Farmers often douse their fields in herbicides to clear the area of weeds. But scientists have found that bees depend heavily on these weeds for food, suggesting that farmers should consider conserving these pesky plants.

We need bees to pollinate crops and flowers. But bee populations have been crashing, probably due to a combination of disease, pesticide exposure, and other factors. Bees living in agricultural areas might also suffer from food scarcity, but “to the best of our knowledge, no study has ever considered both pollen and nectar diet for bees in farmland landscapes,” researchers write in Ecological Applications.

The team monitored apiaries in a 450-square-kilometer area in France, where cereal, corn, rapeseed, and sunflower crops grew. From 2008 to 2012, the scientists set up 50 hives spread over 10 locations each year. They installed pollen traps at the hives and measured the amount of nectar in some hives as well. Then the team analyzed the samples to figure out which plants the bees had visited.

The most common sources of nectar were crop plants, and bees collected the most nectar in April-May and July-August, when the rapeseed and sunflowers bloomed. But bees canvassed a wider variety of plants for pollen, visiting more than 200 species. During the two-month period between crop flowerings, as much as 40 percent of their food came from weeds such as red poppies.

Weeds could be a key part of the bees’ diet because many species are available, and they continue to flower when crop plants do not. Weed pollen can help bees weather a tough period when the number of bees in the hives has peaked but food is scarce, the authors suggest. By using less herbicide, farmers could help maintain weed populations — and support the bees as well. — Roberta Kwok | 27 November 2014

Source: Requier, F. et al. 2014. Honey bee diet in intensive farmland habitats reveals an unexpectedly high flower richness and a major role of weeds. Ecological Applications doi: 10.1890/14-1011.1.

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