Persistence of Heterorhabditis infective juveniles in soil: Comparison of extraction and infectivity measurements

The persistence of Heterorhabditis megidis in soil was studied over a 4-wee k period. On days 0, 2, 14, and 28, infective juveniles (IJ) were extracted by centrifugal flotation, Baermann funnel, and baiting of soil with Tenebr io molitor larvae, which were then dissected. Extraction efficiencies on da y 0 were 82% by centrifugal flotation, 56% by Baermann funnel, and 19.8% by bait insect. The relative efficiency of the three methods changed over tim e. The relationship between the-density of nematodes in the soil and the pr oportion recovered by dissection was non-linear. Up to a dose of approximat ely 60 IJ/insect, less than 12% became established, while at higher doses ( up to 200 IJ/insect) the invasion efficiency was 23%. Mortality of bait ins ects increased from day 0 to day 2, but decreased to day 28. A novel method of assessing soil pathogenicity by preparing a soil density series and cal culating the dose of soil or IJ that kills 50% of the bait insects gave a s imilar pattern. This method is recommended as a means of tracking changes i n pathogenicity over time when bait insect mortality in undiluted soil is a t or near 100%. Two methods of preparing a series of Heterorhabditis IJ den sities in soil, either by diluting the soil itself with IJ-free soil or by adding diluted suspensions of IJ to the soil, resulted in the same bait ins ect mortalities.