Comparison among the responses of the greater wax moth, Galleria mellonella and red palm weevil Rhynchophorus ferrugineus to the entomopathogenic nematodes, Steinernema carpocapsae

Abstract

The entomopathogenic nematode-bacterium complex of Steinernema-Xenorhabdus has high potential as lethal biological control agent against many insect pest species. The Red Palm Weevil (RPW) Rhynchophorus ferrugineus (Coleoptera: Curculionidae) is an important worldwide palm trees pest. This insect is a quarantined pest, accidentally introduced in Sicily in 2005. The pest is killed by Steirnenema carpocapsae, but nematodes are unable to reproduce in the RPW larvae. This research try to understand the reasons of the inability of S. carpocapsae to complete its life cycle in the host comparing what happens in one of the most suitable host, the greater wax moth, Galleria mellonella (Lepidoptera: Pyralidae), for this nematode species. G. mellonella is a pest of beehives; the larvae feed on wax and do considerable damage to the wax and to honey production in Italy. The lethal doses were comparable if pondered at the weight of the hosts. In both G. mellonella and R. ferrugineus there were no encapsulation or melanisation responses against S. carpocapsae. However S. carpocapsae successfully complete its life cycle in G. mellonella producing thousands of offspring while just few moults were recorded in RPW larvae and not male and female were found in the RPW larvae cadavers. Xenorhabdus nematophila is a gram-negative member of the family Enterobacteriaceae that lives in a symbiotic association with S. carpocapsae in a highly effective symbiosis of pathogens. We found that RPW was unable to defence its self against the bacterium. Through viable plate counts we measure the quantity in terms of CFU (Colony forming units)/ml of X. nematophila in the G. mellonella and in the R. ferrugineus hemolymph. The results show that the quantity of X. nematophila was higher in G. mellonella than in RPW already after 24 h from bioassay. The presented combination of nematode species, the two host species and the bacterium studied in terms of reproduction of the complex nematode-bacterium, is unique to this study.