Feeding mode and prey detectability half-lives in molecular gut-content analysis: an example with two predators of the Colorado potato beetle

The time during which prey remains are detectable in the gut of a predator is an important consideration in the interpretation of molecular gut-content 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 delectability, typically expressed as the half-life, depends on many variables; one that has not been explicitly examined is the manner in which the predator processes prey items. The influence of differences in feeding mode and digestive physiology on the half-life of DNA for a single prey species, the Colorado potato beetle Leptinotarsa decemlineata (Say), is examined 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. In order to standardize the amount of DNA consumed per predator, a single L. decemlineata egg was used as the prey item; all predators were third instars. 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 difference in the prey DNA half-lives from these two predators underscores the need to determine detectabilities from assemblages of predators differing in feeding mode and digestive physiology, in order to weight positives properly, and hence determine the predators' relative impacts on prey population suppression.