Susceptibility of Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae) to entomopathogenic nematodes with regard to its immune response

Abstract

The red palm weevil (RPW) Rhynchophorus ferrugineus Olivier is the most devastating pest of date palm, with a severe impact on cultivation and production. The virulence of five species/strains of entomopathogenic nematodes (EPNs), Steinernema scapterisci (Ss), Steinernema sp. (SII), Steinernema abbasi (Ab), Steinernema glaseri (Sg), and Heterorhabditis bacteriophora (HP88) was tested against larval and adult stages of RPW under semi-field laboratory conditions. Total and differential haemocyte counts were determined, and protein analysis and histopathological study of larvae injected with SII and HP88 were performed. Susceptibility results revealed that the larval stage was much more susceptible to Sg and HP88 than the adult stage. Larval mortality rate was as high as 100% and 92%, respectively, with concentrations of 1250infective juveniles (IJs)/pot, while adult mortality reached 48% and 60%, respectively, with concentrations of 20000IJs/pot at 9 d post infection with Sg and HP88. In general, SS, SII, and Ab appeared to be more virulent during the adult stage than the larval stage. They recorded mortality rates of 88%, 88%, and 72%, respectively, for the larval stage, and 100%, 100%, and 84%, respectively, for the adult stage at concentrations of 20000IJs/pot at 9 d post infection. A severe decline in the total and differential haemocyte counts (THCs and DHCs) at 12 h post-injection with SII and HP88 at 20IJs/larvae was observed. Similarly, the effect on total haemolymph protein of the normal and infected larvae was reflected by the appearance and disappearance of protein bands in addition to changes in the protein band concentration. Notably, remarkable changes in the cuticle, in the form of disruption to the outer epicuticle, splitting of the epidermal layer, and disorganization of the musculature region, were observed. In addition, notable histological effects were seen in the midgut region at 12 h post injection with 20IJs/larvae of HP88 and SII. Our results demonstrate that R. ferrugineus is unable to defend itself against EPNs. These results indicate an effective and natural biological control strategy with potential for application as an eco-friendly and inexpensive insecticide against the adult and larval stages of the RPW.