Ground-nesting bees: The neglected pollinator resource for agriculture

Bees have been pollinating a substantial fraction of Earth's flowering plants for over 100 million years, but fewer than 10 of the available 20 000 non-parasitic bee species have been managed for agricultural pollination. These include the honey bee (Apis mellifera): alfalfa leafcutting bee (Megachile rotundata), alkali bee (Nomia melanderi), and several species each of mason bees (Osmia spp.) and Bumblebees (Bombus spp.). Among these managed pollinators, only the alkali bee nests in the ground, although ground-nesting bee taxa predominate outside of the lowland tropics. Despite their many desirable qualities for agricultural pollination, including fixed host fidelity (oligolecty), superior pollination efficiencies, reliable synchronization with bloom and gregarious nesting habits, ground-nesting bees have generally defied attempts at manipulative management. Progress in managing ground-nesting bees has been impeded by our limited insights into the sensory cues and behavioral mechanisms responsible for nest site philopatry and the recruitment of females into conspecific nesting aggregations. Candidate cues include olfactory attractants (mandibular, labial and Dufour's gland secretions), visual landmarks and soil quality. The olfactory cues are likely recognized instinctively or by imprinting, possibly at eclosion. In contrast, visual cues are probably learned during orientation flights, as is known for Apis and several sphecid wasps. Specific hypotheses are reviewed and possible experimental designs are proposed in the hope of encouraging resolution of these long-standing, fundamental facets of bee biology.