Dr David Pattemore, Plant & Food Research
Two papers were published today in the prestigious journal Science reporting negative effects of neonicotinoid pesticides on honey bees and wild bees in realistic field trials. Neonicotinoids are arguably public enemy number one for bees in the mind of the public, but the actual science behind the headline-grabbing stories is far from conclusive about the impacts of these pesticides.
Neonicotinoids are a class of insecticides which have a more specific action against insects and a lower environmental impact than more traditional organophosphate pesticides. As they are taken up by plant tissues, including through the coating of seeds to protect the resulting seedling as it grows, much lower rates can be used. In this sense, they are widely held to be far superior to traditional organophosphate pesticides which are broad spectrum and are applied at higher rates than neonicotinoids to suppress pest insects.
Several studies have reported a negative effect of neonicotinoid pesticides on the health and behaviour of bees in laboratory experiments. This has led to concerns among scientists, the public and policymakers about the harmful effects of neonicotinoid use. Most of their effects, however, have been reported as sub-lethal, in other words, they don’t directly kill the individual bees immediately, but their long-term effects on behaviour or physiology could affect the health of bee colonies and insect populations. A big limitation to date has been that large-scale field trials to test the ‘real world’ effects of the use of neonicotinoids are very difficult to run, and so clear evidence of the effects has been lacking.
I am aware of only four realistic field trials published prior to today. Two studies looked at the effect of real-world neonicotinoid treatments on honey bee colonies and found no negative effect (Cutler et al., PeerJ, 2014; Pilling et al., PLOSOne, 2013). Another study found a negative impact on individual honey bees, but found that the colonies were able to compensate for the loss of these individual bees so that there was no net effect (Henry et al., Proc Roy Soc B, 2015). The fourth study has probably been the most convincing to me to date, finding a negative effect on wild bumble bees and solitary bees, but no effect on managed honey bee hives (Rundlöf et al., Nature, 2015).
Two new studies have been published in Science this week, by Woodcock et al and Tsvetkov et al. The main take home message from these two studies is that they are the first to document negative effects on honey bee colonies from real-world neonicotinoid exposure. So in terms of the number of studies evaluating the hypothesis that real-world use of neonicotinoids have colony-level effects on honey bees, the score in my calculation now stands at 2 for, 4 against. This is how science proceeds: we are likely to continue to see scientific tussles back and forth while we narrow in on the actual suite of factors that affect honey bee health.
Three European countries, three different results
However, when you look a little closer at these new studies, the results are not quite so straightforward. In the Woodcock et al. study, the trial was repeated in three countries. When we look at the honey bee data, where nine parameters were assessed for each of two neonicotinoids, no negative effect was detected in Germany (they actually found a positive effect), two negative effects were detected in Hungary, and two negative and one positive effect were found in the UK.
Even when negative effects were found, this appears to only be linked to one of the two neonicotinoid treatments compared to the control. But all three treatments (the control and the two different neonicotinoid treatments), had different fungicide treatments applied with them, violating the basic scientific rule to control all variables apart from the one you are interested in, or at least account for these other variables. This study, unfortunately, has confounded neonicotinoid treatment with fungicide treatment, so it is not really possible to draw many conclusions on neonicotinoids alone. So my personal opinion is that the effect on honey bees in this study is ambiguous at best.
In terms of wild bees, the evidence is more convincing. Both bumble bees and the solitary Osmia bicornis bees showed reduced reproductive output with higher levels of neonicotinoids. This was based on total neonicotinoid residues in nests, which was not correlated with experimental treatment (i.e. the aim of the experiment to delimit three different exposures to the bees did not work), and included another widespread neonicotinoid not tested in this study. This part of the paper is an important piece of evidence of the negative effects of increased neonicotinoid exposure on the reproductive capacity of wild bees.
Canadian study clearer
The second paper by Tsvetkov et al. is more convincing about the effect of neonicotinoids on honey bees. It starts with an assessment of the exposure of bees to agrichemicals in cropping and non-cropping areas. Like the other study, their exposure was not directly linked to the treatments of the crops in the fields in which the hives were placed, but rather reflected exposure from a much wider surrounding area. Then, based on the levels of the neonicotinoid clothianidin found from these exposure trials, they assessed the effects of feeding pollen contaminated at these levels to honey bees.
The neonicotinoid treatment reduced the longevity of workers (which can hasten colony decline), reduced hygienic behaviour by the colonies (which affects susceptibility to pathogens and parasites), and increased ‘queenlessness’ (a state where the hive is without an active laying queen, so reproductive output stops until a new queen is raised or installed). They also show that combining neonicotinoids with a particular pesticide increased lethality of the neonicotinoid (which could explain some of variability in results found in the other paper).
In terms of the balance of evidence for all six papers now published, one study shows negative effects on colonies, one is ambiguous and four show no negative effects on honey bee colonies. We shall have to continue to wait to see what further studies reveal.
But what about New Zealand?
What does this mean practically for New Zealand? Nothing really at this stage. We can see that there is no net effect of neonicotinoids on honey bees in New Zealand, as numbers of hives continue to rise. We now have something close to 850,000 hives, up from just under 700,000 this time last year and up from 300,000 15 years ago. However, there are localised effects of pesticides on hives. This is usually due to applications of non-neonicotinoid pesticides, rather than neonicotinoids. The colony deaths experienced with these more traditional pesticides are much more severe than the sub-lethal effects of neonicotinoids on overall colony reproductive fitness.
There are two important caveats to this. Firstly, wild bees in the northern hemisphere have been found to be at greater risk than managed honey bees. This could be happening in New Zealand too (or rather, it is unlikely that this is not happening), with native ground nesting bees and introduced bees like the ever-popular bumble bees present at these agricultural sites throughout the year and therefore likely to be more exposed to these pesticides. However, yet again, these wild bees would be affected much more severely by organophosphates than neonicotinoids.
Secondly, the finding of a synergistic effect is really important. Agrochemicals are purposely combined at times for the sake of efficiency, and inadvertent combinations can occur through exposure from different sites, spray drift, or accumulation of residues. The effects of these many combinations of agrochemicals are rarely tested, and are not usually considered in product registrations. Maybe it is time we should start thinking about this.
We don’t have evidence of population-level bee declines in New Zealand due to agrochemicals yet, but do we need to wait for clear evidence of a decline before we start thinking about and testing for some of these potential synergistic effects? We have no long-term monitoring of any bee or other insect pollinator populations apart from managed honey bees; maybe it is time we got some programmes like this up and running.
It is good to see more thorough studies on neonicotinoids in the field coming through, providing further support for the hypothesis that these chemicals negatively affect wild bees. It could be that this is the start of a wave of studies that start to build a consensus that these pesticides also pose an undue risk to honey bee populations, or it could be that further studies reveal this specific threat to be minor compared to other pressing issues like organophosphate pesticides, loss of forage resources, climate change, and pests and pathogens. Sometimes in science you just need to sit and wait as the studies accumulate!
David Pattemore is the Pollination and Apiculture Team Leader at Plant and Food Research. All images: Copyright © Plant & Food Research. All rights reserved.