Effect of soil salinity on entomopathogenic nematode survival and behaviour

Abstract Soil salinity, measured as electroconductivity (dS m–1), is a major problem in crop production, including areas where entomopathogenic nematodes (EPN) are applied as biological control agents. EPN species, primarily Heterorhabditis, have been isolated from coastal areas and agricultural soils with high salinity (> 4.0 dS m–1). Given the aqueous nature of their environment, soil salinity may play an important role in EPN movement and host finding. We assessed the survival of Steinernema riobrave, S. glaseri, Heterorhabditis indica, H. sonorensis and H. bacteriophora exposed to saline soils within the range found in agricultural soils. Survival and infectivity were generally unaffected by salinities ranging from 0 to 50 dS m–1 (50 dS m–1 is similar to the salinity of seawater). Salinity had been shown to negatively impact foraging in S. riobrave so additional experiments analysing the behaviour and attraction to host cues by S. riobrave and H. indica were conducted. Agar-based behavioural assays revealed species-specific responses to salinity. At the higher salinity levels (30 and 50 dS m–1) movement of H. indica decreased, the path taken was more circuitous and individuals did not move toward a host. There was a strong antagonistic effect on H. indica motility and host-finding behaviour. No significant differences were observed for S. riobrave exposed to any of these salinity levels. However, under simulated field conditions, high saline conditions (30 and 50 dS m–1) reduced the distance both H. indica and S. riobrave travelled toward a host. Both species are used for biological control of weevil pests in orchards where salinities have been recorded up to 20 dS m–1. Field efficacy of EPN applied for biological control in saline soils may be improved by timing applications to avoid late season build-up of salts in irrigated crops and applying the appropriate EPN species.