SPATIAL AND TEMPORAL OCCURRENCE OF NATURAL-POPULATIONS OF HETERORHABDITIS SPP (NEMATODA, RHABDITIDA) IN A SEMIARID REGION

The spatial and temporal dynamics of Heterorhabditis spp, in a citrus grove located in the northwest of the Negev region, Israel, were deter mined. Seasonal nematode occurrence was investigated in relation to so il depths, distance from the tree trunk, and shade provided by the tre e canopy. Insects belonging to the orders Coleoptera, Lepidoptera, and Diptera were found in the soil samples. For the entire study period, the number of nematodes as a function of shade by the tree canopy show ed a linear relationship. Nematode densities were higher on those plot s with a high degree of shade. Differences in numbers of nematodes as affected by shade were more obvious during the winter season (specific ally in November 1992 and October 1993) and during May and July 1993. Most nematodes were recovered from the deeper soil layers (15-20 and 2 5-30 cm). Sampling date had a significant effect upon nematode densiti es on different soil depths. In the upper layer (5-10 cm) nematodes we re found mainly during the winter in plots with 75-100% shade. No sign ificant differences were found in numbers of nematodes among distances at any of the sampling dates. However, some trends can be observed. I n all the plots with >25% shade, nematodes were mainly recovered 25 cm from the tree trunk. (C) 1996 Academic Press, Inc.