Resource Effects on Solitary Bee Reproduction in a Managed Crop Pollination System

Abstract

Population density may affect solitary bee maternal resource allocation. The number of Megachile rotundata (F.), alfalfa leafcutting bee, females released for seed production of Medicago sativa L., alfalfa, may limit flower availability for nest provisioning. In turn, pollinator abundance also may affect crop yield. The M. sativa pollination system presents an opportunity to test for effects of density dependence and maternal manipulation on M. rotundata reproduction. A multiyear study was performed on M. sativa fields upon which M. rotundata densities were altered to induce low, medium, and high density situations. Numbers of adult bees and open flowers were recorded weekly; bee reproduction variables were collected once. Fields varied in plant performance for each site and year, and the intended bee densities were not realized. Therefore, the variable density index (DI) was derived to describe the number of female bees per area of flowers over the study period. As DI increased, percentages of pollinated flowers, established females, and healthy brood significantly increased, and the number of pollinated flowers per female and of dead or diseased brood significantly decreased. Sex ratio was significantly more female biased as DI increased. Overwintered offspring weights were similar regardless of DI, but significantly differed by year for both sexes, and for males also by field and year × field interaction. Overall, resource limitation was not found in this field study. Other density-dependent factors may have induced a bee dispersal response soon after bees were released in the fields that circumvented the need for, or impact of, maternal manipulation.

DNA metabarcoding identifies urban foraging patterns of oligolectic and polylectic cavity-nesting bees

“…Studying the larval provisions (nectar and pollen) within trap nests can be a valuable tool to understand foraging resource availability within a season. Forage resource availability for solitary bees partly determines the number, size, and sex ratio of offspring (Pitts-Singer, 2015 ). This is because female bees can control the offspring sex and body size, and a shortage of resources can result in reduced maternal investment favouring the production of fewer young that require less resources, often males (Seidelmann et al 2010 ).…”

Interaction between warming and landscape foraging resource availability on solitary bee reproduction

“…With limited resources, offspring quantity can decrease due to increasing foraging distances needed to obtain resources, thus increasing time and energetic foraging costs. Besides, extended time far from the nests could increase the risk of bee parasitization and predation (Pitts‐Singer, 2015), as well as brood abortion via an indirect increase in exposure to pathogens (Steffan‐Dewenter & Schiele, 2008) or related to low floral abundance (Jauker et al, 2012). Moreover, because sex dimorphism occurs in solitary bees and females control both the offspring sex and body size, a shortage in resources can also lead to a skewed maternal investment favouring the sex that requires less resources, often males (Seidelmann et al., 2010).…”

“…Moreover, because sex dimorphism occurs in solitary bees and females control both the offspring sex and body size, a shortage in resources can also lead to a skewed maternal investment favouring the sex that requires less resources, often males (Seidelmann et al., 2010). Hence, foraging resource availability may affect not only the number and size of the offspring but also their sex ratio, which ultimately could affect pollination performance (Pitts‐Singer, 2015). Overall, despite the current evidence on the deleterious effects of climate change and land‐use change in bee populations, little is known on how these two factors interact when occurring simultaneously, especially on solitary bees.…”