The science of neonicotinoids: guest blog by Dr David Gibbons

Following the UK Government’s recent decision to support a complete ban on neonicotinoids, my colleague Dr David Gibbons, Head of RSPB’s Centre for Conservation Science, provides a personal perspective on these insecticides.

The recent decision by the Environment Secretary, Michael Gove, to support a complete ban on the use of neonicotinoid insecticides came as a delightful surprise to me. After all, it was only in 2013 that the UK Government opposed a European Union moratorium on the use of these chemicals on just flowering crops. I was particularly pleased, as over the last few years I have been one among hundreds, probably thousands of scientists whose research has been raising concerns about the impacts of these insecticides on wildlife. It seemed as though Christmas had come early this year.

So, what was it that led Mr Gove to take such an enlightened stance? The simple answer is the growing body of evidence that has arisen over the last seven or eight years demonstrating the harmful impacts of neonicotinoids on bees, bumblebees and other pollinators. One or two very recent studies seem to have tipped the balance in the minds of Mr Gove and his advisors.

To set this dramatic change of heart in context, it is probably worth delving a little into the recent history of these insecticides.

Neonicotinoids, commonly known as neonics, were introduced to the market in the early 1990s. They are mostly applied as a coating to crop seeds before planting, with the chemical subsequently spreading ‘systemically’ throughout the growing plant. Unlike many other insecticides that are sprayed onto the crop, neonics were designed to kill only those pests that actually fed on the crop, and not those that were harmless to it, and certainly not those that were beneficial to it through providing their services as pollinators. In addition, neonics exert their lethal effect by binding to receptors on nerve cells, and they bind more strongly to insect than vertebrate receptors. Consequently, it was thought that neonics would be much safer for vertebrates - and thus people -than some other insecticides in widespread use at the time. Neonics were thus heralded as a highly-targeted, more environmentally benign class of insecticides, killing only true crop pests and leaving other insects untouched. As an added bonus, they were much safer for farmers to use. It is perhaps not surprising that neonics became the fastest growing of the five classes of insecticide over the intervening two decades, and are now the dominant class. The title of a 2008 scientific paper ‘Neonicotinoids—from zero to hero in insecticide chemistry’ captured the widespread agrochemical industry view at that time.

At about the same time, however, it became apparent that neonics were finding their way into the nectar and pollen of flowering crops, such as oil-seed rape, and that insect pollinators were consequently exposed to them while foraging. The extent to which this was happening had not been foreseen when these chemicals first came to market. Two particular studies, published in the journal Science in 2012, catalysed research into the impacts of neonics on pollinators. One study, from Scotland, showed that neonics reduced the production of bumblebee queens dramatically; the other – a French study - that neonics intoxicated honey bees to such an extent that they were unable to find their way back to the hives, and consequently died.

Over the last decade, the rate of publication of studies of the impacts of these insecticides on pollinators has risen ten-fold, with a new study published every few days. To the follower of the science in this area it has been daunting to keep abreast of it. Despite this, several groups have attempted to do so. I have been involved in one of these, investigating not only the impacts on pollinators, but also on other organisms. My particular focus has been to review the evidence of impacts on vertebrates. The most recent update of this work, which you can read for yourself here in the journal Environmental Science and Pollution Research, was published during the same week as Mr Gove’s announcement. While not all of the evidence points in the same direction, this group nevertheless concluded that there was substantial evidence of harm to a wide variety of organisms through normal use of these insecticides, and called for a marked reduction in their global use. Others have reviewed the evidence, and come to similar conclusions.

Despite this, the view held by the UK Government - until the recent announcement - was that while they accepted pollinators could be harmed by neonicotinoids, those studies that had demonstrated harm had only done so by using concentrations higher than would occur following normal agricultural use. Consequently, they called for more realistic field studies. The scientific community listened, and a number of such studies have now appeared. Two are particularly notable, one from Sweden published in Nature in 2015, the other from field studies in UK, Germany and Hungary published in Science a few months ago. Both were field studies in real-world farming environments in which pollinators fed on neonic-treated crops. Both showed evidence of harm – to bumblebees in the Swedish study, and to bumblebees, mason bees and honey bees in the pan-European study, though for the latter in only two of the three countries. To add to the growing concern, we now know that neonics are present in three-quarters of all honey sampled around the world.

Worryingly, it has become increasingly apparent that pollinators are exposed to neonics even when feeding on plants that have not been treated with these chemicals. Most of the neonicotinoid insecticide attached to crop seeds leaches into the soil, from where wild plants or the following un-treated crop can absorb it. In addition, neonic particles can be knocked off the seed coating during sowing, leading to dusting of neighbouring plants. Neonics have even been found in flowers in the margins of fields specially created to be beneficial to pollinators. None of this was foreseen when these insecticides were originally introduced, and has increased the exposure of wildlife to these chemicals markedly. Non-flowering crops, such as wheat, are still routinely treated with neonics in the EU, as the 2013 moratorium only applied to flowering crops. Thus although non-flowering crops do not attract pollinators, if they are neonic-treated they can still have an impact on them.

This October, the UK’s Expert Committee on Pesticides met to discuss the newly emerged evidence. Their advice to Ministers was that the risk posed to pollinators from the use of neonicotinoids on flowering crops was now greater than in 2013, and that they supported the continuation of the moratorium. But they also went further. They noted that the sowing of treated non-flowering crops could contaminate nearby wild plants, and thus advised that the moratorium should cover all crops, not just those that flower. Given this very strong steer, it is hard to see how Mr Gove could have done anything other than what he did.

As a final postscript, there are suggestions that neonicotinoids may have impacts on vertebrate populations, too, although the evidence is weaker than for invertebrates, partly because of the paucity of studies, and partly because neonics bind less effectively to vertebrate nerve receptors. Despite this, it is theoretically possible that seed-eating birds are being exposed to very high concentrations of neonics by eating treated seeds left on the surface at sowing. One study in France suggested that mortality due to poisoning by neonic-treated seeds was at least likely in 70% of reported wildlife mortality incidents. In addition, recent laboratory studies have shown that bats and birds exposed to field-realistic levels of neonics are unable to echolocate and navigate, respectively.  A further study from the Netherlands linked declines in insectivorous bird numbers to concentrations of neonics in water bodies that were high enough to kill their aquatic insect prey, and is one of the few studies to suggest such an indirect or ‘food-chain cascade’ effect.

The impacts of neonics on vertebrates, and particularly birds, is an active area of research for RSPB’s Centre for Conservation Science.

@David_W_Gibbons