Increased bee extinction associated with neonicotinoid pesticide use

An extensive study looks at data for 62 English bee species over 18 years.

Neonicotinoid insecticides are used worldwide and have been shown to be harmful to commercial honeybees and bumblebees. However, most research has examined only short-term effects on a few species in experimental settings. Now, Ben Woodcock, Nick Isaac, and colleagues have collected data from across the UK, covering 62 species over 18 years. Their results, published in Nature Communications, show that species that feed on oilseed rape, the only flowering neonicotinoid-treated crop in the UK, experience three times the population decline of those that don’t. In conjunction with smaller-scale experimental studies, the data strongly suggests neonicotinoid pesticides are affecting bee populations over long periods at a national scale. 

ResearchGate: Why is it a problem that wild bee populations are declining?

Ben Woodcock: Bee populations have been undergoing decline in the UK and other parts of the world for a very long time. It's raised a lot of concern, because bees have considerable importance. Approximately 9.5 percent of the world's food is supported by bees, and pollination is very important for a wide variety of wild plants. No one's entirely sure what the underlying cause of these widespread declines is. There have been several possibilities proposed: habitat loss, fragmentation, climate change, diseases, and of course pesticides, which is what we’re focusing on. The population declines are likely a combination of multiple factors, but we're looking at a specific pesticide: neonicotinoids.

CC BY-NC Matthias Buehler

RG: What made you focus on neonicotinoid pesticides as a possible culprit?

Woodcock: Neonicotinoids have been widely used as a seed treatment. That is, they are applied directly to the seeds of a wide range of crops, not just those that are fed on by bees. As the plant grows, they are expressed throughout the plant tissue. In some sense, this is really great, because it means any pest that feeds on the plant gets a dose of the pesticide. But the problem is, beneficial pollinators are also feeding on the plant; they're feeding on the pollen and the nectar. There have been a lot of short-term, semi-field studies—and occasionally also field studies—that have shown that when honey bees feed on pollen and nectar of these mass-flowering crops, it can have negative effects on them. So there's been evidence already for honey bees, bumblebees and one other species of solitary bee.

RG: And what does your study contribute?

Woodcock: The issue is that, while previous studies have been really crucial for identifying the negative impact of neonicotinoids, they've been very focused. They've only been able to look at three species of bees, principally because these are the only bee species you can breed under laboratory or field conditions: the domesticated honey bee, the buff-tailed bumble bee, and the red mason bee. So earlier studies have looked only at these three species and never for more than a year at a given time. Usually also over a small area—even the big field trial run by Maj Rundlöf the other year only looked at 20 fields nationally in Sweden.

“We’ve suddenly been able to make this jump from the three-species model over very short-term studies to getting an idea of what the impacts of neonicotinoids might be over longer time periods.”

So our study looks at the effects of neonicotinoids on a larger scale over a longer period. What we've been able to do is use citizen science data and data on pesticides from FERA Ltd. to look at a period of just under 20 years, spanning approximately eight or nine years before the use of neonicotinoids on oilseed rape and about nine years after. Because we've been able to look at this very long time period on a national scale across the whole of England, we've got a really informative way of looking correlatively at how changes in the use of neonicotinoids on oilseed rape have impacted wild bees. And, because we're using this long-term citizen science dataset, we we're able to consider 62 species, which is really such a difference. We’ve suddenly been able to make this jump from the three-species model over very short-term studies to getting an idea of what the impacts of neonicotinoids might be over longer time periods.

RG: What are the results of the study and their implications?

Woodcock: Due to the large number of species we looked at, we were able separate species into those that feed on oilseed rape—these are the species likely to be exposed to this pesticide in the UK—and those that don't feed on oilseed rape.  What we find is that species that feed on oilseed rape are three times more affected by neonicotinoids compared to those that don’t. This is a really important distinction.  Although this is a correlative study, we can really make this distinction as to what is potentially the underlying mechanism, because we've been able to look at these two different functional groups. Particularly in England, where oilseed rape is really the only large, mass flowering crop. You can really narrow it down.

RG: Are your results a definitive indication that neonicotinoid pesticides are to blame for declining bee populations?

Woodcock: I would describe this as being a complementary piece of evidence. Because it's correlative, it has the underlying problem that we can't experimentally manipulate. It’s great because it’s a long timeframe; it's great because it covers the whole of England; it’s great because it covers 62 species. But you can't experimentally manipulate at that scale. Instead it complements experimental studies by Rundlöf, Whitehall, and Henry, papers specifically focused on experimental manipulation. What we're showing here is that the mechanism they identified looking at one or two fields over the period of a year is occurring at national scales across whole communities of bee species over a long time period. And the timeframe is really important. With a single-year study, if you've got a species that’s in a population decline phase or a population increase phase, it can show very different responses to being exposed to a neonicotinoid pesticide. But because we've got these long time periods, we can in this sense get a much greater insight into what's going on in real biological systems.

“We’re working with are datasets collected by people who've got a very high level of taxonomic expertise.”

RG: How exactly does citizen science fit into this study?

Woodcock: In the UK we’re uniquely situated in that we have a strong history of amateur naturalists who are well-versed in taxonomy collecting data sets. The members of the Bees, Wasps and Ants Recording Society have been collecting data for an extremely long time. Like all citizen science data, it’s got its inherent problems, because it's not a long-term monitoring process; it’s not someone going out to the same place and monitoring each year in the same way. Because of that, it requires a very specific modeling approach to understand it. Nick Isaac, who did the statistical analysis in the paper, specifically models the frequency and intensity with which observations and surveys are observed within the dataset. We put a lot of thought into how to take into account that this is citizen science data. But we’re not talking about people who've never identified an insect before going out with an app. That can be valuable too in some circumstances, but what we’re working with are datasets collected by people who've got a very high level of taxonomic expertise.

RG: Based on your findings, do you recommend that we stop using neonicotinoid pesticides?

Woodcock: My perspective on this is that we’re providing an additional piece of evidence for policymakers to take into account. The reality is that a crop like oilseed rape cannot be grown without the use of pesticides. One option is to just not grow this crop anymore, which is probably not very realistic. If you continue growing it, but stop using neonicotinoids, you’re going to have to switch over to something else to control pests. Because neonicotinoids are systemic, they're quite targeted. If you start applying pyrethroids as a spray, then you've got issues of drift and water contamination.  So while we see this this study as being an important contributory piece of evidence, policymakers will need to look at it in the context the needs of farmers to produce crops and weigh it against alternatives in terms of the needs of wildlife and resource protection in general.

Featured image courtesy of Susanne Nilsson.