Models of sterile insect releases for populations under attack by parasitoids

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Five host-parasitoid models are presented to determine the effects of: (a) host stage parasitized, and (b) position of density-dependent population regulation on the efficiency and dynamics of the sterile insect release method for pest control. Previous models indicate a catastrophic decline of the pest population when sterile releases are above a certain threshold. The five models incorporating parasitoids presented here indicate that this catastrophic decline will usually not occur, but rather the parasitoids will be driven extinct whereupon the system reverts to the one-species dynamics. In this situation, either a population explosion or extinction of the pest species will generally follow, depending on the level of sterile releases and on chance events. The models predict that the behavior of the system is similar regardless of whether the parasitoids parasitize host larvae or host adults. The system dynamics do depend, however, on whether density-dependent population regulation is effective in the hosts or the parasitoids. If population regulation is in the hosts, then the parasitoids may or may not reduce the sterile release rate required for eradication; usually the existence of the parasitoids will substantially reduce this sterile release rate. If population regulation is via the parasitoids, then the sterile release rate required for eradication is greatly reduced over that with no parasitoids; however, the possibility of a major population explosion during the release program is more likely than if population regulation is in the hosts. Model stability was much greater with population regulation in the parasitoids than in the hosts.