The success of a habitat specialist biological control agent in the face of disturbance

Natural enemies that can use multiple habitats are thought to better withstand disturbances in agricultural systems than natural enemies that are habitat specialists. This is because habitat generalists have populations in multiple habitats that can serve as sources of immigrants into an agricultural crop following a disturbance. In contrast, the dynamics of habitat specialists are tightly coupled with those of one agricultural crop. Nonetheless, some habitat specialists are successful in highly disturbed environments. To test how the magnitude of within-field disturbance affects biological control agents, we conducted a large-scale field manipulation in alfalfa fields and monitored the response of pea aphids, habitat-generalist predators, a habitat-specialist parasitoid (Aphidius ervi), and hyperparasitoids of A. ervi. The manipulation involved three treatments: harvesting normally (intermediate disturbance); spraying insecticide immediately after harvesting (high disturbance); and harvesting in strips (low disturbance). As a group, the habitat-generalist predator species showed a range of responses to disturbances, from no response to decreases in abundance in the high-disturbance treatment, indicating differences in their response to the density of pea aphids following disturbances. Surprisingly, percentage parasitism by the habitat-specialist parasitoid was little affected by experimental disturbance manipulations. Furthermore, two of the four hyperparasitoids of A. ervi were negatively affected by the magnitude of disturbance, suggesting that disturbance could have an indirect positive effect on A. ervi. These results suggest that a habitat specialist can overcome the detrimental effects of disturbances without using alternative habitats. In addition, disturbance can sometimes benefit biological control agents by disproportionally negatively affecting their enemies from the fourth trophic level.