Hemocytes of Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) and their response to Saccharomyces cerevisiae and Bacillus thuringiensis

Originally from tropical Asia, the Red Palm Weevil (RPW Rhynchophorus ferrugineus (Olivier)) is the most dangerous and deadly pest of many palm trees, and there have been reports of its recent detection in France, Greece and Italy. At present, emphasis is on the development of integrated pest management based on biological control rather than on chemical insecticides, however the success of both systems is often insufficient. In this regard, RPW appears to be one pest that is very difficult to control. Thus investigations into the natural defences of this curculionid are advisable. RPW hemocytes, the main immunocompetent cells in the insect, are described for the first time. We identified five hemocyte cell types from the hemolymph of R. ferrugineus: plasmatocytes (∼50%), granulocytes (∼35%), prohemocytes (∼8%), oenocytes (∼4%) and spherulocytes (∼3%). SEM observations were also carried out. Some aspects of RPW interaction with non-self organisms, such as Saccharomyces cerevisiae and the entomopathogen bacterium, Bacillus thuringiensis (Bt), are discussed. Plasmatocytes and granulocytes were involved in nodules and capsule formation as well as in the phagocytosis of yeast. The hemocyte response of RPW larvae to sub-lethal doses of commercial products containing Bt was examined. In vivo assays were carried out and Bt in vegetative form was found in the hemolymph. After a diet containing Bt, the number of total hemocytes, mainly plasmatocytes, in the RPW larva hemolymph declined sharply (∼12%) and then remained at a low level, while the number of other circulating cells was almost unchanged.

Upper: Yeast (S. cerevisiae) encapsulation and phagocytosis by hemocytes from Rhynchophorous ferrugineus larvae as shown by an in vitro assay. lower: Hemocytes from RPW larvae stained with toluidine blue. (A) Control hemolymph; A1 control Bacillus thuringiensis vegetative form (Btv) in PBS; larvae feed with Bt after 12 h (B) and after 19 h (C). C1 hemocyte debris after bacterial interaction. Pl: plasmatocyte; G: granular haemocyte; S: spherulocytes; Pr: prohemocyte; Db: cellular debris.Research highlights► The RPW hemocytes, the main immunocompetent cells, are described for the first time. ► Hemocytes interaction with Saccharomyces cerevisiae and Bacillus thuringiensis. ► Plasmatocytes and granulocytes involve in capsules formation and yeast phagocytosis. ► Larvae fed with diet plus Bt had a lower number of hemocytes, mainly plasmatocytes. ► Bt in its vegetative form was recorded in high number in the hemolymph of RPW.