BIOLOGICAL CONTROL IN DISTURBED AGRICULTURAL SYSTEMS AND THE RAPID RECOVERY OF PARASITOID POPULATIONS

Abstract

In annual or periodically harvested crops, biological control of pests is aided if natural enemy populations rapidly recover within fields following disturbances. Here, we show that the life history of parasitoids may facilitate their recovery within fields. Because many parasitoids live within their still-living hosts, recovery of parasitoid populations can occur simultaneously with the recovery of their host populations. In alfalfa, periodic harvesting causes crashes in the populations of pea aphids, Acyrthosiphon pisum, and their parasitoid Aphidius ervi. Using laboratory experiments, we showed that the survival of parasitized aphids (before parasitoid-caused death) is little different from the survival of unparasitized aphids. In field experiments, by erecting exclosure cages immediately following harvesting we showed that both aphid and A. ervi populations can recover in the absence of immigration. Furthermore, successful A. ervi recovery suppresses aphid population growth over the ensuing harvesting cycle in the absence of other natural enemies. Therefore, the persistence of parasitoids within their hosts may be a key factor leading to successful biological control by specialist parasitoids in disturbed systems.