Organic farming conserves biodiversity and enhances natural pest suppression

Background/Question/Methods

Organic farmers rely, in part, on natural enemies to control herbivorous pests. This reliance might be misplaced if greater on-farm biodiversity only encourages predators to attack one another, or innocuous prey species, rather than damaging pests. Unfortunately, the biodiversity-biocontrol relationship has proven difficult to study outside of caged arenas. We overcame this obstacle by identifying prey DNA within predatory Nabis and Geocorisbugs. This allowed us to quantify trophic connections for predators hunting entirely naturally in the open field.

Results/Conclusions

Arthropod richness and evenness were higher in organic than conventional potato fields, indicating that a switch to organic farming practices promoted greater arthropod biodiversity. Densities of the two predatory bugs increased on organic farms, while individual predators also increased their per-capita predation rates on several key pests. Therefore, natural pest control remained strong despite a diverse background of non-pest prey. Indeed, predation on the most common detritivore in these communities, a Scaptomyza fly, was lower in organic than conventional fields. Predation of smaller Geocoris by larger Nabis predators also was reduced in organic fields, perhaps reflecting the broader range of non-predator prey that was available in those fields. Our study suggests, with evidence from naturally-foraging predators, that organic farming strengthens natural pest control both by encouraging higher predator densities and per-capita impacts of those predators on pests, and by making intraguild predation less common.