Compatibility of entomopathogenic nematodes with plant extracts and post-exposure virulence test under laboratory condition

Abstract

The efficacy of both botanical pesticides and entomopathogenic nematodes (EPN) is largely dose-dependent and driven by environmental conditions. Combination of the EPNs and botanicals may enhance their efficacy; thus, we investigated the compatibility of the medicinal plants Alepidea amatymbica and Elephantorrhiza elephantina with five locally isolated EPN strains, three Steinernema (S. khoisanae, S. biddulphi and S. innovationi) and two Heterorhabditis (H. bacteriophora and Heterorhabditis sp. SGI 244). The experiments were designed to evaluate EPNs survival in plant extracts extracted using water and ethanol at 1%, 0.75%, 0.50%, 0.25% and 0.125% concentrations and nematode viability post-exposure. A concentration of 1000 IJs/mL in distilled water was used. Incubating extracts of A. amatymbica and E. elephantina with EPNs influenced the survival and virulence of the EPN species examined. The percentage survival of the EPNs post-exposure to the plant extracts was dependent on the plant extraction method and concentration in A. amatymbica but not in E. elephantina. The ethanol extraction method supported high percentage survival both at smaller and larger concentrations. The surviving infective juveniles (IJs) were virulent to Tenebrio molitor in both the aqueous and ethanol extracts of A. amatymbica and E. elephantina. However, virulence was observed to be strain-specific and not IJ concentration-dependent. Percentage mortality (against T. molitor) of strains with low percentage survival in aqueous extract of A. amatymbica compared effectively with those of higher percentage survival. The compatible relationship between extracts of the two plants and the EPN strains may lead to improved pest control in agricultural farming systems relative to either one of them applied individually.