Quantifying the food requirements and effects of food stress on bumble bee colony development

Agricultural intensification has led to a reduction in semi-natural areas and in the abundance of wild flowering plants, reducing the availability of floral resources upon which pollinating insects depend. This is widely accepted as one of the major drivers of pollinator declines, but few studies have directly addressed the effects of dietary restrictions on pollinator fitness. Here, we investigated the effects of restricting pollen and nectar supply on bumble bee (Bombus terrestris) colony growth, adult size and number. Colonies required up to 6 g pollen/1 g protein and 50 g sugar to establish a colony of 5 workers, and consumed in excess of 176 g pollen/31 g protein and 1,186 g sugar in their lifetime. Regardless of restrictions on pollen or nectar availability, colonies consumed a ratio of 1 g protein to similar to 43 g sugar, though free-flying colonies require proportionally more sugar to fuel foraging. Food-limited colonies from an early stage grew little with anything less than ad-lib nectar, while more-established colonies increased in weight even with low levels of nectar suggesting a shortage of resources in early spring may be most damaging to bumble bee colonies. Dietary restriction reduced the number of reproductives produced, but had variable effects on the size of workers and males. Nosema ceranae infection was included as a covariate in analyses and had a significant negative effect on colony growth. This study provides a base line for the developmental requirements of bumble bee colonies, and indicates the effects a resource deficit may have on their development and reproduction.