Effect of soil depth and moisture on the vertical distribution of Steinernema riobrave (Nematoda : Steinernematidae)

The effect of soil moisture on the distribution of Steinernema riobrave in a sand column was determined. Larvae of Pectinophora gossypiella were used to detect S. riobrave infective juveniles (IJ) in each 2.5-cm section of 30 -cm-long soil columns. Soil moisture was determined for each section and re lated to the numbers of nematodes recovered from infected insect baits. Inf ective juveniles of S. riobrave applied on the sand column surface showed s ome degree of positive geotaxis. IJ in soil columns with a consistent moist ure gradient grouped in the upper 12.7 cm within a water potential range of (-)40 to (-)0.0055 MPa (2% to 14% moisture). Nematodes in sand columns tha t were gradually dehydrating moved down the soil column, aggregating on the 28th day between 15-23 cm in depth. Nematode redistribution over time allo wed IJ to remain within a water potential range of (-)0.1 to (-)0.012 MPa ( 5.2% to 9.5% moisture).