Efficacy assessment of entomopathogenic nematodes native to Morocco against the white grubs Rhizotrogus obesus Lucas and Geotrogus olcesii Fairmaire (Coleoptera: Scarabaeidae)

Abstract

The white grubs Rhizotrogus obesus Lucas and Geotrogus olcesii Fairmaire (Coleoptera: Scarabaeidae) are two of the most problematic soil-dwelling pests of some annual crops in Morocco, where they may become a target for entomopathogenic nematodes (EPN). In this study, fourteen EPN strains reported from different fields in Morocco were investigated for their efficacy against both scarab species, under laboratory and greenhouse conditions. In laboratory experiments, seven strains of Heterorhabditis bacteriophora (HB-MOR2, HB-MOR6, HB-MOR1, HB-MOR5, HB-MOR7, HB-MOR4, and HB-MOR8) and one strain of Steinernema feltiae (SF-MOR10), provided remarkable efficacy against R. obesus (38–73% of mortalities). The EPN strains of Steinernema sp.2 (Sfg-MOR12), H. bacteriophora (HB-MOR4), Steinernema sp.1 (Sfg-MOR11), and H. bacteriophora (HB-MOR1) caused mortality rates ranging from 53 to 68% against G. olcesii. In greenhouse experiments where chlorpyrifos-ethyl was used as a positive control, larval mortality of R. obesus caused by both strains of H. bacteriophora, HB-MOR6 (81%) and HB-MOR2 (75%), was higher than that obtained by chlorpyrifos-ethyl (73%). The mortality rate in G. olcesii caused by Steinernema sp.2 (Sfg-MOR12) (77%) was the same as for chlorpyrifos-ethyl treatment (74%). Therefore, it can be concluded that EPN can constitute a promising biological alternative for the sustainable control of R. obesus and G. olcesii.