Biological responses of Rhynchophorous ferrugineus to Steirnenema carpocapase: an example of a model system

Rhynchophorus ferrugineus (Coleoptera: Curculionidae) known as the Red Palm Weevil (RPW) is becoming more and more of a problem in Italy, and especially in Sicily, where it is well adapted. The infestations are mainly in the urban areas, and for that reason, chemical control is not advisable. Data from literature show that entomopathogenic nematodes (EPN) control RPW quite successfully in the field. However, results coming from the laboratories are often in contrast with each other and no data are available on precise doses. In this context we studied the Median Lethal Dose (DL50) and the Median Lethal Time (TL50) of young and older larvae and also of adults. The doses for the adults were twice that of the ones for larvae. Moreover, we investigate the effects of EPN on the immune system of RPW larvae. After a few hours, the nematodes were measured in the hemolymph of the insect and it appeared that the immune system was not activated by the presence of these foreign bodies. The nematodes suddenly mutated in the hemolymph totally undisturbed by the hemocytes. After 24 hours, the number of the hemocytes (THC) recorded in the larvae treated with Steirnenema carpocapsae were dramatically inferior compared to the THC found in the control larvae. EPN had also a detrimental effect on the weight of larvae. The study of the interaction between EPN and RPW could be crucial understanding the mode of action of EPN in the different instars and the reason for the response to different doses.