Estimating carabid beetle population parameters in field corn to identify potential bioindicator species

A field study evaluated two quantitative sampling methods to determine their utility in estimating carabid beetle population densities and community structure for non-target organism monitoring programs. At nine field sites in five Maryland counties, carabid populations were monitored in field corn during three cropping events (canopy close, pollen shed, post-harvest) expected to influence ground-dwelling beetle activity and potential exposure to conventional or genetically modified pesticides. Carabid beetles were point sampled during day and night periods using a modified G-vac (leaf-blower/vacuum), and continuously sampled using pitfall trapping for two week periods surrounding each cropping event. The G-vac sampler unit included a novel collection canister designed for sampling ground-dwelling organisms. To assess the utility of each sampling method the following measures were considered: species abundance ranking, species similarity index values, and differences in body sizes captured. Additionally, carabid species biomass (dry weight) was examined to address the question of whether the most abundant species contained the most biomass. Vacuum sampling and pitfall trapping produced conflicting results, where vacuum sampling ranked small bodied species as dominant and pitfall trapping ranked larger species dominant. Differences in species capture between methods highlight the complementary use of both techniques in estimating total diversity, but suggest that vacuum sampling produces samples with less bias and provides area based densities useful in comparisons within and between studies. Finally, suggestions for candidate bioindicator species are based on synchronicity with crop phenology, population density, sampling feasibility, and trophic affiliation.