In this post Fabrice Requier discusses his recent article, The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands, available in issue 54:4 of the Journal of Applied Ecology.

Over the past 50 years, landscapes have been profoundly modified to meet growing food requirements. Current human-dominated landscapes result in a loss of habitats and associated biodiversity. These often-simplified landscapes limit the availability of flowers – the food resource for bees. While there is no longer any need to demonstrate that bees are critical for pollination of crops and wild plants, to date there has been no evidence of a link between the lack of floral food resources and the current honey bee – Apis mellifera – colony losses. This study shows such evidence for first time.

A third to a half of managed honeybee colonies are lost every winter in Europe and North America. This decline in managed honeybees threatens honey production and crop pollination services in many countries, leading to concerns for negative social, economic and ecological effects. Among the list of stresses known to affect honeybee health, the lack of alternative food resources to mass-flowering crops is only suggested as a potential stress factor in farmlands. However, intensive cereal farmland habitats are characterized by a temporally fragmented succession of mass-flowering crops with a two-month scarcity of flowers in spring.

The concept of carry-over effect was used to test the possible causal link between scarcity of flowers in spring and honeybee colony losses in winter. Carry-over effects are mostly based on some kind of food shortage which has a detrimental effect at a later life history stage and affects the fitness or reproductive value of individuals. Given that insects have several life-history stages (egg, larvae, pupae and adult), a food shortage at an early life-history stage, e.g. larvae, could result in detrimental carry-over effects on subsequent life-history stages, e.g. adult.

By monitoring honeybee colony dynamics, this study showed that a spring shortage in pollen, the main source of protein used to feed the larvae – called brood in bee science – had a direct limiting effect on brood production, leading to a reduction in the size of the adult colony population later in the season and lower honey reserves before the onset of winter. As a final cascading cost of this carry-over effect, the spring pollen shortage weakened the colony health, with higher Varroa mite load and high seasonal and winter colony losses. These results suggest that pollen shortage – as a proxy of the limited flower availability – may have been overlooked as a cause of honeybee colony losses in farmlands.

The most interesting result was that the productive beekeeping management of colonies in the spring – i.e. harvesting honey after rapeseed Brassica napus blooming period – resulted in less honey during the July sunflower Helianthus annuus blooming period and halved the colonies survival probability over winter.  While bees are often fed with sugar syrup in May and June to combat this response, this may not compensate for a deficiency in brood production. The results suggest that avoiding such losses in intensive farmland systems include limiting or avoiding harvest of honey in early spring and increasing the amount of floral resources available through wise land-use management.

The full article The carry-over effects of pollen shortage decrease the survival of honeybee colonies in farmlands is available to read in the Journal of Applied Ecology.