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Exotic plants can negatively impact the fitness of native plants by changing the behavior of flower visitors and thus affecting pollen transfer. The presence of an exotic plant may decrease the visitation rate to native plants and thus increase pollen limitation. Flower visitors may also switch between exotic and native plants and if pollen from an exotic plant is transferred to native plant stigmas this may impede siring by conspecific pollen. As flower visitors forage within a spatial context, the distribution of plants may affect the type and magnitude of pollinator-mediated competition. In this study we examined two questions: 1) does the exotic plant, Carduus nutans (Asteraceae) interact with the native Monarda fistulosa (Lamiaceae) through flower visitors by changing visitation rate and/or through heterospecific pollen transfer, and does this affect seed set of the native plant? 2) Does spatial context affect how the native and exotic plants interact through flower visitors? We created plots containing potted M. fistulosa with and without the presence of potted C. nutans. In the presence of C. nutans, M. fistulosa stigmas had significantly fewer conspecific and more C. nutans pollen grains. Visitation rate and seed set tended to be lower in these invaded plots, however they were not significant. In a second experiment, we examined whether changes in visitation rate to M. fistulosa due to the presence of C. nutans was a function of M. fistulosa distance from C. nutans. We found that visitation rate did not decrease in the presence of C. nutans when M. fistulosa were adjacent to C. nutans or 15 meters from C. nutans. However, floral visitation rate to M. fistulosa decreased at 1 and 5 meters from C. nutans. Our results suggest interactions between plant species through flower visitors may depend on spatial scale. [ABSTRACT FROM AUTHOR]

Abstract: The distribution of phytophagous insects is influenced by their preference for and performance on their host plants. Biological control agents of invasive plants that prefer and perform better on their target hosts are more likely to be both effective and safe.Brachypterolus pulicarius is an herbivore used in North America to combat two invasive plants, yellow toadflax and Dalmatian toadflax (Linaria vulgaris and Linaria dalmatica). Adult beetles prefer yellow toadflax over Dalmatian toadflax, and when beetles are redistributed onto Dalmatian toadflax, populations do not consistently establish. This leads to the hypothesis that beetle larvae will perform best on yellow toadflax. A reciprocal transfer experiment was conducted to test this hypothesis. Development rate, pupal mass and percent survival were measured to assess larval performance. Development time was influenced by an interaction between the source host and the test host, a pattern suggesting that it is important to consider both the collection host and redistribution host when releasing this beetle for the control of toadflax. Pupal mass of larvae reared on yellow toadflax was, on average, 13% greater than that of larvae reared on Dalmatian toadflax, supporting the hypothesis. Survival rate was not significantly influenced by source host, test host, or their interaction, suggesting that survival rates will be similar no matter the combination of collection host and redistribution host. These results, along with the preference that adult beetles show for yellow toadflax, do not support the redistribution of B. pulicarius onto Dalmatian toadflax in North America. [Copyright &y& Elsevier]

Evolution has contributed to the successful invasion of exotic plant species in their introduced ranges, but how evolution affects particular control strategies is still under evaluation. For instance, classical biological control, a common strategy involving the utilization of highly specific natural enemies to control exotic pests, may be negatively affected by host hybridization because of shifts in plant traits, such as root allocation or chemical constituents. We investigated introgression between two parent species of the invasive shrub tamarisk (Tamarix spp.) in the western United States, and how differences in plant traits affect interactions with a biological control agent. Introgression varied strongly with latitude of origin and was highly correlated with plant performance. Increased levels of T. ramosissima introgression resulted in both higher investment in roots and tolerance to defoliation and less resistance to insect attack. Because tamarisk hybridization occurs predictably on the western U.S. landscape, managers may be able to exploit this information to maximize control efforts. Genetic differentiation in plant traits in this system underpins the importance of plant hybridization and may explain why some biological control releases are more successful than others. [ABSTRACT FROM AUTHOR]