As more farms shift to organic methods, natural means of pest control are becoming an increasingly important aspect of agriculture. The degree to which resident populations of natural enemies can control agricultural pests may depend on the landscape context of the farm. To test the effect of surrounding natural habitat on farm insect populations, weekly insect surveys were conducted on 16 organic broccoli farms on the Central Coast of California in the summers of 2006 and 2007. The field sites were chosen to represent a landscape gradient, ranging from less than 5% to more than 80% natural habitat in the 1.5 km surrounding the farms. Cabbage aphids (Brevicoryne brassicae) are important pests of broccoli and can be controlled by a suite of natural enemies when not disrupted by pesticides.
Results/Conclusions The most conspicuous of these natural enemies, fly larvae in the family Syrphidae, increased with the proportion of natural habitat in the surrounding landscape in both years of the study. This effect became magnified toward harvest, when syrphids on “more natural” farms (>60% natural habitat in the surrounding 1.5km) reached levels three times that at “less natural” farms (with <30% natural habitat). More natural farms also had higher abundance of other generalist predators, including spiders, ladybugs, and lacewings. However, aphid parasitism was highest in the mid-range of the landscape gradient (30-60% natural habitat in the surrounding 1.5km), where crop habitat accounted for more of the total proportional area. This suggests that crop habitat may favor specialist parasitoids more than generalist predators. Despite these fairly pronounced trends for natural enemies, the effect of natural habitat on aphid distributions was not consistent across growing season or years. In the first year, aphid densities were erratic throughout the season. In the second year, aphid densities were initially higher at more natural sites than less natural sites but by mid-season this trend had dramatically reversed. Identifying source habitat for aphids may be an equally important consideration in predicting aphid distributions as understanding the role played by their natural enemies.
