Quantification of sperm transfer during mating in two solitary bee species, Megachile rotundata (Megachilidae) and Nomia melanderi (Halictidae)

Both male and female adult solitary bees have recently been shown to regularly consume pollen across their flight season, and it has been suggested that pollen may be important for sexual maturation, especially in females. However, no studies have examined whether pollen consumption by males impacts sperm production. This study had three goals. The first goal was to develop a hemocytometric method to count sperm in the female spermatheca (sperm storage organ) of solitary bees in order to quantify the number of sperm transferred by males during mating. A method was successfully designed based on protocols used in honeybee queens. The second goal was to conduct sperm counts on field-collected bees of Megachile rotundata Fabricius (Megachilidae) and Nomia melanderi Cockerell (Halictidae). Sperm counts varied between the two species, with N. melanderi females having a mean of 1,579±270 sperm and M. rotundata having 417±83 sperm. These values are much lower than the millions of sperm found in the spermatheca of honeybee queens. The third goal was to conduct sperm counts on lab-emerged M. rotundata females that were mated with males that either did or did not have access to pollen. The data show that pollen consumption had no effect on sperm transfer, but did impact male vigor: pollen feeding males were much more successful in mating than males with no access to pollen. Overall, the findings of this study outline a method for counting sperm in solitary bees and present data that suggest that spermathecal sperm counts may vary greatly among bee species. Furthermore, the effect of pollen feeding on male reproductive behavior and hence vigor calls for further investigation to identify the role of pollen in male bee biology.