Long-term effects of an insecticide application on non-target arthropods in winter wheat - a field study over 2 seasons

The effects of the insecticide lambda-cyhalothrin (Karate) on non-target ar thropods in winter wheat were studied throughout two successive seasons in 1998 and 1999. The study particularly focussed on the crop in the growing s eason after insecticide application (also winter wheat) for detection of po tential long-term effects and for determination of the suitability of diffe rent sampling methods. The investigations were based on the assumption that arthropod immigration from surrounding areas is limited in large fields. For this reason a simple approach seemed to be feasible. Two plots of equal size (10 ha, adjusted t o each other) were defined in a 100ha field and designated control and trea tment plots. Ten sampling points were established on each plot. The followi ng monitoring methods were utilised: visual counting, sweep netting and pit fall trapping. In the first year of investigation, countings and catches we re carried out 2 days prior to insecticide application and 2, 16, 30 and 44 days after application. In the next year, they were carried out 365 days a nd 384 days after insecticide application in the successive crop of winter wheat. At the time of the first sampling prior to insecticide application, the two plots showed significant differences with respect to arthropod density or activity, particularly in visual counting and to a minor degree to sweep ne tting and pitfall trapping. Lower densities or activities were observed in the plot reserved for treatment. Measures for mathematical equalisation of the results of population densities before pesticide treatment should be co nsidered. After insecticide application, the densities or activities of non-target ar thropods decreased, particularly in visual counting and sweep netting. Afte r one year, these effects disappeared to a large extent. Several groups of arthropods reached even higher levels in the treated plot than in the untre ated one. The pitfall traps revealed weak activity-decreasing effects in ca rabids and spiders in the treated plot, but the opposite tendency for staph ylinids. Hence, it seems that the conditions in a large field are less homo geneous, and that smaller scale conditions can support processes of recover y in non-target populations.