The effect of introduced honeybees on native bee visitation and fruit-set in Dillwynia juniperina (Fabaceae) in a fragmented ecosystem

The endemic shrub Dillwynia juniperina is found in fragmented woodlands on the Northern Tablelands of New South Wales, Australia. The species obligatorily relies on pollinators to effect fruit-set and in this study the effect of fragmentation and the presence of the introduced honeybee on fruit-set was examined at two locations. Over two seasons Dillwynia juniperina was not pollen-limited indicating that flowers were saturated with pollen and that adequate bee servicing was occurring. Two native bee species ( Leioproctus sp. 1 and Lasioglossum sp.) and the introduced honeybee, Apis mellifera L., were the most common visitors to flowers. Bee abundance varied between sites with honeybees being more common than native bees at one site. Native bees were never the most dominant pollinator. Visitation data show that native bees spend more time at flowers than introduced bees, although on average honeybees visit slightly more flowers on a bush than do native bees. Visitation data also revealed that native bee presence at bushes is negatively correlated with the presence of honeybees at the same bushes. At one of the study sites, honeybees were very abundant, but very few native bees were ever recorded over the 3 years. Results show that flowers can be pollinated from a single visit by a honeybee or native bee. Extrapolation of visitation data showed that native bees could on their own adequately service flowers in some years at some sites while at other times introduced honeybees may be necessary to augment pollination services.