The Plight of the Bumblebee

Without bumblebees, we're in trouble. As they fly between flowers in our gardens and fields, bees do a very important job.

Flowering plants rely on cross-pollination for their survival. Bumblebees are pollinators: they transport pollen from one plant to another on their hind legs. They've been doing this for over 145 million years, so many plants, including some our most important crops, have evolved to depend on bees and other pollinators for successful reproduction.

Throughout the world, bee populations are suffering alarming declines. In spite of their vital role as pollinators, not enough people realise the severity of the issue (read this article about workers in China struggling to pollinate crops by hand). Research and media attention often focus on the afflictions of honeybees. However, bumblebees are a different type of bee that in some circumstances (read about the fascinating process of buzz pollination) are a more effective pollinator than the honey bee. 

Bumblebee queens spend up to nine months in diapause, a hibernation-like state which allows them to survive the harsh winter weather. My research demonstrated that queens have reduced immune function during this time, leaving them vulnerable to infections and parasitic attack.

Sphaerularia bombi is a common yet poorly studied parasitic roundworm found primarily in the Northern hemisphere. It can infect up to 50% of queen bumblebees in some populations. Adult female Sphaerularia live in the soil and infect unsuspecting bumblebee queens during their diapause. My project showed that, with their guards down, the queens cannot defend themselves from invading parasites.

The problems for infected queens start when they emerge from diapause. During this time the parasite everts its uterus, massively increasing the organ's size to an astonishing 300 times the volume of the rest of the worm’s body. This enormous uterus can release up to 100,000 eggs into the queen, while it also extracts essential nutrients from the bee.

Infection with Sphaerularia bombi has serious consequences for bumblebee populations. Sphaerularia castrates queens so they can't form new colonies. Sphaerularia also manipulates the infected queens’ behaviour; causing them to shed the parasite’s eggs into soil where new bumblebee queens are likely to spend their diapause. This mechanism ensures the parasites' survival.

The relationship between parasite and host appears to exist down to a molecular level. S.bombi infection seems to change the amount of key proteins which are produced by the bumblebee queens. Further investigations of interactions at the molecular level may give us clues about how the parasite causes such dramatic behavioural and physiological changes in its hosts.

Bumblebees are important pollinators with high ecological and commercial value, yet Sphaerularia bombi remains poorly studied and has received almost no attention from the media so public awareness is all but absent. Research at Trinity College Dublin aims to address the significant gaps in our understanding of this important yet often overlooked parasite. It is essential to tease apart the complex interactions between bumblebees and Sphaerularia if we are to have any chance of addressing such a key factor which contributes to the plight of the bumblebee.

For more information about protecting bumblebees in the UK, visit the Bumblebee Conservation Trust.

Sive Finlay is a PhD student at Trinity College Dublin. Her research into Sphaerularia bombi won her the European biology student of the year at the SET Awards in 2012. Her study was part of her BA in Zoology and was in collaboration with Joe Colgan’s PhD research at Trinity College Dublin, under the supervision of Professor Celia Holland.