Submitted to: Bulletin of Entomological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2006
Publication Date: 4/11/2007
Citation: Greenstone, M.H., Rowley, D.R., Weber, D.C. and Hawthorne, D.J. 2007. Feeding mode and prey detectability half-lives in molecular gut-content analysis: An example with two predators of the Colorado potato beetle. Bulletin of Entomological Research 97: 201-209. Interpretive Summary: Predatory insects can kill millions of pest insects in U.S. crops every year. However, it is extremely difficult to determine predators’ impact on pest control at any given time or place because the remains of the pest in the predator’s gut are usually formless and liquid, preventing visual identification. We can identify the pest’s DNA in the predator’s gut by polymerase chain reaction (PCR), in a process akin to DNA fingerprinting in human forensics work. We expected that predators that attack and consume pests in different ways would differ in the amount of time during which the pest’s DNA can be detected in their gut. We studied this problem in two predators of a major U.S agricultural pest, the Colorado potato beetle. The pink ladybeetle chews up the pest and swallows the pieces, while the spined soldier bug harpoons the pest, injects enzymes into it, and sucks the liquefied remains into its gut. We found a seven-fold difference in the amount of time during which the pest’s DNA can be detected in these two predators, dramatically illustrating that differences in half-lives can be quite large. This is important, because the larger the difference, the larger the potential error in evaluating the role of each predator in controlling the pest. This information will enable scientists developing biological control methods for the Colorado potato beetle to accurately evaluate the importance of the different predators feeding upon it.
Technical Abstract: The time during which prey remains are detectable in the gut of a predator is an important consideration in the interpretation of molecular gut analysis data, because predators with longer detectability times may appear on the basis of unweighted data to be disproportionately important agents of prey population suppression. The rate of decay in detectability, typically expressed as the half-life, is likely dependent on many variables; one that has not been examined is the feeding mode of the predator. We examine the influence of differences in feeding and digestive morphology and physiology on the half-life of DNA for a single prey species, the Colorado potato beetle, Leptinotarsa decemlineata (Say), in two predators that differ dramatically in these attributes: the pink ladybeetle, Coleomegilla maculata (DeGeer), which feeds by chewing and then ingesting the macerated material into the gut for digestion; and the spined soldier bug, Podisus maculiventris (Say), which physically and enzymatically processes the prey extra-orally before ingestion and further digestion in the gut. The PCR assay yields estimated prey DNA half-lives, for animals maintained under field temperatures, of 7.0 h in C. maculata and 50.9 h in P. maculiventris; the latter is the longest half-life for an insect predator in the literature. The dramatic, seven-fold difference in the prey DNA half-lives from these two predators underscores the need to determine DNA detectability half-lives from assemblages of predators differing in feeding morphologies and digestive physiologies, in order to weight positives properly and hence determine their relative impacts on prey population suppression.