Honey bees ( Apis mellifera ) decrease the fitness of plants they pollinate

Most flowering plants require animal pollination and are visited by multiple pollinator species. Historically, the effects of pollinators on plant fitness have been compared using the number of pollen grains they deposit, and the number of seeds or fruits produced following a visit to a virgin flower. While useful, these methods fail to consider differences in pollen quality and the fitness of zygotes resulting from pollination by different floral visitors. Here we show that, for three common native self-compatible plants in Southern California, super-abundant, non-native honey bees ( Apis mellifera L.) visit more flowers on an individual before moving to the next plant compared to the suite of native insect visitors. This likely increases the transfer of self-pollen. Offspring produced after honey bee pollination have similar fitness to those resulting from hand self-pollination and both are far less fit than those produced after pollination by native insects or by cross-pollination. Because honey bees often forage methodically, visiting many flowers on each plant, low offspring fitness may commonly result from honey bee pollination of self-compatible plants. To our knowledge, this is the first study to directly compare the fitness of offspring resulting from honey bee pollination to that of other floral visitors.