Foraging efficacy of the entomopathogenic nematode Steinernema riobrave in different soil types from California citrus groves

Entomopathogenic nematodes (EPNs) are lethal insect parasites that develop inside the body of their insect host. The EPN Steinernema riobrave infects many different types of soil insects, including larvae of the invasive citrus root weevil, Diaprepes abbreviatus, which is a key pest in citrus production. Soil characteristics have a great impact on nematode foraging efficacy and therefore on the ability of EPNs to be used successfully in biological control programs. Our specific objectives were to: (1) survey for the presence of native EPNs in California citrus groves; and (2) identify soil parameters that would allow us to predict EPN efficacy. The long-term objective of this study is to develop biological control methods to address the invasion of the citrus root weevil, D. abbreviatus, into California using EPNs. We collected soils from 36 sites in California where citrus is produced commercially, sampled them for native nematode presence and tested the foraging efficacy (movement and host finding) of S. riobrave in these soils at three different soil depths; 100, 250 and 500mm. Soil textures and physical characteristics were analyzed. We found five isolates of endemic EPNs (Heterorhabditis spp.) from southern California. Three were identified as Heterorhabditis bacteriophora, and two as Heterorhabditis sonorensis, which were recorded for the first time in California. S. riobrave's ability to infect hosts varied among soil types and depths. We also found significant correlations between soil characteristics (texture and physical characteristics) and S. riobrave foraging efficacy. Synthesis and applications. Our results and analyses can play a role in the prediction of S. riobrave success in various soil types. Perhaps more importantly, we suggest an approach to predict biological control potential based on soil characters that might be used with other insect pathogens. While soil texture is the main factor that affects EPN efficacy, we found that other parameters like organic matter level and dissolved solids are also important.