (Aporia crataegi L.), death of individuals was noted on the second day of the experiment. However, mortality rate was signifi cantly different between species and increased over time. The diamondback moth (Plutella xylostella L.) was more susceptible to the pathogen. Signifi cant mortality of the larvae of this phytophage was observed already on the fi rst day after infection; while on the second day of the experiment all individuals died. Lethal time LT90 of more resistant species (black-veined white) and more sensitive ones (diamondback moth) differed sevenfold. Semi-lethal concentrations of B. thuringiensis strain for the tested insect species were determined: on the third day of the experiment, lethal concentration LC50 for the cabbage moth larvae, the greater wax moth and blackveined white ranged from 1.7 to 4.5 × 108 spores/ ml. For the cabbage white butterfl y, it was much lower (1.8×107 spores/ml). For diamondback moth caterpillars, LC50 was 4.25×107 spores/ml already one day after infection. It was noted that the specifi city of B. thuringiensis strain is associated with various mechanisms of insect protection from pathogens, and is due to individual characteristics of the species. In particular, the pH of excrement of intact caterpillars of the cabbage white butterfl y, a species sensitive to B. thuringiensis, was 2 times higher than that of caterpillars of the greater wax moth – 8.9 and 4.3, respectively, which is one of the reasons for the susceptibility (or resistance) of the species to bacterial entomopathogen.