Effect of Bacillus thuringiensis as vegetative form on hemocytes of Rhynchophorous ferrugineus (Coleoptera Curculionidae) larvae

The bacterium Bacillus thuringiensis (Bt) is a pathogen of many insect species and is actively used in biocontrol. The vegetative form as been reported to be involved in insect septicemia process. Bt during the vegetative stage of growth, is known to secern a new family of insecticidal proteins. Moreover recently evidence has been provided, that B. thuringiensis can establish itself in replicative and vegetative form on the leaf surface. Little is known on the interaction of pathogens with the defense responses of phytophagous insects. Insect circulating hemocytes are primarily responsible for the immune defense against parasites and pathogens. We use as model Rhynchophorous ferrugineus Olivier (Coleoptera: Curculionidae) to study the interaction between Bt and insect hemocytes. R. ferrugineus is a key pest of palm trees who are still difficult to combat with both chemical and biological control. Here, we report on the response of larvae of R. ferrugineus circulating hemocytes following Bt vegetative form inoculation. In the hemolymph, plasmatocytes, granulocytes, prohemocytes, oenocytes and spherulocytes were identified. After injection of a sub lethal dose, of Bt, RPW larvae had a decrement in the total number of circulating hemocytes. Particularly there was a reduction in the plasmatocytes which also lost their typical spindle-shape as shown by light and TEM microscopy. However further research are necessary to clarify the role of plasmatocytes in the larvae and their interaction with Bt.