Effects of Insecticide Exposure on Movement and Population Size Estimates of Predatory Ground Beetles (Coleoptera: Carabidae)

Authors: Prasifka, Jarrad; Lopez, Miriam; Hellmich Ii, Richard; PRASIFKA, PATRICIA - IOWA STATE UNIVERSITY

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2007
Publication Date: 10/2/2007
Citation: Prasifka, J.R., Lopez, M.D., Hellmich II, R.L., Prasifka, P.L. 2007. Effects of Insecticide Exposure on Movement and Population Size Estimates of Predatory Ground Beetles (Coleoptera: Carabidae). Pest Management Science. 64:30-36.

Interpretive Summary: Numbers of beneficial insects sometimes may appear to increase after applications of insecticides. This is an unexpected result that may occur because the insects become more active after insecticide applications. When insect activity increases, they are more likely to be collected in insect traps. This is particularly common for some predatory beetles, which are generally collected by pitfall traps while walking on the soil. Previous researchers have disagreed whether the insecticides directly alter the beetles' behavior (causing hyperactivity) or if insecticides affect the prey that beetles consume, encouraging scavenging or eliminating food for the beetles (which can cause beetles to move more in search of food). Results from experiments using computer-aided tracking of beetles in the lab show that a short time of exposure to insecticides causes immediate increases in beetle movement that last for 1-2 hours. After 4 hours, beetle movement decreases, but can be stimulated again by a second brief exposure to insecticides. This suggests the constant exposure to insecticides in crop fields might directly cause increased collection in traps. However, field experiments with insecticide applications intended to produce hyperactivity did not show an increase in the number of beetles collected. The disagreement between results of lab and field tests suggests (1) indirect causes are more likely to explain unexpected increases in insect numbers after insecticide applications, and (2) information other than that provided by traps may be needed to understand changes in insect numbers after insecticides are used. This information is useful for industry, government, and academic stakeholders interested in evaluating the potential effects of agricultural practices (like the use of insecticides or planting of transgenic crops) on beetles and other insects.

Technical Abstract: Population size estimates of arthropod predators and parasitoids may paradoxically increase following insecticide applications. Previous research with ground beetles (Coleoptera: Carabidae) suggests such unusual results reflect increased arthropod movement and capture in traps rather than real changes in population size. However, it is unclear whether the effects on movement are produced by direct (hyperactivity) or indirect (prey-mediated) mechanisms. Video-tracking observations of Scarites quadriceps Chaudior indicated brief exposure to pyrethroid insecticides produced increases in total distance moved, maximum velocity, and percentage of time moving. Repeated-measurements of individual beetles indicated a drop in movement 240 min after initial lambda-cyhalothrin exposure, but another increase following a second exposure period. Because beetles used in laboratory experiments were not food-limited, video-tracking results suggest hyperactivity could lead to increased trap captures in the field. Ground beetles were collected after insecticide application in two separate field experiments intended to detect increases in population size estimates resulting from hyperactivity. Field trials used mark-release-recapture methods in small plots and natural carabid populations in larger plots, but detected no significant short-term (< 6 d) increases in beetle trap captures. The disagreement between laboratory and field results suggests mechanisms other than hyperactivity might better explain unusual changes in population size estimates in field research. When traps are used as a primary sampling tool, unexpected population-level effects should be interpreted carefully or interpreted along with additional data that are less influenced by arthropod activity.