HABITAT MANIPULATIONS INCREASING SPIDER DENSITIES IN AGROECOSYSTEMS - POSSIBILITIES FOR BIOLOGICAL-CONTROL

Field experiments with habitat manipulations were conducted in Belgian crop fields in order to follow changes in abundances and species composition of the spider taxocoenosis. Spider densities increased spectacularly in plots with holes (up to 13 times) in the soil. Moreover, this density increase depends on the diameter of the holes, with diameter 9.5 and 5 cm being the most effective. Species composition in the plots with holes also changed considerably compared to control plots. Exclusively web building Linyphiidae (Bathyphantes gracilis and Lepthyphantes tenuis) took far more advantage of the new possibilities offered by the habitat for web construction than non web-building spiders. B. gracilis preferred holes with a diameter of 5 cm whereas L. tenuis was more abundant in the D9.5 cm plots. It is concluded that this kind of relatively simple habitat manipulations are advantageous for population levels of certain beneficial polyphagous predators, such as spiders. This might offer possibilities for biological control of pest populations.