Mass flowering oilseed rape improves early colony growth but not sexual reproduction of bumblebees

Pollination is a vital ecosystem service, which is endangered by the ongoing declines of pollinators. These declines also affect bumblebees (Bombus spp.), which are important generalist pollinators in agricultural landscapes. Most studies focussing on the conservation of bumblebees have investigated the effects of local flower-rich habitats on bumblebee density and diversity. However, bumblebee densities do not necessarily correlate with the colonies' reproductive success (i.e. the presence or absence of males and/or queens). We analysed the effects of landscape-wide availability of mass flowering oilseed rape Brassica napus on the growth and sexual reproduction of Bombus terrestris colonies. Thirty-two young colonies were established and monitored in different resource environments represented by 16 landscapes (circular study areas with 3000 m radius) with large or small amounts of oilseed rape. As an indicator of colony growth, we used weight gain, which was strongly correlated with the numbers of brood cells in the colonies. The colonies gained significantly more weight in study areas with large amounts of oilseed rape particularly during early colony stages. Despite early weight gain, the colonies in study areas with large amounts of oilseed rape did not reproduce more successfully. The frequencies of colonies that produced males and/or queens did not differ between the two resource environments. Synthesis and applications. Early mass flowering oilseed rape has a beneficial effect on colony growth, which however, does not translate into a greater likelihood of colonies producing sexual offspring. This may be due to food plant scarcity later in the colony cycle. Conservation measures should enhance food plant availability in agricultural landscapes, particularly during the most critical phases of the colony cycle: the colony establishment in spring and the reproductive phase in mid- to late summer.