Nectar sugar limits larval growth of solitary bees (Hymenoptera: Megachilidae).

The bottom-up effects of plant food quality and quantity can affect the growth, survival, and reproduction of herbivores. The larvae of solitary bee pollinators, consumers of nectar and pollen, are also herbivores. Although pollen quantity and quality are known to be important for larval growth, little is known about how nectar quality limits solitary bee performance. By adding different levels of nectar sugar directly to solitary bee provisions in the subalpine of Colorado, we tested the degree to which larval performance (development time, mass, and survival) was limited by nectar sugar. We found that larval growth increased with nectar sugar addition, with the highest larval mass in the high nectar-sugar addition treatment (50% honey solution). The shortest larval development time was observed in the low nectar-sugar addition treatment (25% honey solution). Neither low nor high nectar-sugar addition affected larval survival. This study suggests that, in addition to pollen, nectar-sugar concentration can limit solitary bee larval growth and development, and nectar should be considered more explicitly as a currency governing foraging decisions related to producing optimally sized offspring. The availability and sugar content of nectar may scale up to affect bee fitness, population dynamics, and plant-pollinator mutualisms.