Seasonal trends in the condition of nesting females of a solitary bee: wing wear, lipid content, and oocyte size

Authors: ONEILL, KEVIN - Montana State University; DELPHIA, CASEY - Montana State University; Pitts Singer, Theresa

Submitted to: PeerJ
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2015
Publication Date: 5/7/2015
Publication URL: https://dx.doi.org/10.7717/peerj.930
Citation: Oneill, K.M., Delphia, C.M., Pitts Singer, T. 2015. Seasonal trends in the condition of nesting females of a solitary bee: wing wear, lipid content, and oocyte size. PeerJ. 3:e930.

Interpretive Summary: During the nesting season, alfalfa leafcutting bee female activities could increase the wear and tear on their bodies and cause them to lose fat reserves that were stored during their immature (larval) stages. Consequently, their reproductive performance may be affected not only by environmental factors (e.g., weather and flower abundance), but also changes that occur to their own bodies. Because of potential fitness effects of seasonal changes in female body condition, the objectives of this study were to determine how wing wear, body fat reserves, and oocyte (i.e., developing eggs) sizes vary during nesting seasons, beginning when females emerge as adults. As nesting seasons progressed, females in two populations experienced a steady increase in wing wear, which is known to reduce food collection efficiency and increase risk of mortality in other bees. Soon after emergence from cocoons, females exhibited a sharp decline in fat content that remained low for the rest of the season. Newly-emerged females fed heavily on alfalfa pollen, perhaps to supplement nutrients lost during the body’s production of fat content. Additionally, the early summer drop in fat stores was correlated with an increase in the size of the oocytes they carried. However, by ~6 weeks after emergence, oocytes began to decline in length and volume, perhaps due to nutrient deficiencies related to loss of stored fats. Our results suggest that alfalfa leafcutting bee management should include rearing bees at temperatures that maximize stored fat reserves in adults and timing bee release so that significant pollen resources are available.

Technical Abstract: During the nesting season, adult females of the solitary bee Megachile rotundata (F.) face considerable physical and energy demands that could include increasing wear and tear on their bodies and loss of lipid reserves accumulated during larval stages. Consequently, their reproductive performance may be affected not only by extrinsic factors (e.g. weather and floral resource availability), but intrinsic changes in their own bodies. Because of potential fitness effects of seasonal changes in female body condition, the objectives of this study were to determine how wing wear, body lipid reserves, and oocyte sizes vary during nesting seasons, beginning when females emerge as adults. As nesting seasons progressed, females in two populations experienced a steady increase in wing wear, which is known to reduce foraging efficiency and increase risk of mortality in other bees. Soon after emergence females exhibited a sharp decline in lipid content which remained low for the rest of the season. Newly-emerged females fed heavily on alfalfa pollen, perhaps to supplement nutrients lost during metabolism of fat bodies. Additionally, the early summer drop in lipid stores was correlated with an increase in the size of the oocytes they carried. However, by ~6 weeks after emergence, oocytes began to decline in length and volume, perhaps due to nutrient deficiencies related to loss of stored lipids. Our results suggest management of M. rotundata should include rearing bees at temperatures that maximize stored lipid reserves in adults and timing bee release so that significant pollen resources are available.