Make way for maggots: The landscape ecology of aphid predators in California broccoli

Pesticides present a serious risk to humans, wildlife, and ecosystems. Enhancing communities of the natural enemies of agricultural pests could curb our dependence on these chemicals while providing a much more sustainable means of pest control. An important consideration in understanding the ecology of natural pest control is the effect of the surrounding landscape on a farm’s insect populations. This research investigates the insect communities on 17 organic broccoli farms over a land-use gradient in coastal Central California. Cabbage aphids (Brevicoryne brassicae) are a major pest of broccoli, and their most conspicuous natural enemies in this system are generalist predators, fly larvae in the family Syrphidae. This preliminary study shows that farms surrounded by more natural habitat had between 3 and 20 times higher syrphid to aphid ratios than farms in simpler agricultural landscapes. The difference between the natural and agricultural landscapes became more pronounced later in the season, when aphid populations were at their peak for most sites. Thus, when pest control matters the most (i.e., during high pest loads), the natural sites appear to have the greatest advantage over the agricultural sites. This study is a first step toward quantifying the ecosystem service of pest control provided by natural habitat to nearby agriculture, with the goal of informing land-use decisions to better manage for these pest control services.