Evolutionary Ecology of Black Pineleaf Scale Infestations on Ponderosa Pine

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The black pineleaf scale insect (Nuculaspis californica Coleman) is a parasite of western yellow pine (Pinus ponderosa Lawson. The insects are shortlived relative to their host and largely sedentary, and their populations are subdivided and genetically differentiated over extraordinarily short distances in association with both individual host trees, and different branches within a single pine (Alstad and Corbin, in press). Three different kinds of evidence resulting from recent research (Alstad and Edmunds 1989, Alstad and Corbin in press, and Alstad unpublished) compromise our earlier view (Edmunds and Alstad 1978, 1981; Alstad and Edmunds 1983a, 1983b) that scale population structure reflects adaptation of the scale demes to intrinsic, genotypic characteristics of individual host trees. The proposed research will test a new hypothesis about the mechanistic causes of local differentiation in black pineleaf scale insects infesting ponderosa pine: that scale adaptation results from selection related to extrinsic phenotypic traits which (a) are the product of interactions between the pine genotype and its environment, and (b) vary both within and between trees. These opposing hypotheses represent major themes in the literature of plant/herbivore interactions; they are of basic interest to evolutionary ecologists and have clear and important implications for forest management.