Effetti di Bacillus thuringiensis nella sua forma vegetativa sulle larve di Rhynchophorous ferrugineus (Coleoptera Curculionidae)

Abstract

The bacterium Bacillus thuringiensis (Bt) is a pathogen of many insects and is widely used, in spore form, in biological control. The vegetative form (Btv) is considered to be responsible for host septicemia after toxins (Cry) released from the parasporal body of the sporulated Bt (Bts) perforate the host's mesenteron. However, little is known about the insect's defense mechanisms in the stages following the action of Cry toxins. The aim of this study is to study the effects of Btv on the larvae of the phytophagous Rhynchophorous ferrugineus Olivier (Coleoptera: Curculionidae) following sublethal injections of Btv. This insect, known as the red palm weevil, has already been used by us as a model to study the interaction between Bt and its host, and we have highlighted Bt's ability to invade, to transition from the sporulated to the vegetative form, and to replicate in the curculionid. Sublethal doses of Btv were then injected into the dorsal vessel of R. ferrugineus larvae weighing approximately 3 grams. After 24–48 hours, a lack of weight gain, and in some cases even a reduction in weight, was observed in the treated larvae compared to the control. The effects on hemocytes challenged with sublethal doses of Btv were also evaluated both in vivo and in vitro. The results show that following the Btv injection, not only was the total number of hemocytes drastically reduced, but also the composition of the hemocyte population was different from that of the control. In particular, plasmacytes appeared to be the most modulated cells. Finally, scanning electron microscopy analysis also showed that hemocytes lost their typical morphologies and emitted cytoplasmic extensions that connected them together.