We report behavioral evidence that Eurosta solidaginis, a stem-galling tephritid fly, has formed host races on its two goldenrod hosts, Solidago altissima and S. gigantea. Previous work has shown that flies from each host plant differ electrophoretically at the level of host races. The two host-associated populations were truly sympatric and were frequently found on host plants of the two species growing interdigitated with each other. Each host-associated population demonstrated a strong preference for ovipuncturing its own host. The S. gigantea-associated population emerged 10 to 14 d earlier than the S. altissima-associated population, contributing to the reproductive isolation between populations. Partial reproductive isolation is also maintained by a preference for mating on the host from which the fly emerged. The populations meet the criteria established for host races, suggesting that they may be in an intermediate stage of sympatric speciation.

Although it is widely acknowledged that a plant's tolerance of herbivore damage depends on resource availability in the plant's environment, there is no consensus on whether higher resource levels lead to greater or to lower tolerance. The prevailing model, the compensatory continuum hypothesis (CCH), predicts that tolerance of herbivory should be greater in high-resource or low-competition conditions. The main rival hypothesis, the growth rate model (GRM), makes the opposite prediction: tolerance of herbivory should be greater in more stressful conditions. The tolerance predictions of a recently introduced model, the limiting resource model (LRM), are more flexible and depend on the type of resource and herbivore under consideration. We reviewed 48 studies (from 40 published articles) of plant tolerance of leaf damage in conditions differing in levels of light, inorganic nutrients, water stress, or competition. The results of 31%, 48%, and 95% of the studies were consistent with the predictions of the CCH, GRM, and LRM, respectively. Thus, by considering which resource is primarily affected by herbivory and which resource is limiting a plant's fitness, the LRM offers a substantial advance in predicting how tolerance will be affected by environmental differences in resource availability.

Wise, M. J. and Abrahamson, W. G. 2005. Beyond the compensatory continuum: environmental resource levels and plant tolerance of herbivory. Á/ Oikos 109: 417 Á/428.The impact that herbivore damage has on plant fitness depends on the environmental conditions in which the plant is growing. It has long been assumed that a plant's tolerance of herbivory should be greater in low-stress, resource-rich environments, and this assumption has been formalized in what has become known as the compensatory continuum hypothesis (CCH). Despite the widespread acceptance of the CCH, recently accumulating results from numerous studies relating resource levels to tolerance of herbivory show that the relationship is not as simple as this hypothesis predicts. In fact, plants have often been found to be less tolerant of a given amount of herbivory in resource-rich environments than they are of the same amount of herbivory in resourcepoor environments. In this article, we present a new model, the limiting resource model (LRM), that can more reliably explain the range of observed effects of resource levels on tolerance. The main advance of this model is that it specifically considers which factors are limiting plant fitness and which resources are affected by particular herbivores. The LRM is presented in a general and flexible form as a flowchart with seven pathways to three potential outcomes: greater tolerance, equal tolerance, or lower tolerance in low-vs high-resource environments. We then review a specific example to illustrate each of these seven pathways with a study from the published literature. We finish with a description of four general criteria to direct the design of future studies to test the predictions of the LRM.

The successful colonization of novel host-plant species by herbivorous insects may be facilitated by a reduction in natural-enemy attack on insect populations associated with the novel (derived) host plant. This is particularly true if natural enemies use host-plant or habitat cues in searching for their herbivore prey. In order to test whether the acquisition of enemy-free space could have influenced the host shift in the goldenrod ball gallmaker, Eurosta solidaginis, we estimated levels of natural-enemy attack in 25 host-race populations associated with Solidago altissima and S. gigantea (Compositae) spanning the zone of sympatry between S. altissima and S. gigantea host races in New England. Mortality due to attack by the parasitoid wasp Eurytoma obtusiventris was significantly higher for the ancestral than for the derived host race (30.5% versus 0.4%) across the transect, which is consistent with the enemy escape hypothesis. Contrary to this hypothesis, mordellid beetles caused significantly higher mortality on the derived than ancestral host race (17.1% versus 2.6%). Mortality by a second parasitoid wasp and birds showed no significant differences between the two host races. Overall, the derived host race had significantly higher survivorship across the transect (36.6% versus 20.8%). An analysis of survivorship and parasitoid mortality levels from sympatric sites in this study and previous studies showed a highly significant correlation between the levels of Eurytoma obtusiventris attack and the survivorship advantage of the derived host race. Observations of this parasitoid's searching behavior confirmed that it preferentially searches the ancestral host for fly larvae. Current patterns of host-race mortality and naturalenemy behavior and abundance are consistent with the facilitation of the host shift by escape from a specialist parasitoid.

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