Herbal remedies may aid bumblebees

Parasites, viruses, and all manner of pestilence are thought to play a major role in the dramatic ongoing declines of both wild bees and managed honeybee hives. But researchers from Dartmouth College and the University of Massachusetts at Amherst say one solution to these problems might be right under our noses – or under the bees’ noses at least, in the nectar of flowers where they forage for food.

The researchers inoculated the common eastern bumblebee, Bombus impatiens, with a single-celled intestinal parasite called Crithidia bombi, which has been causing trouble for numerous North American bumblebee species. Then, they fed the infected bees sugar water laced with one of eight chemical compounds produced by plants common in the bumblebee’s native range, which spans the eastern half of North America from Ontario to Florida.

After a week, the researchers counted the number of parasites in the bees’ guts. Four of the eight compounds significantly reduced parasite loads, they report in a paper published 18 February in Proceedings of the Royal Society B. These were catalpol, found in turtlehead flowers; thymol, from basswood tree nectar; and nicotine and anabasine, produced by plants in the tobacco family. Anabasine is the most powerful of the chemicals, reducing the number of parasites in bees that ate this diet by 81% compared to the controls that got plain sugar water.

The compounds that the researchers tested are among a broad group of chemicals called plant secondary metabolites, many of which help plants protect themselves from herbivores. So it could be that chemicals that help bees fight off parasitic infections might harm the bees themselves as well.

To test whether such tradeoffs occur, the researchers reared trios of infected or uninfected bumblebee sisters on either sugar water or the anabasine-containing elixir. Being infected with Crithidia shortens a bee’s lifespan, but consuming anabasine doesn’t, the researchers found.

The bees fed anabasine started laying eggs 2 days later than controls, but they were just as likely to reproduce in the end. And neither the total number nor mass of eggs laid was affected by anabasine treatment.

Nevertheless, for infected bees, consuming anabasine rather than sugar water doesn’t yield a survival benefit. The researchers suggest that instead of extending the lifespan of individual bees, the chemical may help by reducing transmission of parasites between members of a colony or between bees that forage on the same flower.

Anyone who has tasted honey knows that bees are excellent chemists. But can a bee tell the difference between an alkaloid and an iridoid glycoside? It’s not yet known whether the bumblebees seek out or preferentially consume flower nectar that can help rid them of parasites, the researchers acknowledge. But other animals, including honeybees, are known to self-medicate.

At the very least, the study points out the need for lots of diversity in our bee landscapes (hint: any habitat that contains flowering plants is a bee landscape). So, for example, incorporating hedgerows into agricultural lands may provide bees not only with places of refuge and supplemental nectar sources, as has previously been found, but with a medicine cabinet as well. It’s one more argument in favor of the tangled bank– Sarah DeWeerdt | 24 February 2015

Source: Richardson L.L. et al. 2015. Secondary metabolites in floral nectar reduce parasite infections in bumblebees. Proc. Roy. Soc. B DOI: 10.1098/rspb.2014.2471

Header image: Bumblebee collecting nectar on a turtlehead flower, which produces a compound that helps bees fight a common parasitic infection. Credit: Leif Richardson, Dartmouth College.