Temperature plays an important role in effective management of the alfalfa leafcutting bee [Megachile rotundata (F.); Megachilidae], the major commercial pollinator of seed alfalfa [Medicago sativa (L.); Fabaceae] in North America. To improve our understanding of threshold and optimum rearing temperatures of M. rotundata, we examined the effect of temperature on postwintering development by using a greater number of temperature treatments than applied in previous studies (19 versus eight or fewer) and analytical tools formulated to model nonlinear relationships between temperature and insect development rates. We also tested the hypothesis that rearing temperature influences adult body lipid content at emergence, which could affect adult survival, establishment and performance as a pollinator, and reproductive success. We found that the Lactin-2 and Briere-2 models provided the best fits to data and gave reasonable estimates of lower (16-18°C) and upper (36-39°C) developmental thresholds and optimum (33-34°C) rearing temperatures for maximizing development rate. Bees successfully emerged over a broad range of temperatures (22-35°C), but variation in development rate among individuals reared at the same temperature was lowest at 31-33°C. The optimum rearing temperature to maximize the proportion of body lipids in adults was 27-29°C. Our results are discussed in relation to previous findings and speak to the difficulties in designing practical rearing guidelines that simultaneously maximize development rate, survival, and adult condition, while synchronizing adult emergence with alfalfa bloom.
The alfalfa leafcutting bee, Megachile rotundata (F.) (Hymenoptera: Megachilidae), is widely used in western North America to enhance pollination of alfalfa, Medicago sativa L., grown for seed production. Feral populations have been documented but have been little-studied, We used trap-nests of a wide range of diameters (3–9 mm) to study a feral population at a wildflower seed farm in Montana. Adult females accepted all hole sizes but tended to provision nest tunnels of smaller diameters (3–4 mm) than those provided in managed populations (5–7 mm). Progeny, especially females, from intermediate- and large-diameter nests averaged larger body size that those in the two smallest nest-diameter classes. Offspring were often larger in nests with greater numbers of offspring, indicating that there was no trade-off between offspring size and number within nests. Individual nests tended to contain a relatively small range of progeny sizes and those sizes tended to vary among nests of the same diameter. Previous studies of M. rotundata indicate that heritability of body size is low, most offspring within nests are full siblings, and variation in offspring size is due to variation in the amount of provision provided by mothers. Thus, the fact that body size varied among families probably related to variation in provisioning capabilities among females. We also found evidence that offspring size varied spatially but that seasonal trends in offspring size were weak. Sex ratio varied among different nest sizes, but only the smallest and largest nests produced biased ratios. Sex ratios were less male-biased than in commercially managed populations of M. rotundata.
We examined how the species composition and volumes of pollen loads of Megachile rotundata varied seasonally and among females of different body sizes. Alfalfa and mustards (Brassicaceae) made up, on average, 88-95% of the pollen load volumes in each of three seasonal samples; in total, the 300 females sampled carried ten different pollen types. Because of variation in pollen grain size among plant species, estimates of the contribution of different species to pollen loads, based on pollen counts only, differed from those adjusted for pollen grain size. The overall size of pollen loads, as well as the contribution of alfalfa declined seasonally, while that for mustards (with smaller individual grain sizes) increased. The type of pollen carried by females did not vary with body size, but we found a significant relationship between female size and the size of the largest loads. Surprisingly, females of intermediate size carried the largest pollen loads, whether measured by counts or volume.Megachile rotundata / alfalfa / pollen load composition / seasonal variation / body size
Females of solitary, nest-provisioning bees have relatively low fecundity, but produce large eggs as part of their overall strategy of investing substantially in each offspring. In intraspecific comparisons of several species of solitary, nest-provisioning bees and wasps, the size of the mature eggs produced increases with female body size. We further examined oocyte size–body size correlations in the solitary bee Megachile rotundata (F.), an important crop pollinator. We hypothesized that larger females carry larger basal oocytes (i.e., those next in line to be oviposited) but that body size–oocyte size correlations would be absent soon after emergence, before their first eggs fully matured. Because egg production is likely affected by the quantity of stored lipids carried over from the bees’ immature stages, we also tested the hypothesis that female body size is correlated with the body lipid content at adult emergence, the time during which oocyte growth accelerates. We found significant correlations of body size with oocyte size variables chosen to reflect: (1) the magnitude of the investment in the next egg to be laid (i.e., the length and volume of the basal oocyte) and (2) the longer term potential to produce mature oocytes (i.e., the summed lengths and volumes of the three largest oocytes in each female). Positive correlations existed throughout the nesting season, even during the first week following adult emergence. The ability to produce and carry larger oocytes may be linked to larger females starting the nesting season with greater lipid stores (which we document here) or to greater space within the abdomen of larger females. Compared to other species of solitary bees, M. rotundata appears to have (1) smaller oocytes than solitary nest-provisioning bees in general, (2) comparable oocyte sizes relative to congeners, and (3) larger oocytes than related brood parasitic megachilids.
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