A refuge for red scale: the role of size-selectivity by a parasitoid wasp

The presence of a physical refuge (an area where mortality due to parasitism is very low) has been demonstrated for California red scale, a pest of citrus. Density of red scale is very high, and parasitism by the principal parasitoid wasp, Aphytis melinus DeBach, is low on the trunk and woody branches of citrus trees as compared with the exterior twigs and leaves. One possible explanation for the low rate of parasitism is that scale in the refuge are less frequently parasitized due to their smaller size. Laboratory experiments show that Aphytis selects larger hosts, and the size distribution of parasitized scale in samples from a citrus grove shows a bias towards larger host individuals in the field. However, size-selectivity can account for only @?10% of the observed difference in parasitism rate and thus cannot be the primary explanation for the presence of the refuge.