Effects of Steinernema carpocapsae (Nematoda: Steinernematidae) on biological parameters of larvae responses of Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

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) particularly Steirnenema carpocapsae, have a quite successfully control of RPW. However, results coming from the laboratories are often in contrast with each other and no data are available on precise doses and S. carpocapsae seems not be able to reproduce itself in the host. The effect of EPN on the RPW immune system is totally unknown. Different dosages of S. carpocapsae and varying durations of exposure were assessed. Larval mortality showed a positive linear correlation with both nematode dosage and the duration of exposure. Median Lethal Dose (DL50) and the Median Lethal Time (TL50) were calculated for older larvae. The number of nematodes that gained access to the haemocoel of larvae was always low, but increased with dosage and exposure time. EPN had also a detrimental effect on larval weight. In this paper we also investigate in vivo and in vitro effects of administered S. carpocapsae on the phagocytic responses of R. ferrugineus later-instar larval haemocytes. 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 moulted in the hemolymph totally undisturbed by the hemocytes but they were unable to complete the life cycle and to reproduce. After 24 hours, the number of the hemocytes (THC) recorded in the larvae treated with S. carpocapsae was dramatically inferior compared to the THC found in the control 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. We also evaluated the defence ability of RPW humoral and cellular immunity system in vitro against the bacterium Xenorhabdus nematophila associated with EPN, through the Minimum Inhibitory Concentrations (MICs) assay. To our knowledge, this is the first time that such assay has been used to evaluate the ability of insect immune system against entomopathogenic bacterium.