LAUSR

Resource availability has been disturbed for many organisms in agricultural landscapes including pollinator species. Abundance and diversity in flower availability benefit bee populations, however, little is known about which of protein or carbohydrate resources may limit their growth and reproductive performance. Here, we test the hypothesis of complementary resource limitation using a supplemental feeding approach. We applied this assumption with bumble bees (Bombus terrestris), assuming that colony growth and reproductive performance should depend on the continuous supply of carbohydrates and proteins, through the foraging for nectar and pollen respectively. We placed wild-caught bumble bee colonies along a landscape gradient of semi-natural habitats, and monitored the colonies' weight, foraging activity and reproductive performance during the whole colony cycle. We performed supplemental feeding as an indicator of landscape resource limitation, using a factorial design consisting of the addition of sugar-water (carbohydrate, supplemented or not) crossed by pollen (protein, supplemented or not). Bumble bee colony dynamics showed a clear seasonal pattern with a period of growth followed by a period of stagnation. Higher abundance of semi-natural habitats resulted in reducing the proportion of pollen foragers relative to all foragers in both periods, and in improving the reproductive performance of bumble bees. Interestingly, the supplemental feeding of sugar-water positively affected the colony weight during the stagnation period, while the supplemental feeding of pollen mitigated the landscape effect on pollen collection investment. Single and combined supplementation of sugar-water and pollen increased the positive effect of semi-natural habitats on reproductive performance. This study reveals a potential co-limitation in pollen and nectar resources affecting foraging behavior and reproductive performance in bumble bees, and indicates that even in mixed agricultural landscapes with higher proportions of semi-natural habitats, bumble bee populations face resource limitations. We conclude that the seasonal management of floral resources must be considered in conservation to support bumble bee populations and pollination services in farmlands.