Survival and infectivity of entomopathogenic nematodes under environmental stress

Abstract

At two experiments, infective stage juveniles of three heterorhabditid nematode isolates andSteinernema riobravis were placed in sterilized sandy soil in sealed containers buried in corn field during summer. Containers were removed daily for 10 consequent days and surviving juveniles were recovered from the soil by Baermann funnel method, then the remainder of soil was subjected toSpodoptera littoralis-baiting technique. Apparently, the low level of soil moisture (3%) and ceiling temperature levels (up to 42°C) as well as faint electric conductivity (0.48 mmhos/cm) of the soil containing the nematodes resulted in a rapid decline in the number of nematodes recoverable from the soil over time by Baermann method. The percentage of recovery for the nematode isolates EAM8, EIS7 and EAS59 by the Baermann funnel was 35.6%, 43.9% and 20.5% for the total nematodes recovered in the first trial, respectively. The nematode numbers increased when the recovery time extended from 24 to 48 h at 23°C. Insignificant differences were found between nematode numbers recovered daily in both trials. The average number of the recoveredS. riobravis was more (P≤0.05) than that of any other nematode isolate. Nematodes which were not recoverable by Baermann technique were infective when bioassayed over the period of the two experiments. The infective juveniles which survivied the stressed conditions are recommended to be subcultured through selection regime in order to obtain an improved nematode strain.