Agricultural pests can be reduced or controlled by naturally-occurring predators and parasitoids, but effective management of this ecosystem service requires an understanding of the factors contributing to its delivery. Pest control services are thought to be enhanced by natural habitat in agricultural landscapes, based on a large body of research demonstrating positive relationships between predator or parasitoid abundance on farms and the amount of nearby natural habitat. However, there is little or no evidence for a concomitant increase in pest control--if "control" is defined as the maintenance of pest populations below a certain level. The difficulty in measuring pest control by pest densities is that it is unknown how much more abundant pests would be in the absence of their enemies. This dissertation investigates different aspects of pest control to assess whether and under what conditions natural habitat can provide this ecosystem service to farms, using the study system of cabbage aphids (Brevicoryne brassicae) and their syrphid predators (Diptera: Syrphidae) in broccoli (Brassica oleracea) on California's Central Coast.
A major factor that may affect the potential for natural habitat to provide pest control services is the presence of alternate host plants for pests within the weed community surrounding the farm. Physiological experiments show that one weedy relative of broccoli, black mustard (Brassica nigra), could serve as more than merely an alternate host. Due to its toxic chemistry, B. nigra allows cabbage aphids to develop more quickly than when on broccoli, and to build a mustard bomb that can compromise or kill their syrphid predators. Weedy B. nigra may provide a refuge from an important predator and thus could be serving as a source of aphid pests to crops. The potential for habitat around the farm to benefit pests as well as natural enemies is an important consideration in understanding the impact of habitat complexity on pest control services.
Underlying spatial variation in pest distributions (whether due to alternate host plants or a number of other factors such as microclimate and dispersal) makes it difficult to detect the contribution of natural enemies to overall pest control. A cage experiment holding initial aphid densities constant across a landscape gradient measures the effect of habitat complexity on pest reduction by natural enemies, and reveals the importance of habitat at the landscape level as well as at the local on-farm level late in the growing season. Reduction of cabbage aphid densities is up to four times higher at more complex sites. The degree to which habitat at one scale could compensate for a lack of it at the other is dependent on aphid colonization and population growth early in the season, determined by abiotic factors such as temperature and dispersal patterns.
In addition to considering such variation across spatial scales, this dissertation tackles variation in pest populations and the delivery of pest control services across temporal scales. A three-year survey involving weekly measurements of aphid and syrphid densities assesses the effect of natural habitat on different indicators of pest control services. Syrphid larval abundance increases strongly with the proportion of natural habitat surrounding the farm, and cabbage aphid population growth is reduced on farms with higher syrphid densities, resulting in a weak but significant decline of aphid densities with natural habitat. Pest control services delivered by natural habitat may be masked by inter-annual variation in environmental factors and by competing direct and indirect effects (i.e., landscape effects on the pests themselves versus on the natural enemies' control of pests). Therefore, longer term datasets are necessary in order to detect the true magnitude of pest control services provided by natural habitat.
The results of this dissertation indicate that pests are indeed constrained to some extent by ecosystem services, but the degree to which this constraint can contribute to overall pest control is dependent on pest colonization and population growth early in the season. The role of natural habitat in promoting natural enemy communities is an important consideration for pest management, but must be employed with other methods of pest management for truly effective and reliable control.