Specificity of pollen collection and consumption by solitary bees oligolectic to Helianthus annuus L.

Females of many solitary bees are specialized in the plants they visit (oligolectic) to collect pollen for their larvae, and it has been recently shown that adult bees also consume pollen. To determine if oligolectic bees are as specialized in their pollen consumption as in their pollen collecting, this study focused on pollen collection and diet of three solitary bee species collected in Walla Walla, WA, where both females and males were collected when possible. The species studied, Melissodes agilis, Melissodes robustior, and Diadasia enavata, are reportedly oligolectic on and important pollinators of the oilseed crop Helianthus annuus L. Samples of pollen carried on specialized hairs called scopal hairs were collected from females, and gut pollen samples were taken from both males and females. The composition of pollen was determined based on the number and volume of different pollen species sampled from the gut and scopae. The findings show that female M. agilis were specialized on collecting H. annuus, whereas female M. robustior, and D. enavata were specialized on collecting Asteraceae pollen in general, but strongly preferred H. annuus. Female Melissodes agilis consumed only H. annuus pollen, while female M. robustior and D. enavata consumed a mix of H. annuus and other Asteraceae pollen when it was present. Additionally, all male M. agilis and D. enavata (no males were collected for M. robustior) exclusively consumed H. annuus pollen, suggesting that H. annuus is important in the bees’ biology. These findings have implications in the conservation of these species in revealing H. annuus is necessary to their survival and could be useful as supplementary pollinators of the crop. Analysis of data on species in this locality and other localities will help clarify pollen collection and consumption patterns seen in this study.