Insect Digestive Physiology
Many studies have attributed the success of bees to their mutualistic pollenivorous lifestyle, but we still know very little about how pollen-feeding affects bee health and fitness. Several studies have suggested that oligolecty (pollen specialization) is the ancestral condition in bee groups and that bees need to overcome physiological or neurological constraints in order to broaden their diet. Physiological constraints are most likely related to the pollen quality and the mechanisms of pollen digestion. Pollen nutritional content varies widely and is known to affect bee survival, especially under conditions of stress. Plant secondary defense chemicals can also have an impact on bee health. Additionally, it appears that bees have differential abilities to digest pollen, meaning that not all bees have the same capacity to extract nutrients from any given pollen source. This complexity of adaptations to the pollen diet makes bees an excellent system for investigating digestive adaptations to a unique diet.
My current work in this areas is assessing the digestive adaptations that generalist and specialist bees use to deal with the pollen diet, focusing on North American cucurbit species.
Pollen Defenses in Cucurbita plants
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My colleagues and I observed that Eucera (Peponapis) pruinosa, a specialist bee on cucurbit plants, collected pure loads of cucurbit pollen while generalist honey bees and bumble bees collected negligible amounts of cucurbit pollen, even though all groups of bees visited cucurbit flowers. We hypothesized that plants may benefit from limiting the community of generalist floral visitors if the species that remain are more effective pollinators. Since cucurbit flowers have no morphological adaptations to limit pollen collection by bees, we assessed the potential for physical, nutritional, and chemical pollen defenses of cucurbit plants to limit pollen loss to their generalist pollinators. Bumble bee (Bombus impatiens) microcolonies fed non-cucurbit pollen showed increased pollen consumption over time, while microcolonies fed natural cucurbit pollen and crushed cucurbit pollen (to remove physical defenses) showed markedly decreased pollen consumption over time. Microcolonies fed crushed and natural cucurbit pollen treatments never reared offspring to adulthood, while all other treatments were more likely to do so, and more larvae were ejected from microcolonies fed natural cucurbit pollen than in any other treatment. Furthermore, bees from microcolonies fed crushed cucurbit pollen had higher proportions of hindgut expansion, gut melanization and mortality. Together, these results suggest that generalist bumble bees avoid collecting cucurbit pollen due to the physiological costs of physical and chemical pollen defenses.
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Download my protocol for microcolony assays here.