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Title: Tracking the role of alternative prey in soybean aphid predation by Orius insidiosus: a molecular approach

item Rowley, Daniel
item Greenstone, Matthew

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/12/2007
Publication Date: 9/27/2007
Citation: Harwood, J.D., Desneux, N., Yoo, H.S., Rowley, D.L., Greenstone, M.H., Obrycki, J.J., O'Neil, R.J. 2007. Tracking the role of alternative prey in soybean aphid predation by prius insidiosus: a molecular approach. Molecular Ecology. 16:4390-4400.

Interpretive Summary: The soybean aphid is an exotic invasive insect pest that causes extensive damage and millions of dollars in yield loss and costs of control in US soybean crops. Short-duration experimental field studies have suggested that predatory insects could be important in reducing soybean aphid populations, but without longer-term direct studies of predation it is impossible to demonstrate that they are actually feeding on the pest. In a method analogous to DNA-fingerprinting, we used the polymerase chain reaction to identify the gut contents of a widespread and abundant insect predator, the pirate bug, during the summer of 2005. We found that large numbers of pirate bugs consumed the soybean aphid early in the season, even when the aphid was at low densities, and thereby delayed the buildup of the pest, as suggested by ecological theory. Later in the season they added an additional insect, the soybean thrips, to their diet. This so-called prey-switching, also predicted by theory, reduced the pirate bugs’ aphid consumption, and their effectiveness as biological control agents of the soybean aphid later in the season. Theory also predicts that different predator species may feed upon one another, thereby reducing total biological control by the community of predatory species. However, we showed by DNA gut analysis that the pirate bug does not feed on the most conspicuous other aphid predator in this soybean system, the Asian multi-colored lady beetle. This research is of interest to soybean growers and consultants, and to ecologists interested in the role of predators in biological control.

Technical Abstract: The soybean aphid, Aphis glycines, is a pest of soybeans in Asia and in recent years has caused extensive damage to soybean crops in North America. Within these agroecosystems, generalist predators form an important component of the assemblage of natural enemies, and can exert significant pressure on prey populations. Due to the complex nature of these food webs, molecular gut-content analyses offer valuable opportunities for examining trophic linkages in the field. We describe the development of a molecular detection system to examine the feeding behaviour of Orius insidiosus, and the role of alternative prey, Neohydatothrips variabilis, and a potential intraguild predator, Harmonia axyridis, on the dynamics of soybean aphid predation. Specific primer pairs were designed to target prey and were used, for the first time, to examine this food web in soybean fields. In total, 32% of O. insidiosus were found to have preyed upon Aphis glycines, but disproportionately high consumption occurred early in the season, when aphid densities were low. The intensity of early-season predation indicates that O. insidiosus are important biological control agents of A. glycines, although data suggest that soybean thrips could disrupt pest consumption by these generalist predators. No Orius were found to contain DNA of H. axyridis, suggesting intraguild predation upon these important late-season predators was low. In their entirety, these results implicate O. insidiosus as a valuable natural enemy of A. glycines in North American food webs.