Quantification of pH tolerance levels among entomopathogenic nematodes

In crop production, soil pH affects the availability of nutrients, which impacts plant growth. Soil pH may also influence microorganisms in the soil. One such group of beneficial microorganisms are entomopathogenic nematodes (EPN). In nature, EPNs are parasites of soil-inhabiting insects. Additionally, EPNs are used as biocontrol agents to suppress insect pests. The aim of this study was to investigate pH tolerance of eleven steinernematids [Steinernema khoisanae, S. riobrave, S. tophus, S. innovationi, S. feltiae, S. beitlechemi, S. glaseri. S. carpocapsae (ScCxrd), S. carpocapsae (ScAll), S. carpocapsae (ScItalian), S. biddulphi] and six heterorhabditids [Heterorhabditis bacteriophora (SASRI75), H. bacteriophora (SGI151), H. bacteriophora (HbHb), H. bacteriophora (HbVS), H. indica and Heterorhabditis sp. (SGI245)] to varying pH levels (pH2 – pH11) prepared using two solutions, ammonium-acetate and citrate-phosphate. The highest infective juvenile survival was obtained from pH3 – pH10 in citrate-phosphate, where all strains displayed >50% survival. Steinernema carpocapsae strains had >90% survival at pH3 – pH11 in citrate-phosphate. Citrate-phosphate solutions were discriminative of the species pH tolerance.  Overall, the steinernematids had a higher survival range in ammonium-acetate pH solutions compared with the heterorhabditids. Moreover, Steinernema spp. S. carpocapsae and S. riobrave showed consistently higher survival in acidic and alkaline solutions suggesting that they may be applied in both acidic and alkaline soils. Of all the species, S. khoisanae survived in acidic pH but not in alkaline pH solutions. These findings can be important when selecting an EPN for biological control purposes.