Aggregation by Parasitoids in the Successful Control of the California Red Scale: A Test of Theory

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Abstract
(1) We examined the interaction between red scale [Aonidiella aurantii], a pest of citrus, and its parasitoid Aphytis melinus, in a lemon grove in Southern California. The scale is under successful control by the parasitoid and the interaction appears to be stable. (2) We tested existing theory tht parasitoid aggregation in space is a key feature of such successful and stable control. The critical variable is the distribution of the parasitism rate. Theory shows that a strong relationship between parasitism rate and local pest density may stabilize a parasitoid-host system, and also that strong clumping of parasitism among pests, independent of their local density, is stabilizing. (3) No evidence was found at any spatial level for aggregation of parasitism in response to local host density, or for sufficiently strong aggregation independent of local host density. Parasitism tended to be homogenous in space. Parasitoids themselves also did not appear to aggregate to local host density. (4) The relevance of these results to the theory is discussed, together with other possible explanations for the system''s apparent stability.

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