A Refuge for Red Scale Under Control by Aphytis: Structural Aspects

Red scale populations in eight grapefruit trees in a grove in southern California were sampled over 18 mo. We established that the interior of the trees was an area of partial refugee from parasitism by the major control agent, the parasitoid Aphytis melinus, and also by Encarsia, which was the second major parasitoid in the system. The refuge (interior) population contained > 75% of the scale and > 90% of the adult scale in the average tree. Parasitism by Aphytis in the exterior (twigs) was 27 times as high as in the refuge for second—instar scale, and 6 times as high for third instars. The differences in instantaneous parasitism rates were greater. Parasitism by Encarsia in the exterior was about twice that in the refuge. A field experiment showed that Aphytis could search in the interior and that it parasitized scales that had been placed there on lemons. Low parasitism rates in the interior may have been caused by the parasitoids' response to the bark substrate. The refuge population may account for the observed stability of Aphytis—red scale interaction in some citrus groves.