True fruit flies in the Rhagoletis pomonella sibling species group are at the center of a long-standing debate concerning modes of speciation. The allopatric separation of populations is widely thought to be a prerequisite for speciation in sexually reproducing animals. However, speciation in the R. pomonella group appears to have occurred sympatrically as a consequence of these flies shifting and adapting to new host plants. The sympatric shift of R. pomonella from its native host hawthorn to introduced domestic apple, which occurred approximately 150 years ago, provides a test of whether host specialization is sufficient to allow populations to differentiate in the absence of geographic barriers to gene flow. We report the results of a geographic study of allozyme variation for hawthorn and apple infesting populations of R. pomonella across the eastern United States and Canada. Six loci consistently show significant allele frequency differences at paired apple and hawthorn sites. These six loci map to three different regions of the genome, and linkage disequilibrium exists between non-allelic genes within each of these regions. Allele frequencies for five of the six loci displaying host associated differences also co-vary significantly with latitude. Inter-host divergence is, therefore, superimposed on north-south clinal patterns of intra-host variation such that the magnitude of genetic divergence between hawthorn and apple flies is a function of latitude. The findings suggest that partially reproductively isolated "host races" can evolve in sympatry as a consequence of R. pomonella infesting new host plants. Host recognition and host associated developmental traits are discussed as important factors differentiating apple and hawthorn flies.

Effects of various single and two species diets on the performance of gypsy moth (Lymantria dispar (L.)) were studied when this insect was reared from hatch to population on intact host trees in the field. The tree species used for this study were red oak (Quercus rubra L.), white oak (Q. alba L.), bigtooth aspen (Populus grandidentata Michaux), and trembling aspen (P. tremuloides Michaux). These are commonly available host trees in the Lake States region. The study spanned two years and was performed at two different field sites in central Michigan. Conclusions drawn from this study include: (1) Large differences in gypsy moth growth and survival can occur even among diet sequences composed of favorable host species. (2) Larvae that spent their first two weeks feeding on red oak performed better during this time period than larvae on all other host species in terms of mean weight, mean relative growth rate (RGR), and mean level of larval development, while larvae on a first host of bigtooth aspen were ranked lowest in terms of mean weight, RGR, and level of larval development. (3) Combination diets do not seem to be inherently better or worse than diets composed of only a single species; rather, insect performance was affected by the types of host species eaten and the time during larval development that these host species were consumed instead of whether larvae ate single species diets or mixed species diets. (4) In diets composed of two host species, measures of gypsy moth performance are affected to different extents in the latter part of the season by the two different hosts; larval weights and development rates show continued effects of the first host fed upon while RGRs, mortality, and pupal weights are affected strongly by the second host type eaten. (5) Of the diets investigated in this study, early feeding on red oak followed by later feeding on an aspen, particularly trembling aspen, is most beneficial to insects in terms of attaining high levels of performance throughout their lives.

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