A survey of entomopathogenic nematodes and their symbiotic bacteria in agricultural areas of northern Thailand

Entomopathogenic nematodes (EPNs)SteinernemaandHeterorhabditisand their symbiotic bacteria,XenorhabdusandPhotorhabdus, have been successfully used for the control of insect pests. The objectives of this study were to survey the EPNs and symbiotic bacteria in the agricultural areas of the Phitsanulok province, Thailand, and to study the association between the soil parameters and presence of EPNs. We collected 200 soil samples from 40 soil sites in agricultural areas (field crops, horticulture crops and forest). The prevalence of EPNs was 8.0% (16/200). Fifteen of the EPN isolates were molecularly identified (based on 28S ribosomal DNA and internal transcribed spacer regions) asSteinernema siamkayai. Seven isolates ofXenorhabdus stockiaewere identified using recombinase A sequencing. Phylogenetic analysis revealed that all theSteinernemaandXenorhabdusisolates were closely related toS. siamkayai(Indian strain) andX. stockiae(Thai strain), respectively. Significantly more EPNs were recovered from loam than from clay. Although the association between soil parameters (pH, temperature and moisture) and the presence of EPNs was not statistically significant, the elevation levels of the soil sites with and without EPNs were found to be different. Moreover, statistical comparisons between the agricultural areas revealed no significant differences. Therefore, we concluded thatS. siamkayaiis associated withX. stockiaein agricultural areas and that there is no association between the soil parameters of agricultural areas and presence of EPNs, except for soil texture and the elevation.Steinernema siamkayaimay be applied as a biocontrol agent in agricultural areas.