Tillage differentially affects the capture rate of pitfall traps for three species of carabid beetles

The influence of tillage, gender, and microclimate on capture rates of pitfall traps for the beetles Poecilus scitulus LeConte, Poecilus lucublandus (Say), and Pterostichus melanarius Illiger (Coleoptera: Carabidae) were assessed in mark-release-recapture experiments in spring pea and spring wheat. Experiments were conducted during June, July, and August of 2003 in the Palouse region of northern Idaho, USA. Rates of capture in pitfall traps for the three carabid species were differentially affected by crop-tillage systems. Capture rates for P. scitulus and R lucublandus were higher in no-till (NT) than in conventional tillage (CT) peas, whereas capture rates for R scitulus and Pt. melanarius were higher in CT than in NT wheat. Ground-level temperatures and relative humidity (r.h.) differed little among tillage systems. Nevertheless, capture rates were generally positively correlated with ground-level temperature and negatively correlated with r.h., with correlations more often significant in NT than in CT systems. The response of the thermophilic Poecilus spp. to temperature provides a possible mechanistic explanation for capture rate patterns in legumes during June, but not in other months for peas or any month in wheat during the experiments. Movement impedance due to residue could explain lower capture rates of R scitulus and Pt. melanarius in NT than in CT spring wheat. These results suggest that researchers using pitfall trapping for carabid populations should take into account potential capture biases their treatments can introduce.