Bumble bee pollinators in red clover seed production

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Bumble bees pollinate red clover (Trifolium pratense L.) but impact on seed production depends on the species, abundance, and synchrony with bloom. The objectives of the current study were to examine pollination by a native bumble bee, Bombus vosnesenskii (Radoszkowski), determine the bumble bee fauna associated with red clover in Oregon, and assess if seed set is limiting. In a cage study, yields with B. vosnesenskii (average = 661 kg ha–1) and honey bees (Apis mellifera L.) (average = 640 kgha–1) were comparable. Yields were lower compared with open pollinated plots (average = 1127 kg ha–1), which was likely due to cage effect, as seed set (# seeds/# florets per seed head) was similar in all three treatments (excluding control). Examination of the diversity of endemic bumble bees through field counts and trapping in red clover fields indicated the presence of six species. Over 92% of these were B. vosnesenskii indicating that it is the key pollinator in Oregon. Seed set across four commercial fields was high (0.84–0.88) documenting that existing pollinators, including rented honey bees, and indigenous bumble bees and solitary bees, provide close to maximum pollination of red clover in Oregon. Higher yields will require improved production practices and new cultivars with more heads per plant. Sustainability of high yields in Oregon will depend on protection of indigenous bee pollinators through conservation of habitats that provide nesting sites, judicious pesticide use, and provision of floral resources before red clover bloom.

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