Many bee species are dietary specialists and restrict their pollen foraging to a subset of the available flowers. However, the reasons for specialization—and the reasons certain plant taxa support numerous specialists—are often unclear. Many bees specialize on the plant family Asteraceae, despite evidence its pollen is a poor food for non-specialists. Here, we studied six mason bee (Osmia) species, including three Asteraceae specialists, to test whether observed pollen-usage patterns reflect larval nutritional requirements, to investigate what aspects of Asteraceae pollen make it unsuitable for non-specialists, and to understand how Asteraceae specialists tolerate their seemingly low-quality diet. We reared larval bees on host and nonhost pollen and found that Asteraceae specialists could develop on nonhost provisions, but that other bees could not survive on Asteraceae provisions. These effects did not seem related to nutritional deficiencies, since Asteraceae provisions were not amino acid deficient, and we found no consistent differences in digestive efficiency among pollen types. However, Asteraceae specialists completed more foraging flights per larva, generally collected relatively larger provisions, and produced more frass (waste) than the other species, suggesting quantitative compensation for low food quality. Toxins, deficiencies in unmeasured nutrients, or aspects of pollen grain structure might explain poor survival of non-specialists on Asteraceae provisions. Our results suggest that floral host selection by specialist bees is not related to optimizing larval nutrition. We recommend further investigation of host-selection behaviour in adult bees and of pollen digestion in larvae to better understand the evolution of bee–flower associations.

许多蜜蜂都是饮食专家，并且将它们的花粉觅食限制在可用花朵的一部分上。但是，专业化的原因（以及某些植物分类支持众多专家的原因）通常不清楚。尽管有证据表明其花粉对非专业人士而言是差劲的食物，但许多蜜蜂还是专攻菊科植物。在这里，我们研究了六只梅森蜂（Osmia）物种，包括3名菊科专家，以测试观察到的花粉使用方式是否反映幼虫的营养需求，调查菊科花粉的哪些方面使其不适用于非专家，并了解菊科专家如何耐受其看似低质量的饮食。我们在寄主和非寄主花粉上饲养了幼虫，发现菊科专家可以在非寄主条件下发育，但是其他蜜蜂在菊科的条件下无法生存。这些作用似乎与营养缺乏无关，因为菊科的规定并非氨基酸不足，而且我们发现花粉类型之间的消化效率没有一致的差异。但是，菊科专家完成了每个幼虫更多的觅食飞行，通常收集了相对较大的食物，并且产生了比其他物种更多的苦味（废物），表明对低质量食品进行了定量补偿。毒素，无法测得的营养素缺乏或花粉粒结构的某些方面可能解释了非专家对菊科的生存能力差。我们的结果表明，由专业蜜蜂进行的花寄主选择与优化幼虫营养无关。我们建议进一步调查成年蜜蜂的寄主选择行为和幼虫的花粉消化行为，以更好地了解蜜蜂与花的关联的演变。我们的结果表明，由专业蜜蜂进行的花寄主选择与优化幼虫营养无关。我们建议进一步调查成年蜜蜂的寄主选择行为和幼虫的花粉消化行为，以更好地了解蜜蜂与花的关联的演变。我们的结果表明，由专业蜜蜂进行的花寄主选择与优化幼虫营养无关。我们建议进一步调查成年蜜蜂的寄主选择行为和幼虫的花粉消化行为，以更好地了解蜜蜂与花的关联的演变。