Comparison of two field-scale approaches for the study of effects of insecticides on polyphagous predators in cereals

In the UK, recommended field trial protocols for assessing within-season ef fects of insecticides on non-target arthropods in cereals utilise either la rge (1 ha or greater) open plots or small (not less than 10 m x 10 m) enclo sed plots. Prior to this study, no direct comparison of the relative effect iveness and reliability in discerning such effects of these two approaches had been attempted. In a 2-year study, the effects of dimethoate and pirimi carb on polyphagous predators were investigated using both small enclosed p lots and large open plots in the same experiment. The activity-density of C arabidae, Staphylinidae and Linyphiidae was measured using pitfall traps ov er at least five pre- and four post-treatment weeks in each year. More spec ies were caught in greater numbers in large open plots than in small enclos ed plots. Both approaches caught sufficient individuals to analyse effects of insecticides on whole taxonomic families but too few individuals were ca ught in small enclosed plots to analyse effects on species other than those most abundant. More individuals were caught per trap and catch variability was less in large open plots than in small enclosed plots. The impact of p lot type on pitfall trap catch was greatest for Carabidae and least for Sta phylinidae. Treatment with dimethoate led to significant short-term reducti ons in catches of Carabidae and Linyphiidae, while treatment with pirimicar b had no significant effect on polyphagous predators. Neither of the insect icides applied in 1993 affected pitfall trap catches, in the following year , of predators that were active pre-treatment. No significant interactions were recorded between plot type and insecticide treatment, but low and vari able pitfall trap catches in small enclosed plots makes the detection of su ch interactions difficult. Small enclosed plots, with pitfall traps placed centrally, should not be used in field trials as an alternative to large op en plots without modifying sampling methods to increase trap captures and d ecrease overall variability in numbers caught. The use of more traps, more efficient trapping and greater replication all need to be investigated. (C) 2001 Elsevier Science B.V. All rights reserved.