Our previous work has shown that an invasive thistle, Carduus acanthoides, is strongly preferred by bee communities of agroecosystems in central Pennsylvania. Our objective for this study was to determine what traits of this plant allowed it to preferentially attract pollinators compared to four confamilial native species and one native legume. We established 30 2x2m experimental plots with a controlled background density of five native annuals, then divided these plots into six blocks, composed of five experimental treatments. These treatments involved the invasion of the thistle into the plant community at two different timings (early and late in the summer) and intensities (high and low abundance), as well as a control plot which was not invaded and comprised only the five native plant species. We measured bee visitation to each plant species throughout the summer by collecting bees in the morning and afternoon, as well as plant traits including: above-ground biomass, number of flowers, size of floral display, and pollen protein, sugar, and lipid content. We chose to contrast this invasive thistle to four native asters and one native legume because asters are generally considered to have poor quality and legumes to have high quality pollen.
Results/Conclusions
Although all treatment plots (with the thistle) had a higher total number of bee visits than control plots across the summer, we found no effect of experimental treatment on the rate of visitation (number of bees per flower per minute) to the native plant species. Within a given plant species, we found the number of flowers was a significant predictor of bee visitation; however, this trend was no longer significant when we looked across different plant species. In other words, the number of flowers did not predict bee preference for different plant species. The above-ground biomass, size of the floral display, pollen sugar content, and pollen lipid content were also not significant predictors of bee preference among different flower species. We found a statistically significant and strong (R2 = 0.9) correlation between the average protein content of each plant species’ pollen and rate of visitation. The unusually high protein content of the thistle pollen may allow it to compete for pollinators with closely related native species. In addition, the high protein and low lipid content is very similar to the protein:lipid ratios of legumes, previously found to be a strong predictor of bumblebee visitation, while providing a higher protein concentration.