Are carabid beetles suitable biotic indicators of insecticide impact in potato fields?

Carabid assemblages in insecticide-free (=N) and conventionally sprayed (=S) potato fields were compared for five years using pitfall traps to evaluate the potential of carabid beetles as biotic indicators of insecticide impact in potato fields. Direct and indirect exposure of adults of major carabid species to several aphid insecticides was also examined in the laboratory. In one out of the five years, the trap catch of Bembidion morawitzi, one of the dominant species, was larger in plot N than in plot S. The species diversity indices for plot S were significantly higher than those for plot N in two years; however, on the whole, insecticides did not have a marked impact on the number of adults trapped, the number of species, indices of species diversity or seasonal changes in daily trap catches. Direct dipping tests showed that B. morawitzi was more susceptible than other carabid species and that fenitrothion was the most toxic to the carabids, but in-field soil-contact toxicity tests indicated that even fenitrothion had weak toxicity. It is likely that ground-dwelling carabids were protected from the insecticide droplets by the potato canopy. This would partly explain why the abundance of carabids in plot S was not much less that in plot N. In conclusion, carabids in potato fields were sometimes adversely affected by insecticide application, but the extent of damage was not so serious. Carabids are therefore not suitable biotic indicators of insecticide impact in potato fields.