Near-death experiences: sublethal effects of pesticides on pollinating insects

Featured image: A bumblebee visiting a field of oilseed rape: one of the most important flowering crops in Europe. Image source: nautilus64/pixabay (CC0) 

References

1Rundlöf, M. et al. 2015. Seed coating with a neonicotinoid insecticide negatively affects wild bees. Nature 521: 77-80. https://doi.org/10.1038/nature14420

2Siviter, H., Brown, M.J.F., Leadbeater, E. 2018. Sulfoxaflor exposure reduces bumblebee reproductive success. Nature (in print). https://doi.org/10.1038/s41586-018-0430-6

N-E-O-N-I-C…?

This spring, I worked on a government report about the status of Swedish pollinators – the bees, flies, butterflies and other animals that pollinate wild and cultivated plants, at great benefit to human society. Being a relative newcomer to the world of environmental policy, it was a great learning experience for me, both in terms of dissolving some of my own personal confusion regarding how such matters are communicated in “official” channels, and also in terms of understanding the intricacies of our society’s dependence on the relationship between plants and their pollinators. In addition to this, when inspecting my writing, I learned that I am maybe not as good a speller as I think I am-

Throughout the report I found a colorful variety of spellings of a certain fourteen-letter word. Neonictonoids. Neonicontids. Neonictinoids. And so on. The word I was trying – and failing – to get my swiftly typing fingers to grasp was neonicotinoids, a word containing far too many ‘o’s and ‘i’s for my brain to accept it. But like it or not, brain: ”neonicotinoids” is an important word to grasp for anyone wanting to understand the problems that wild pollinators are facing. In those environmental policy discussions of preserving pollinating insects, this widely used group of broad-spectrum pesticides sit front and center. Their negative impacts on pollinators – demonstrated for example in a widely publicized study from my home region of Skåne1 – have led to a near-complete ban on their use in EU member states.

Sublethal weapons

Negative impacts of pesticides on pollinators can take different forms. The pesticides can either kill the pollinators upon contact, thereby directly impacting the number of pollinators – this is called lethal effects – or, the impact can be on the pollinators’ abilities to reproduce. These kinds of indirect impacts on pollinator abundances are called sublethal effects, and are generally not spotted by regulatory bodies through traditional ecotoxicological tests.

Citizens celebrate in Brussels the EU’s decision to ban three key neonicotinoid compounds (Source: avaaz.org via flickr)

In the case of neonicotinoids, these sublethal effects proved to be especially alarming. It turned out that it didn’t matter if the pesticide was applied indirectly through seed-coating or sprayed directly on the crop. In theory, seed-coating should be a more pollinator-friendly pesticide application technique, since the pesticide slowly disperses within the plant as it grows and doesn’t come in direct contact with the pollinators, as a spray does. But the aforementioned study in Skåne demonstrated how oilseed rape fields that had received a neonicotinoid seed-coating were not very attractive homes for pollinators. The treated fields displayed dramatically reduced densities of wild bees, as well as reduced bumblebee colony growth and reproduction. Additionally, one common species of solitary bee entirely avoided building nests in these fields. The only bees that seemed unaffected by the pesticide were the domesticated honeybees, which in itself was a surprise, and a clear example of why honeybees and wild bees can’t necessarily be treated as ecological equivalents.

Out of the frying pan, into the fire?

A more recent study from a group of scientists at Royal Holloway, University of London2, shows us that available alternatives to neonicotinoids might not be much better. The scientists have investigated the impact of sulfoxaflor (another spelling challenge!) on the reproductive success of Bombus terrestris, one of the most abundant European bumblebee species, and an important wild pollinator in most flowering crops in Europe. Sulfoxaflor belongs to a pesticide group called sulfoximines, a likely successor to neonicotinoids, following the latter’s near-ban. These substances are either already licensed for use or being considered for licensing in several markets around the world.

Just as with neonicotinoids, the scientists here found considerable sublethal effects on bumblebee colonies in treated fields. Colonies that had been exposed to sulfoxaflor produced fewer workers, and later on, fewer reproductive offspring. Such a reduction in reproductive output sets the colony on a downward growth trajectory and can ultimately lead to its collapse. The scientists observed these dramatic effects despite using pesticide application levels lower than the US Environmental Protection Agency’s estimates for field-realistic post-spray nectar concentrations of sulfoxaflor. The results from their study might therefore actually be less severe than what we would see in practice, when this compound is used in flowering crops.

Pesticides are only one of many underlying reasons why ecosystems around the world are facing dwindling pollinator numbers. But given the fact that we can readily control the availability and use of different agrochemical compounds, pesticide effects might be easier to address relatively quickly, compared to, for example, optimizing the amount and availability of flowering resources at a landscape scale over the season (another key issue for pollinators). If we want to avoid negative impacts of pesticides, evidence like that presented by these two research groups is crucial. It can inform regulators on what compounds are available on the market, and also when and how they are used. Documenting sublethal effects is an entirely necessary step for developing reliable ecotoxicological assessments in the future.

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Pernilla Borgstrom

Pernilla Borgstrom

I recently finished my PhD in ecology at the Swedish University of Agricultural Sciences (SLU). In my PhD research, I investigated the role insect herbivores might play in mediating nitrogen eutrophication effects on grassland ecosystems. I'm especially interested in interactions between plants and insects, but not too picky about which plants or which insects.

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