Response of larval Ephestia kuehniella (Lepidoptera: Pyralidae) to individual Bacillus thuringiensis kurstaki toxins mixed with Xenorhabdus nematophila.

Bacillus thuringiensis kurstaki strain BNS3 produces parasporal crystals formed by Cry1Aa, Cry1Ac and Cry2Aa delta-endotoxins. In a previous work, we showed that the latter exhibited individually, a weak insecticidal activity against Ephestia kuehniella. In order to improve their toxicities, we studied the combined effect of each delta-endotoxin with X. nematophila cells on E. kuehniella larvae growth. Xenorhabdus cells were used in combination with spore crystal mixture of the wild strain BNS3, known to be active against E. kuehniella, but no improvement in toxicity was observed. This could be due to the high efficiency of BNS3 crystals against this insect. However, when X. nematophila was combined with each of Cry1Aa, Cry1Ac and Cry2Aa, improvement of toxicity was noticed. The best improvements were obtained with Cry1Ac and Cry2Aa, which are more toxic to E. kuehniella than Cry1Aa. The difference in toxicity improvement was attributed to the low affinity of Cry1Aa to BBMV receptors, compared to those of Cry1Ac and Cry2Aa. This synergism between Cry toxins and Xenorhabdus cells could be exploited on control target insect, particularly in case of resistance to Cry toxins.