Fungicides cause bumblebee declines – or do they?

Lynn Dicks explores the drivers of rapid declines in US bumblebees populations.

Check out the full paper here.

US bumblebee declines

An important new study came out this week exploring the causes of rapid bumblebee declines in the US (McArt et al. 2017). Once again, it’s a story of interacting threats revealed by statistical wizardry, leaving scientists like me concerned, but cautious. This time there is a single culprit – a widely used fungicide called chlorothalonil.

rustypatched bumblebeeThe study looks for statistical associations with land use and habitat that might explain prevalence of a bumblebee gut parasite (Nosema bombi) or disappearance from previously occupied sites, for eight US bumblebee species – four known to be declining , and four stable. The declining species include the beautiful rusty-patched bumblebee Bombus affinis (pictured), which has disappeared from almost all its former range and was recently listed on the US Endangered Species Act.


Revealing the drivers of declines

The statistics test a wide range of possible factors – 24 in all – including different measures of habitat loss, areas of agriculture, built up area, insecticide, herbicide and fungicide use. The bumblebee dataset is based on 10,745 bumblebees from 284 sites across the USA, collected systematically between 2007 and 2009. For declines over time, the presence of each species at each site is compared with expected presence according to distributions of the species, modelled using locations of 73,759 specimens from US natural history museum collections.

The findings are startling. Only three factors are associated with prevalance of Nosema infection in the four declining bumblebee species – built up area, latitude (it’s worse further north) and the expected usage of one particular fungicide, chlorothalonil (more fungicide is worse for bees). Weirdly, more buildings were associated with less infection – maybe because bees are better nourished when they are feeding in gardens?

Just two factors were associated with disappearance of bumblebees from previous sites – latitude and total expected fungicide use. For the four bumblebee species that were not declining, no such associations could be found.

What do these findings mean?

This is a story of interacting threats. Fungicides are very widely used in agriculture, and emerging science is showing that they negatively impact on bee health, possibly by reducing their immune response or affecting their gut microbiome. Fungicides have also been shown in honey bees to kill gut cells and allow the honey bee version of Nosema to proliferate. These effects would not be picked up by risk assessment processes that test for direct toxic effects of chemicals, but not for complex interacting effects over longer time scales.

The authors quite rightly caution against over-interpreting these findings. They are statistical associations only. Fungicide usage is predicted, using surveys of actual use on specific crops from a sample of farms, and extrapolated to the whole country based on areas of each type of crop. So the association is with areas of certain crops with high fungicide use, rather than with actual fungicide use. There are suggested mechanisms, but a lot more work needed to prove a causal link with the disappearance of these bees.

What about the UK?

Could the same thing be happening in the UK? There are similar levels of fungicide use (chlorothalonil is approved for use here), and our wild bees are exposed to fungicides and normal gut parasites in the same way. We have some rapidly declined bumblebee species, although the fastest range contractions happened last century here, rather than more recently.

So the same has potentially happened in the UK, but as far as I know, no-one has done this particular analysis. A really interesting question is – why does this association with fungicide only appear for some bees, not others? The bumblebees that are apparently not declining in the UK, like the buff-tailed bumblebee Bombus terrestris and the common carder bee Bombus pascuorum, have become extremely important for crop pollination. Are they less susceptible to interacting threats from fungicides, insecticides and pathogens? Are they somehow protected by their large numbers? Or have their numbers also decreased a lot since intensive agriculture starting using chemicals, and we haven’t noticed because we didn’t count them and they are still common enough to be widespread?

Getting involved

If you want to get involved in scientific monitoring of insect numbers, there are plenty of opportunities, such as the Bumblebee Conservation Trust’s BeeWalk and the Flower Insect Timed Count (FIT Count) organised by the Pollinator Monitoring and Research Partnership (”

About the author

Lynn Dicks

Lynn Dicks is a NERC Independent Research Fellow in the School of Biological Sciences. She works on pollinator conservation and farmland biodiversity, doing research at the interface between agro-ecology, policy and the food and farming industry. She also works on how to summarise and communicate scientific knowledge.