ESTIMATING SAMPLE-SIZE AND PERSISTENCE OF ENTOMOGENOUS NEMATODES IN SANDY SOILS AND THEIR EFFICACY AGAINST THE LARVAE OF DIAPREPES-ABBREVIATUS IN FLORIDA

In two studies to estimate sampling requirements for entomogenous nema todes in the field, highest persistence of Heterorhabditis bacteriopho ra after application occurred beneath the canopies of mature citrus tr ees. Nematode persistence declined with distance from the center-line of the tree row toward the row-middles. Immediately after nematode app lication to soil, 32 samples (15 cm deep, 2.5-cm diameter) beneath a s ingle tree were required to derive 95% confidence intervals that were within 40% of mean nematode population density. The estimated probabil ity of measuring the mean density within 40%, using 32 samples, declin ed to 88% at 2 days post-application and to 76% at 7 days. The persist ence in soil of Steinernema carpocapsae, S. riobravis, and two formula tions con raining H. bacteriophora and their efficacy against the larv ae of Diaprepes abbreviatus were compared in a grove of l-year-old cit rus trees, Within 6 days, the recovered population densities of all ne matodes declined to < 5% of levels on day 0. The recovery of H. bacter iophora during the first 2 weeks was lower than that of the other two species. Steinernema riobravis and both formulations of H. bacteriopho ra reduced recovery of D. abbreviatus by more than 90% and 50%, respec tively. Steinernema carpocapsae did not affect population levels of th e insect.