Although for many plants the amountof pollen grains that arrive on the stigma is far in excess of theamount required to fertilize all the ovules, poor seed set or low yieldis often caused by insufficient pollination (Burd, 1994; Larson andBarrett, 2000; Ashman et al., 2004) both in animal-pollinated andwind-pollinated plants (reviewed in Friedman and Barrett, 2009).