Submitted to: Molecular Ecology Resources
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/26/2010
Publication Date: 10/7/2010
Citation: Greenstone, M.H., Weber, D.C., Coudron, T.A., Payton, M.E. 2010. Unnecessary roughness? Testing the hypothesis that predators destined for molecular gut-content analysis must be hand-collected to avoid cross-contamination. Molecular Ecology Resources. http://dx.doi.org/10.1111/j.1755-0998.2010.02922.x. Interpretive Summary: Insect predators tend to be small, hard to observe, and feed by sucking juices out of their prey, all of which make it very difficult to determine their importance in controlling insect pests. Scientists have approached this problem by using molecular techniques to identify minute quantities of pest remains in predators’ guts. However, these methods are so sensitive that concern has been raised that the manner in which predators are collected in field research could lead to contamination with material from other insects that had not been consumed. We studied this question by performing an experiment in which predators that had been starved or fed only one of two insect prey species were released into potato fields and then re-collected by two standard methods: individually by hand, or in groups by knocking them off the plants onto drop cloths and collecting them from the cloths with an aspirator. When the predators were later tested by polymerase chain reaction for the presence of remains of the prey insects they had been fed, cross-contamination was low in hand-collected animals but as high as 39% in those that had been knocked off the plants and collected by aspiration, indicating that at least one common method for collecting insect predators can compromise the interpretation of feeding data, and should be modified. These results are of interest to insect ecologists and insect pest managers.
Technical Abstract: Molecular gut-content analysis enables direct detection of arthropod predation with minimal disruption of on-going ecosystem processes. Mass-collection methods, such as sweep-netting, vacuum sampling, and foliage beating, could lead to regurgitation or even rupturing of predators along with uneaten target prey, thereby potentially compromising the resultant gut-content data. Proponents of this ‘cross-contamination hypothesis’ advocate hand-collection as the best way to avoid this potential problem. However, hand-collection is inefficient when large samples are needed, as is usually the case for ecological research. We tested the cross-contamination hypothesis by setting out Coleomegilla maculata larvae and Podisus maculiventris nymphs that had been fed larvae of either Leptinotarsa decemlineata or L. juncta, or unfed individuals of these predator species along with L. decemlineata larvae, and then immediately re-collecting the predators by knocking them vigorously off the plants onto a beat cloth and capturing them en masse with a pooter. Collected predators were transferred in the field to individual vials of chilled ethanol and subsequently assayed by PCR for fragments of Cytochrome Oxidase I of L. decemlineata and L. juncta. Between 10 and 39% of re-collected fed predators were positive by PCR for DNA of both Leptinotarsa species, and between 14 and 38 % of re-collected unfed predators contained L. decemlineata DNA. The cross-contamination hypothesis is therefore supported for this method. We propose measures to reduce the incidence of cross-contamination. Other so-called ‘harsh’ methods, such as sweep-netting and vacuum collecting, should also be tested to see if they are as problematic as beating.