Effectiveness and importance of wild bees for apple pollination

Recent declines in honey bee health and increasing demand for pollination services highlight a need to optimize pollination by wild bees in agriculture.  Apple requires insect pollination, and growers rent increasingly expensive honey bees to insure adequate pollination. Surveys over the last 4 years reveal that apple is visited by a diverse and abundant wild bee community in New York State; however, an assessment of the pollination services contributed by wild bees is lacking. To quantify the per-visit effectiveness  of native bees in apple pollination, in 2010, we interviewed dominant wild pollinators Andrena (Melandrena) spp. and honey bees (Apis mellifera) with plucked, emasculated apple blossoms to compare pollen deposition.  In addition to bee type, we tested the effects of visit duration, approach (front or side-working) and foraging type (land only, pollen or nectar foraging) on number of pollen grains transferred.  We calculated pollinator importance of Melandrena and Apis at the orchard level as the product of per-visit effectiveness and relative abundance.  In 2011, we improved these calculations, by modeling the relationship between pollen deposition and fruit and seed set. Immobilized Melandrena and Apis were hand-applied to emasculated apple flowers on trees; pollen was allowed to germinate for 48 hours before stigmas were mounted for counting. Fruit and seed set were recorded one and two weeks post pollination, respectively. Wild Melandrena moved more pollen than Apis by contacting stigma more often, and were more important pollinators in 70% of study orchards.  Accounting for resulting fruit and seed set, Melandrena were found to be as good a pollinator as Apis, and its importance based on pollen deposition alone inflated. Our results support the importance of wild bees in New York apple pollination. Identifying the biophysical factors that support abundant and diverse wild bee populations, notably ground-nesting Andrena, is, therefore, key to developing management strategies to optimize wild pollination services in orchards.