Spatial and Temporal Occurrence of Natural Populations ofHeterorhabditisspp. (Nematoda:Rhabditida) in a Semiarid Region

The spatial and temporal dynamics ofHeterorhabditisspp. in a citrus grove located in the northwest of the Negev region, Israel, were determined. Seasonal nematode occurrence was investigated in relation to soil depths, distance from the tree trunk, and shade provided by the tree 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 showed a linear relationship. Nematode densities were higher on those plots with a high degree of shade. Differences in numbers of nematodes as affected by shade were more obvious during the winter season (specifically in November 1992 and October 1993) and during May and July 1993. Most nematodes were recovered from the deeper soil layers (15–20 and 25–30 cm). Sampling date had a significant effect upon nematode densities on different soil depths. In the upper layer (5–10 cm) nematodes were found mainly during the winter in plots with 75–100% shade. No significant differences were found in numbers of nematodes among distances at any of the sampling dates. However, some trends can be observed. In all the plots with >25% shade, nematodes were mainly recovered 25 cm from the tree trunk.