Invasion Efficiency as a Measure of Efficacy of the Entomogenous Nematode Steinernema carpocapsae (Rhabditida: Steinernematidae)

Abstract

Potential of entomogenous nematodes for biological control generally is assessed by host mortality; nematode efficacy often is estimated by LC₅₀. Nematode invasion efficiency (i.e., the percentage of the infective-stage juveniles that invade and establish in the host) has been proposed as an alternative to LC₅₀ as a measure of nematode efficacy. The relationship between these two measurements of nematode efficacy was explored in studies with Mexican and All strains of Steinernema carpocapsae (Weiser) tested against last instars of three lepidopteran species: Agrotis ipsilon (Hufnagel), Spodoptera frugiperda (J.E. Smith), and Galleria mellonella (L.). Estimates of LC₅₀ ranged from 4 to 91 infective-stage juveniles (infectives) per host. Invasion efficiency (percentage invading) ranged from 11 to 31% among the six host-nematode combinations when concentrations of 10 to 100 infectives per host larva were tested and ranged from 1 to 28% when single infectives were tested against individual larvae in one-on-one bioassays. We observed no obvious host- or nematode strain-related patterns in invasion efficiency and no correlation between LC₅₀ estimate and invasion efficiency. We recommend that determinations of percentage host mortality and nematode invasion efficiency be done simultaneously for assessments of nematode efficacy.