Neonicotinoid pesticides have, not for the first time, been hitting the news over the past few weeks. The commonly used chemicals help keep pests from decimating crops, but have been linked with negative effects on other organisms, in particular bee colonies. This graphic and article take a look at what we know about the neonicotinoids, and the evidence for their suggested impacts.
Neonicotinoids are actually relative newcomers on the pesticides scene. Their development began in the 1980s, but they only became widely used in the 1990s. The name ‘neonicotinoids’ stems from their similarity in terms of chemical structure to nicotine, the well-known stimulant used in cigarettes; they are a family of compounds, with the three most commonly used being Imidacloprid, Thiamethoxam, and Clothianidin. There are also another four of the compounds currently on the agrochemical market: Acetamiprid, Thiacloprid, Dinotefuran, and Nitenpyram.
From their initial use in the 1990s, neonicotinoids have gone on to become the most used insecticides worldwide – their total share of the global market for insecticides in 2007 was 24%. Imidacloprid, first marketed in 1991, accounted for almost 42% of that market share, and is the largest selling insecticide in the world. Their popularity, as well as being down to their effectiveness, is a consequence of their versatility. Most of the seven approved neonicotinoids can be applied either as a spray, by seed treatment, or by direct application to soil.
This leads us on to how neonicotinoids exert their effects on insects. They are effective against a wide range of different pests, and all act in a similar manner. Neonicotinoids are systemic insecticides – meaning they are water-soluble, and can be absorbed by plants and distributed through their tissues. When insects ingest them, they bind to and block nicotinic receptors for the neurotransmitter acetylcholine in the central nervous system of insects. Acetylcholine is a neurotransmitter in many organisms, including humans. The effect of blocking the receptors for this neurotransmitter is overstimulation, which leads to paralysis and eventual death for the insects.
We also have nicotinic acetylcholine receptors, both in our central nervous system and the peripheral nervous system, so you might wonder why the neonicotinoids don’t pose just as big a danger to us. This is because, although both insects and us have the receptors, they are differently structured, and the upshot of this is that the neonicotinoids don’t bind to our receptors as strongly as they do to those of the insects. As such, they are much more toxic to insects than they are to us, or other mammals.
An insect doesn’t need to ingest a great deal of a neonicotinoid pesticide for it to exert its deadly effect. The exact figure is, of course, variable, depending on the specific species of insect. Values for the median lethal dose (the dose that kills 50% of test subjects) range from 1 to 90 nanograms per insect. For comparison, the lethal dose figure for neonicotinoids is several orders of magnitude lower than for older insecticides such as the controversial DDT.
Despite their effectiveness and widespread use, neonicotinoids have come in for increasing criticism over the past decade. Concerns are increasing that, as well as eradicating pests from crops, they can also have harmful effects on other, more beneficial organisms. Chief amongst these concerns is that they may be playing a role in bee colony collapses.
Due to these concerns, there’ve been a lot of studies examining the potential effects of neonicotinoids on bees, both in labs, and in field trials that examine bees in their natural habitats. The evidence from these is, to an extent, conflicting. Firstly, there’s no doubt that neonicotinoids can have detrimental effects on bees. For instance, lab studies have shown that reaching the median lethal dose in honeybees for the main three neonicotinoids used requires ingestion of only 4 to 5 nanograms. Exposure of honeybee colonies to a high range of field doses of Imidacloprid have been shown by other studies to have sublethal effects, with impacts on their navigation and learning skills, along with worse colony health and reduced ability for these colonies to survive winters.
However, the debate around many of these studies centres on whether they are representative of the doses actually encountered by bee colonies. Some lab studies have previously been criticised for using doses of the compounds much higher than it is expected bees would be exposed to normally. Although neonicotinoids are systemic insecticides, reported concentrations in the nectar of seed treated crops tend to be less than 5 micrograms per kilogram of nectar – much lower than reported median lethal doses. In cases, they can, however, be approaching the concentration at which sublethal effects have been seen.
The difficulty in confirming the extent of the effects of neonicotinoids on bees in their natural habitat stems from the difficulty in staging controlled field trials. It’s almost an impossibility to control every factor around trials in the wild, and a number of attempted field trials have had the results of control colonies, meant to be isolated from neonicotinoid exposure, contaminated by bees bringing neonicotinoid-containing pollen back to the colony. A UK government study fell victim to this issue in 2013, making its results and, by association, conclusions less robust.
What we can say, and what an EU report considering multiple studies concluded this month, is that there is increasing evidence that neonicotinoids can have negative effects on pollinating insects such as bees. Whether they are the major cause of bee colony collapses is another question entirely. Entomologists have pointed to mites, viruses, and habitat loss as other factors. Particularly, Varoa mites have been highlighted as a major cause of some colony collapses, although in some cases no mites were present in collapsed colonies.
Despite the conflicting evidence, neonicotinoids have been subject to a two year partial ban in the EU since December 2013. The ban prevents use of the three most commonly used neonicotinoids on flowering crops, but still allows them to be used on winter crops (when bees are dormant) and in greenhouses. However, there doesn’t seem to have been any planned monitoring of bee or colony numbers during the duration of the ban, so assessing its impact is likely to be difficult. Recently, the Environmental Protection Agency in the USA has also restricted new uses of four neonicotinoids, but has yet to place restrictions on existing uses.
The concern from some regarding these bans is that they may merely encourage the use of older pesticides that neonicotinoids replaced by virtue of their lesser impact on the environment. These include pyrethroid insecticides, commonly used in commercial household insecticides, which are also toxic to other organisms as well as the pests they seek to control.
In conclusion, the evidence implicating neonicotinoid pesticides as being a major player in the collapse of bee colonies is still far from conclusive, but there are legitimate concerns. There’s certainly an argument that we should do all we can to preserve pollinator populations – after all, we’re reliant on either direct or indirect insect pollination for the success of a significant proportion of our food crops worldwide. Still, other potential causes of bee decline have to be identified and tackled too, and a ban on neonicotinoids alone is unlikely to prove the pivotal change that turns around bees’ fortunes.
There are a number of free to access reviews on neonicotinoids and their impacts on bee colonies provided in the ‘Further Reading’ links below, if you want to peruse them and draw your own conclusions. I’ve marked those that are open access with (OA), and those which are paid access as (£).
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References & Further Reading
- Ecosystem services, agriculture and neonicotinoids (OA) – European Academies Science Advisory Council
- Overview of the environmental risks posed by neonicotinoid insecticides (£) – D Goulson
- Assessment of sublethal effects of Imidacloprid on honeybee colony health (OA) – G P Dively & others
- Direct & indirect effects of neonicotinoids in vertebrate wildlife (OA) – D Gibbons & others
- Neonicotinoids in bees: A review (£) – T Blacquiére & others
- Overview of status & global strategy for neonicotinoids (£) – P Jeschke & others
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