As the concentration of sugar in a solution increases, the viscosity increases exponentially. A dilute sugar solution will “pour” rapidly like water, but high concentrations are “as slow as molasses”.
Kim and colleagues have devised a mathematical model* to calculate the optimal concentration of sugar in nectar based on different styles of feeding. Honey bees and ants have a “tongue” that is dipped into the nectar and the viscous liquid pulled into the mouth. Butterflies have a long tube that is placed in the nectar like a straw, and the nectar pumped with muscles. The honey bee mouthparts can accommodate more viscous liquids than butterflies. The Kim-Gilet-Bush model predicts an optimal concentration of about 55% sugar for honey bees while most butterflies have an optimal concentration closer to 30% sugar.Plants attract insect pollinators by offering rewards that include sugar and other nutrients. It has been noted that insects differ in their preference for sugar concentration. Honey bees and ants typically prefer flowers with nectar that has higher concentrations of sugar while butterflies prefer flowers with nectar sugar concentrations that are somewhat lower. In the coevolution of flowering plants with their insect pollinators, the quality of the nectar is selected to meet the needs of the plant to attract pollinators and the needs of the pollinator for nutrients.
*Optimal concentrations in nectar feeding. Wonjung Kim, Tristan Gilet, and John W. M. Bush. 2011. PNAS 108: 16618–16621.