Enhancing predation of a subterranean insect pest: A conservation benefit of winter vegetation in agroecosystems

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. 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, from which 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). A diverse predator community consumed D. virgifera in maize fields, and predation was significantly greater in maize following cover crops (as measured with sentinels, but not gut content analysis). Predation was particularly intense during the 3rd stadium of the pest, especially in the cover-cropped maize. qPCR-based gut content analysis of natural populations functioned well in determining which predators consumed D. virgifera, but was only correlated with their impact on the pest and its damage when the relative frequency of detection, quantity of DNA calculated, and predator abundance were combined into a predation index. In support of these observations, 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. Cover crops reduced D. virgifera populations by increasing predation levels on this pest, which indicates that conserving predation as an ecosystem service 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 conservation efforts at specific key predator taxa.