Models of sympatric speciation for phytophagous insects posit a central role for host plant-associated mating as a premating isolating mechanism in lieu of geographic barriers to gene flow. Here, by means of three markand-recapture studies, we confirm that host fidelity (i.e., the tendency of an insect to reproduce on the same host species that it used in earlier life-history stages) restricts gene flow between sympatric apple-and hawthorn-infesting races of Rhagoletis pomonela (Diptera: Tephritidae) to =6% per generation. Genetically based differences in host preference, adult eclosion under the "correct" host species, and allochronic isolation contribute to host fidelity in various degrees in the races. The results verify that host-associated adaptation can produce reproductive isolation as a correlated character (a key premise of sympatric speciation). The study also represents one of the few or perhaps only example in animals where the intra-specific isolating effects of specfic phenotypes have been quantified in nature. although we refer to host-specific taxa, our comments also apply to sympatric divergence based on habitat specialization.) (ii) The semiautonomous nature of host-specific subpopulations ("host races") permits the refinement of hostassociated adaptations (e.g., traits involved in survivorship or performance on a host) that produce reproductive isolation as a correlated character, by either pleiotropy or negative genetic tradeoffs (3, 4). (iii) These host-associated adaptations either isolate the host races to such an extent that they represent distinct species or favor the evolution of additional prezygotic barriers to gene flow that eventually lead to speciation; in the latter case, the additional premating isolation is most likely to involve traits increasing host fidelity, but it could also be caused by the evolution of assortative mating traits not directly tied to host selection (i.e., pheromones, cuticular hydrocarbons, or mating structures).Allozyme studies suggest that hawthorn (Crataegus spp.)-and recently derived apple (Malus pumila)-infesting populations of Rhagoletis pomonella, the apple maggot fly, represent host races in the initial stage of sympatric divergence (5-7). Because these flies mate exclusively on or near the fruit of their host plants (8, 9), a test for the existence and effectiveness of host fidelity (the tendency of an insect to reproduce on the same host species that it used in earlier life-history stages) as a premating isolating mechanism is possible (requirement i for sympatric speciation). Here, by means of three mark-and-recapture studies conducted at a

Intra- and interspecific resource competition are potentially important factors affecting host plant use by phytophagous insects. In particular, escape from competitors could mediate a successful host shift by compensating for decreased feeding performance on a new plant. Here, we examine the question of host plant-dependent competition for apple (Malus pumila)- and hawthorn (Crataegus mollis)-infesting larvae of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) at a field site near Grant, Michigan, USA. Interspecific competition from tortricid (Cydia pomonella, Grapholita prunivora, and Grapholita packardi) and agonoxenid (subfamily Blastodacninae) caterpillars and a curculionid weevil (Conotrachelus crataegi) was much stronger for R. pomonella larvae infesting the ancestral host hawthorn than the derived host apple. Egg to pupal survivorship was estimated as 52.8% for fly larvae infesting hawthorn fruit without caterpillars and weevils compared to only 27.3% for larvae in harthorns with interspecific insects. Survivorship was essentially the same between fly larvae infesting apples in the presence (44.8%) or absence (42.6%) of interspecific insects. Intraspecific competition among maggots was also stronger in hawthorns than apples. The order or time that a larva exited a hawthorn fruit was a significant determinant of its pupal mass, with earlier emerging larvae being heavier than later emerging larvae. This was not the case for larvae in apples, as the order or time that a larva exited an apple fruit had relatively little influence on its pupal mass. Our findings suggest that decreased performance related to host plant chemistry/nutrition may restrict host range expansion and race formation in R. pomonella to those plants where biotic/ecological factors (i.e. escape from competitors and parasitoids) adequately balance the survivorship equation. This balance permits stable fly populations to persist on novel plants, setting the stage for the evolution of host specialization under certain mitigating conditions (e.g. when mating is host specific and host-associated fitness trade-offs exist).

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