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We examined the role of nonnative honey bees (Apis mellifera) as pollinators of the invasive, nonnative plant species yellow star-thistle (Centaurea solstitialis), both introduced to the western United States in the early to middle 1800s. Using four different treatments (three exclosure types) at flower heads, we observed visitation rates of different pollinators. Honey bees were the most common visitors at each of three transects established at three study locales in California: University of California at Davis, Cosumnes River Preserve, and Santa Cruz Island. A significant correlation existed between honey bee visitation levels monitored in all these transects and the average number of viable seeds per seed head for the same transects. Selective exclusion of honey bees at flower heads using a 3 mm diameter mesh significantly reduced seed set per seed head at all locales. Seed set depression was less dramatic at the island locale because of high visitation rates by generalist halictid bees Augochlorella pomoniella and Agapostemon texanus that penetrated the 3mm mesh. The introduced megachilid bee Megachile apicalis occurred at all three locales as well (though in much lower numbers) and may contribute to pollination. In an ancillary study, seed set of plants with bagged heads was compared with that of plants without any bagged heads to test for resource shunting effects. These results showed that seed set differences observed between treatments within a single plant were not exaggerated due to resource shunting induced by the bagging technique. Yellow star-thistle may have low or variable levels of self-compatibility (as reflected by low seed set levels in small-mesh bags), increasing the importance of pollination in its breeding system. These results suggest that honey bees and yellow star-thistle may act as invasive mutualists, an association that may extend to other nonnative plant and pollinator species from Eurasia.

We examined the role of nonnative honey bees (Apis mellifera) as pollinators of the invasive, nonnative plant species yellow star‐thistle (Centaurea solstitialis), both introduced to the western United States in the early to middle 1800s. Using four different treatments (three exclosure types) at flower heads, we observed visitation rates of different pollinators. Honey bees were the most common visitors at each of three transects established at three study locales in California: University of California at Davis, Cosumnes River Preserve, and Santa Cruz Island. A significant correlation existed between honey bee visitation levels monitored in all these transects and the average number of viable seeds per seed head for the same transects. Selective exclusion of honey bees at flower heads using a 3 mm diameter mesh significantly reduced seed set per seed head at all locales. Seed set depression was less dramatic at the island locale because of high visitation rates by generalist halictid bees Augochlorella pomoniella and Agapostemon texanus that penetrated the 3‐mm mesh. The introduced megachilid bee Megachile apicalis occurred at all three locales as well (though in much lower numbers) and may contribute to pollination. In an ancillary study, seed set of plants with bagged heads was compared with that of plants without any bagged heads to test for resource shunting effects. These results showed that seed set differences observed between treatments within a single plant were not exaggerated due to resource shunting induced by the bagging technique. Yellow star‐thistle may have low or variable levels of self‐compatibility (as reflected by low seed set levels in small‐mesh bags), increasing the importance of pollination in its breeding system. These results suggest that honey bees and yellow star‐thistle may act as invasive mutualists, an association that may extend to other nonnative plant and pollinator species from Eurasia.

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