Submitted to: Journal of Applied Entomology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/5/2006
Publication Date: 3/5/2007
Citation: Prasifka, J.R., Lopez, M.D., Hellmich II, R.L., Lewis, L.C., Dively, G.P. 2007. Comparison of pitfall traps and litter bags for sampling ground-dwelling arthropods. Journal of Applied Entomology. 131:115-120. Interpretive Summary: The pitfall trap, which refers to a container placed with its open top at ground level, is a simple and effective technique to collect insects and other small animals. Screen or mesh bags filled with leaves or other plant material, called litter bags, have also been used to collect some small ground-dwelling animals, which crawl into the decaying plant material in each bag. To help future studies evaluate the potential effects of agricultural practices (like the planting of transgenic crops), a comparison of these two trapping methods is appropriate. Comparisons of side-by-side pitfall traps and litter bags showed litter bags placed above ground more frequently succeeded in collecting centipedes and beetle larvae, which are associated with moisture and sheltered area. However, pitfall traps more often captured adult ground beetles or harvestmen (daddy-longlegs), groups considered active at ground level. For groups collected in 40% or more of all trap types (pitfall traps, above- and below-ground litter bags) the numbers of individuals collected per trap were more precise (less variable) for above-ground litter bags than pitfall traps for some springtails and adult rove beetles, but only in one out of two years. While the performance of below-ground litter bags generally appeared similar to one or both of the other trap types, in no case were below-ground litter bags best. Differences were not always consistent between years, but the additional effort required to use litter bags may be justified for some insects and related groups. This information is useful for industry, government, and academic stakeholders interested in testing for potential non-target effects of transgenic crops.
Technical Abstract: For their simplicity and effectiveness, pitfall traps have become a standard technique to measure the activity and relative abundance of ground-dwelling arthropods. Permeable screen or mesh bags filled with plant material, referred to as litter bags, have also recently been employed as a complementary sampling technique for epigeal taxa. The anticipated need for increased field research on arthropod populations, particularly in transgenic crops with insecticidal properties, suggests that a relative assessment of both sampling methods could contribute to the design of future studies. Comparisons among pitfall traps, and litter bags placed above- or below-ground indicated that above-ground litter bags most frequently succeeded in collecting certain groups of arthropods associated with moisture and sheltered areas, including centipedes (Chilopoda) and beetle larvae (Carabidae, Staphylinidae). Conversely, pitfall traps most often captured taxa considered active at ground level, such as adult carabids or harvestmen (Opiliones). For taxa collected in >40% of all three trap types, bootstrap confidence intervals for the coefficient of variation (CV; used to assess precision or sampling efficiency) showed that above-ground litter bags were significantly more precise than pitfall traps for sampling elongate springtails (Collembola) and adult rove beetles (Staphylinidae), but only during the first year of sampling. While below-ground litter bags often appeared similar to one or both of the other trap types, in no case were below-ground litter bags best based on frequency of collection or coefficient of variation. Though differences were not always consistent between years, results suggest that the additional effort required to sample using litter bags may be justified for collection of some ground-dwelling taxa.