ๆ10๑bndฉศwZ~i[@uBiological control of insect pests: Multiple vs single enemy speciesv@Dr. William W. Murdoch (Univ. of California, Santa Barbara)Entomologists and theoretical ecologists have argued for at least 50 years whether it is better to release only one natural enemy to control a pest insect, or whether it is more effective to release multiple species of enemies. Where conditions allow the pest-enemy interaction to come to a stable equilibrium locally in homogeneous space, competition theory shows that coexistence of natural enemies is difficult to obtain. Coexistence is more likely if each of the enemies is inefficient, in the sense that each limits its own density; such self-limitation reduces the enemyfs effectiveness. In spite of this prediction, there are usually several natural enemy species present in successful cases of biological control of insect pests in agriculture.
I will describe experiments on a highly successful case of biological control where several natural enemies are present: the pest is California red scale and there are two parasitoid species and two predator species. The experiments suggest that control is achieved by the main parasitoid, Aphytis melinus, and that the other parasitoid (Encarsia) and the predators are not important. A model of the system also shows that Aphytis can control the scale in the absence of Encarsia and predators. These results suggest that the mere presence of multiple species of natural enemy is not good evidence that multiple enemies are needed for control. I will suggest, however, that it is likely that multiple enemy species are needed when the pest attacks temporary crops that are harvested frequently.