Crab spiders show mixed effects on flower-visiting bees and no effect on plant fitness components1

Predation on flower-visiting insects can potentially reduce pollination levels and hence plant fitness. We tested this prediction in a field experiment with 10 matched pairs of milkweed patches in which one patch in each pair contained crab spiders and the other patch was predator free. Significantly fewer honeybees were observed in the spider than no spider patches, but there was no spider effect on the two species of bumblebees. A possible explanation for this difference is that the honeybees suffered three times higher predation rates than the bumblebees. There were no spider effects on either the rate of pollinia removal from milkweed flowers or the number and mass of seeds produced. The results suggest that crab spiders may have only small effects on bees and plant fitness, at least where bees are abundant.