Promotion of seed set in yellow star-thistle by honey bees: Evidence of aninvasive mutualism

We examined the role of nonnative honey bees (Apis mellifera) as pollinator s of the invasive, nonnative plant species yellow star-thistle (Centaurea s olstitialis), both introduced to the western United States in the early to middle 1800s. Using four different treatments (three exclosure types) at fl ower heads, we observed visitation rates of different pollinators. Honey be es were the most common visitors at each of three transects established at three study locales in California: University of California at Davis, Cosum nes River Preserve. and Santa Cruz Island. A significant correlation existe d between honey bee visitation levels monitored in all these transects and the average number of viable seeds per seed head for the same transects. Se lective 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 depre ssion was less dramatic at the island locale because of high visitation rat es by generalist halictid bees Augochlorella pomoniella and Agapostemon tex anus that penetrated the 3-mm mesh. The introduced megachilid bee Megachile apicalis occurred at all three locales as well (though in much lower numbe rs) and may contribute to pollination. In an ancillary study, seed set of p lants with bagged heads was compared with that of plants without any bagged heads to test for resource shunting effects. These results showed that see d set differences observed between treatments within a single plant were no t exaggerated due to resource shunting induced by the bagging technique. Ye llow star-thistle may have low or variable levels of self-compatibility (as reflected by low seed set levels in small-mesh bags), increasing the impor tance of pollination in its breeding system. These results suggest that hon ey bees and yellow star-thistle may act as invasive mutualists, an associat ion that may extend to other normative plant and pollinator species from Eu rasia.