Abundance and genetic structure of nucleopolyhedrovirus populations in greenhouse substrate reservoirs

The abundance and genetic structure of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) occlusion body (OB) populations in greenhouse substrates in southern Spain was determined during an 18 month period over an area of 426 km2. To detect the presence of OBs, substrate samples were incorporated into a semi-synthetic diet and fed to first instar S. exigua. SeMNPV OBs were detected in 34% of substrate samples (N = 267). The prevalence of substrates positive for OBs did not differ significantly between samples from different crops or from four different geographical zones. Seasonally, Spring and Summer samples had significantly greater OB concentrations than samples taken in Autumn and Winter, concurrent with changes in the host insect population. Restriction fragment length polymorphism analysis revealed at least nine different variants. Substrate pH was identified as a major factor influencing the abundance and genetic composition of the soil OB population. Certain variants were more prevalent in substrates of more alkaline pH suggesting genotype-specific differences in survival outside the host. The prevalence of mixed variant infections was inversely related to an increasing substrate pH. Greenhouse substrates of southern Spain represent an abundant and diverse reservoir for the survival of SeMNPV. Studies are required to determine the degree to which agropractices that conserve soil OB populations would favor long-term microbial control of lepidopteran pests in greenhouse crops.