Biological control in disturbed agricultural systems and the rapid recovery of parasitoid populations

In annual or periodically harvested crops, biological control of pests is a ided if natural enemy populations rapidly recover within fields following d isturbances. Here, we show that the life history of parasitoids may facilit ate their recovery within fields. Because many parasitoids live within thei r still-living hosts, recovery of parasitoid populations can occur simultan eously with the recovery of their host populations. III alfalfa, periodic h arvesting causes crashes in the populations of pea aphids, Acyrthosiphon pi sum, and their parasitoid Aphidius ervi. Using laboratory experiments, we s howed that the survival of parasitized aphids (before parasitoid-caused dea th) 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 absenc e of immigration. Furthermore, successful A. ervi recovery suppresses aphid population growth over the ensuing harvesting cycle in the absence of othe r natural enemies. Therefore, the persistence of parasitoids within their h osts may be a key factor leading to successful biological control by specia list parasitoids in disturbed systems.