Submitted to: Applied Soil Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/19/2011
Publication Date: 9/18/2011
Citation: Lundgren, J.G., Fergen, J.K. 2011. Enhancing predation of a subterranean insect pest: a conservation benefit of winter vegetation in agroecosystems. Applied Soil Ecology. 51:9-16. Interpretive Summary: Generalist predators are a pervasive component of farmland, and contribute to free insect pest suppression. But even though predators are abundant and diverse, pests still persist as a problem for farmers; managing farms in ways that promote predator populations could help increase their impacts on pests. Here, we tested whether winter cover crops could help increase predation on the subterranean larvae of the corn rootworm. Using PCR, we analyzed the stomachs of predators collected from corn that followed bare soil or winter cover crops, looking for rootworm-specific DNA sequences. Also, we examined which predators were eating restrained rootworm larvae in these two different treatments. We also quantified population densities of rootworms and their damage to corn roots. PCR-based gut analysis helped us to identify dozens of predator species that were consuming corn rootworms under field conditions. Moreover, we found that predation intensity on restrained larvae was greater in the cover crop treatment than in the bare soil plots. Finally, as predation intensity increased in each plot, rootworm larvae and their damage diminished. We conclude that cover crops may be a viable method for conserving predators within cropland and reducing the impact of this key pest of corn.
Technical Abstract: 1. Generalist predator communities are abundant and diverse in agroecosystems, but pests often persist nevertheless. Winter vegetation (e.g., cover crops) provides an agronomically sound opportunity to conserve predator communities and promote their impact on pests. We evaluate whether winter vegetation increases predation of Diabrotica virgifera, a key subterranean pest of maize. 2. Fields of maize were preceded by a winter cover crop (slender wheatgrass) or a fallow period (bare soil) over two years. Pest populations and root damage were measured in each field. In each field, the gut contents of predators aspirated from the soil surface, or extracted from the soil column, were analyzed using qPCR and primer sets specific to D. virgifera COI gene sequences. Predation intensity on restrained D. virgifera larvae (sentinels) was observed during the three larval stadia of the pest (n = 400 3rd instars per plot per stadium). 3. A diverse predator community consumed D. virgifera in maize fields, and predation was significantly greater in maize following cover crops (as measured with sentinels). Predation was particularly intense during the 3rd stadium of the pest, especially in the cover-cropped maize. 4. qPCR-based gut content analysis of natural populations functioned well in determining which predators consumed D. virgifera, but was poorly correlated with their impact on the pest or its damage. Conversely, predation intensity on sentinels was negatively correlated with D. virgifera populations and plant damage, but did not provide an accurate picture of the community involved. 5. Cover crops reduced D. virgifera populations by increasing predation levels on this pest, which indicates that predation is a mechanism for how this form of habitat diversification functions. Also, we conclude that employing diverse methods provides the best insight into trophic relationships within subterranean systems. Finally, because of the dynamic and diverse interactions between pests and their natural enemy complexes, we advocate conserving diverse predator communities within agroecosystems, rather than targeting specific key predator taxa.