Increasing vegetative diversity in a conservation program to enhance weed seed predation services

Background/Question/Methods

On-farm conservation programs that support fallow vegetation do not provide adequate habitat for beneficial insects, including biological control agents. Increasing vegetative diversity in these conservation programs with plantings of native forbs and grasses may increase the populations of these beneficial organisms and enhance the ecosystem services they provide. Weed seed predation by post-dispersal granivores, an important component of ecological weed management, may be enhanced by these diverse plantings. Carabid beetles, a significant group of weed seed predators, can use these habitats for overwintering sites and refuges from in-field disturbance. Research on similar groups of ground beetles has shown that their numbers can be increased by planting perennial grasses within and around crop fields.

We studied weed seed predation in nine organic crop fields (between 2.5 and 4 ha each) (three fields each of corn, soybeans and hay) in Eastern North Carolina from September to November 2009 and 2010. Each field was surrounded by four field border treatments (native warm season bunch grasses and prairie flowers, planted prairie flowers only, fallow vegetation, or mowed vegetation). We used predator exclusion cages to determine the amount of weed seed removal caused by invertebrates. Activity density of invertebrates was measured with pitfall traps.

Results/Conclusions

Weed seed predation rates were heavily affected by crop type but not by the surrounding field border treatment. Similarly, crop type but not field border treatment affected the activity density of granivorous invertebrates in our study, including the most common Carabid beetle, Harpalus pensylvanicus De Geer. Increasing vegetative diversity along the field borders may not have had an effect on weed seed predators for a number of reasons specific to our research site. First, resource abundance for granivorous organisms is already high in organic fields because of elevated weed densities. Second, while many studies show that Carabid beetles seek overwintering sites in non-crop vegetation, our research site was in a lower latitude that most of these previous Carabid studies, and our warmer climate may reduce or eliminate the need for overwintering sites. Enhancing ecosystem services through conservation programs will need to account for these types of local conditions.