Ecological considerations for biological control of aphids in protected culture

Several braconid and aphelinid parasitoids, midges, lacewings, and ladybird beetles are used to control aphids in greenhouses. Here, I review three topics as ecological bases for the biological control of aphids in a protected culture: the preliminary evaluation of biological control agents, natural enemy release strategies, and the effects of intraguild predation (IGP) on biological control. A comparison of several parasitoid species was conducted to select agents for the biological control of aphids; the intrinsic rate of natural increase was a useful criterion in the preliminary evaluation. To compare predators as biological control agents, the aphid-killing rate must be considered as a critical criterion, rather than reproductive criteria. The banker plant system (open rearing system) is used as a release method for Aphidius colemani and other natural enemies of aphids. Continuous release of parasitoid adults, which is the important characteristic of this method, has a stabilizing effect on population fluctuation in the aphid–parasitoid system. Two species of natural enemies can be used to control aphids in greenhouses. When one parasitoid and one predator are used simultaneously in a greenhouse, IGP of the parasitoid by the predator can occur, but the effect of IGP is less important in greenhouses than in the field.