Pitts-Singer Theresa L
USDA ARS Pollinating Insects Research Unit, Utah State University, Logan, UT 84322.
Environ Entomol. 2015 Aug;44(4):1125-38. doi: 10.1093/ee/nvv088. Epub 2015 Jun 13.
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.
种群密度可能会影响独居蜂的母体资源分配。苜蓿切叶蜂（Megachile rotundata (F.)），即用于紫花苜蓿种子生产而释放的雌性蜜蜂数量，可能会限制筑巢所需花朵的可获得性。反过来，传粉者的数量也可能影响作物产量。紫花苜蓿授粉系统为测试密度依赖性和母体操控对苜蓿切叶蜂繁殖的影响提供了一个机会。我们在紫花苜蓿田进行了一项多年研究，在这些田地上改变苜蓿切叶蜂的密度，以营造低密度、中等密度和高密度的情况。每周记录成年蜜蜂数量和开放花朵数量；蜜蜂繁殖变量只收集一次。每个地点和年份的田地在植物表现方面存在差异，且预期的蜜蜂密度并未实现。因此，我们推导出可变密度指数（DI）来描述研究期间每单位面积花朵上的雌性蜜蜂数量。随着DI的增加，授粉花朵的百分比、已定居的雌性蜜蜂数量和健康幼虫数量显著增加，而每只雌性蜜蜂授粉的花朵数量以及死亡或患病幼虫的数量显著减少。随着DI的增加，性别比例明显更偏向雌性。无论DI如何，越冬后代的体重相似，但两性的体重在不同年份存在显著差异，雄性的体重还因田地以及年份×田地的交互作用而存在显著差异。总体而言，在这项田间研究中未发现资源限制。其他密度依赖性因素可能在蜜蜂被释放到田间后不久就引发了蜜蜂的扩散反应，从而避免了母体操控的必要性或影响。
