Effects of perennial grass strips on spiders (Araneae) in cereal fields and impact on pesticide side-effects

The effects of cultivation methods, pesticide use and perennial grass strips on spider fauna of cereal fields were studied in a large-scale field experiment. The effects of two pesticide regimes (conventional and reduced) on spiders were contrasted with effects of two types of cultivation systems (customary and integrated farming practice). The plots under the conventional pesticide regime were sprayed with herbicides, insecticides, fungicides and growth regulators annually. The plots on reduced pesticide regime were treated only when the control thresholds were exceeded tin 1992 with insecticide and in 1994 with herbicide). The perennial grass strips (sown with a mixture of timothy, meadow fescue, red clover and white clover) at the margins of the treated plots provided an additional element. Seasonal patterns of spider catches were quite synchronous in the field and in the perennial strip. Number of spiders were clearly higher in perennial strips than in the field. Furthermore, the composition of spider fauna differed between cerela plots and perennial grass strips. Lycosidae dominated in grass strips, whereas in cereal fields the family Linyphiidae was common. The dominance of Lycosidae in grass strips increased from year to year. The perennial strips affected the spider abundance of the field; total spider catches and Lycosidae catches, but not Linyphiidae, in pitfalls near the strips were higher than in pitfalls situated further away. However, the effects of pesticide treatments on spider numbers did not depend on the distance from the perennial strip. Reduced pesticide use plots had greater total spider catches than conventional plots in 1992 and 1994. In both years the difference was obvious for more than 3 weeks after insecticide (dimethoate) treatment. In 1992, pirimicarb was nearly harmless to spiders compared to dimethoate. In 1994, the dimethoate treatment tin Tune) did not decrease clearly the abundance of spiders, but deltamethrin treatment tin July) had dramatic effects on the total spider abundance and the difference was still obvious in the last sampling period. At family level, the effects of pesticide treatments varied between families. Statistically significant differences between pesticide treatments were found in all years in Linyphiidae numbers, but in Lycosidae only in 1994. During the period of 3 years, no differences were found in the numbers of spiders between the two cultivation systems tested.