Pesticide harms bees at ‘safe’ levels – experts respond

Two new studies published in Science this week show that honey and bumble bees are negatively affected by a widely-used class of pesticides – even at supposedly safe doses.

The pair of research articles examines the effects of neonicotinoid pesticides, insect neurotoxins which were introduced in the early 1990s and have now become one of the most widely-used crop pesticides in the world. The specific chemicals examined in the research, imidacloprid and thiamethoxam, are both licensed for use in New Zealand.
In one study, Penelope Whitehorn of the University of Stirling, UK and colleagues exposed developing colonies of bumblebees, Bombus terrestris, to low levels of  imidacloprid. The doses were comparable to what the bees are often exposed to in the wild.

The researchers then placed the colonies in an enclosed field site where the bees could forage under natural conditions for six weeks. At the beginning and end of the experiment, the researchers weighed each of the bumblebee nests – which included the bees, wax, honey, bee grubs and pollen – to determine how much the colony had grown.

Compared to control colonies that had not been exposed to imidacloprid, the treated colonies gained less weight, suggesting less food was coming in. The treated colonies were on average eight to 12 percent smaller than the control colonies at the end of the experiment. The treated colonies also produced about 85 percent fewer queens, lowering the likely numbers of nests next season.

The researchers called for urgent re-evaluation of  neonicotinoid use on flowering crops because it posed a threat to bumblebee health.

In the other Science report, a French team found that exposure to another neonicotinoid pesticide impaired honey bees’ homing abilities, causing many of the bees to die. Free-ranging honeybees were tracked as they came and went from their hives after some of them were given a sublethal dose of the pesticide thiamethoxam.

The treated bees were about two to three times more likely to die while away from their nests, probably because the pesticide interfered with the bees’ homing systems, the researchers said. Mortality caused by the homing failure meant honeybee populations exposed to this pesticide was likely to drop to a point from which it would be difficult to recover.

Studies used to determine this lethality level had probably underestimated the ways that pesticides can kill bees indirectly, for example by interfering with their homing systems. “Authorisation procedures … basically ignore the consequences of doses that do not kill them but may cause behavioural difficulties,” said study author Mikaël Henry.

The Science Media Centre contacted New Zealand experts for further comment. Feel free to use these quotes in your reporting. If you would like to contact an expert, please contact the SMC (04 499 5476;

Dr Linda Newstrom-Lloyd, Research Associate, Biodiversity and Conservation, Landcare Research, comments:

“The pesticide issue in New Zealand for honey bees is a serious issue.  Misuse of harmful chemicals is certainly a factor in weakening bees further on top of varroa, which is devastating enough.  The pesticide issue needs to be evaluated in the larger context of additional problems such as increases in diseases and lack of good bee forage which further stress the bees.  Any actions that increase the risk of new diseases entering New Zealand or the risk of malnutrition from eliminating bee forage for nectar and pollen will worsen the situation.  It is the combination these three stressors that are overwhelming for the bees and make beekeeping more difficult and challenging.

“Such a combination of increasing threats to bees, and ultimately the pollination services industry, needs to be addressed with good policy awareness and more research on what is happening in New Zealand to the bees.  If we look to North America, Europe and many other regions dealing with serious problems of bee losses we can see the path that New Zealand can hopefully avoid.  It is important to realise that these countries have had varroa for 20 years or more.  New Zealand has only had varroa for just over 10 years.  Do we have the policy awareness and enough New Zealand based scientific evidence to be able to say that similar large scale problems of bee losses would not happen in New Zealand?”

Assoc Prof Peter Dearden, Director of Genetics Otago, University of Otago:

“Neonicotinoid insecticides are being increasingly used in New Zealand and overseas because they are versatile and very effective. This increase in use, however, has been accompanied by controversy as to their effects on bees, and links to a devastating bee disease affecting the Northern Hemisphere called Colony Collapse Disorder. New Zealand does not yet seem to have Colony Collapse Disorder, but its causes are still unclear, making it hard to know if we will be able to avoid it.

“Insecticide manufacturers and users know that if their products affect bees, then they will have problems getting them licensed, so they determine the lethal dose of that insecticide to bees, and try to ensure that the chemicals are kept below that level. What the two papers published in Science show is that both Bumble bees and Honeybees are affected by doses below that lethal dose.

“In the case of the Honeybee, (as shown by Henry et al), exposure of sub-lethal doses to a commonly used Neonicotinoid insecticide, causes some forager bees- those that collect food for the colony- to lose their way and not return. So while the rest of the hive seems fine, those bees that provide food for the colony aren’t doing that efficiently, because they are becoming lost, causing the colony to starve, stop thriving, and eventually die.

“For the Bumblebees (Whitehorn et al), while it is not clear quite what is causing the trouble, again the hives fail to thrive. Bumblebee hives generally die out in winter, but before that, new queen bees are produced, and they go and find somewhere to hibernate ready for next spring. If the colonies do not reach a particular size during summer, then they will not produce queens for the next year. This means the hives will die without reproducing. Exposure to sub-lethal doses of the insecticides seem to cause this to occur more frequently.

“This is important. Honeybees contribute, in honey exports, $70-80 Million to the NZ economy each year. But this is not about honey for your toast. Pollination by honeybees underpins NZs biologically based economy, and are estimated (by the National Beekeepers Association) to support >$5B pa of pastoral, arable, horticultural and vegetable exports through their pollination services.

“This pollination is under threat already from the incursion of the Varroa mite. In New Zealand there has been a slow, silent, mass-extinction of feral (non-managed) bees because of Varroa mite. All those bee colonies in holes in trees and banks you used to see have died out, leaving us with a deficit in pollination, particularly of marginal land that requires nitrogen fixation by clover, pollinated by bees, to be productive.

“It is vital that we manage this situation. We must protect what bees we have left, both honey and bumble, and the science implies that sub-lethal Neonicotinoid insecticides have detrimental impacts on bees. It is urgent that we find solutions to minimise that impact, either through better control of these very useful chemicals, or through banning them, if necessary, to maintain and improve our pollination security.”

Dr Mark Goodwin, Team Leader, Pollination and Apiculture Plant & Food research, comments:

“In New Zealand, we don’t see any significant losses of beehives that we could attribute to insecticide use or any significant losses due to unexplained causes.  That said,  most of the losses that do occur in New Zealand are not usually investigated.

“Traditionally most testing of insecticides has been on whether they kill  adult bees in short term trials and at what concentrations.  Their effect on honey bee larvae  has often been tested directly.   The effect on whole colonies is often tested. Testing chemicals  on other animals, e.g. mammals  or birds, looks for any  non lethal affects however this is usually not done when testing chemicals on honey bees.

“There has been  a trend recently, at least in the research community,  to look at sub lethal effects  of a range of chemicals on honey bees  which, as with Henry et al., are starting find these effects. It is likely that the more we look for these sub lethal  effects the more we will find.

“Possibly at some time in the future the registration process around the world  will required not just information on the effect of chemicals on honey bee survival directly but will require the sub lethal affects to be investigated as well.”

Barry Foster, President, National Beekeepers Association, comments:

“In areas of arable and horticultural farming in NZ, bees are at risk from the same risk factors that are found overseas: errors in application, chronic effects, accumulation in soil, synergistic effects with other chemicals, and the increased toxicity of new generation chemicals like the neonicotinoids and others that are being developed that may replace them. Unlike Europe, we do have plenty of areas of agricultural and forested land that is not subject to these pesticides and beekeepers can and do move into these areas to get away from at-risk areas of arable land.

“Considering NZ honeybees in managed hives are already stressed by varroa mite, extra care should be taken over other potential environmental stresses or toxin exposures. Right now we are following trends observed overseas that are likely at some point to lead to cases of large scale colony losses in NZ.”

Prof Christian Krupke, Associate Professor of Entomology, Purdue University, USA, comments:

“This work fills a notable gap in the literature with some compelling studies documenting the sub-lethal effects of these pesticides on both individual and colony levels. To that extent, they provide some data that regulators in any country can use. The concentrations of pesticide are the key – if similar concentrations (in/on bee attractive crops) are commonly seen in NZ in areas where bees will forage then the potential certainly exists for similar problems. Bees follow bloom. If that bloom (nectar and/or pollen is toxic, even at sub-lethal levels, these studies show colony-level negative effects can occur.

“These studies are also valuable because they clearly show how the living bee/dead bee paradigm used by regulators (in other words, where lethal dose is the main parameter for registration) is an outdated and oversimplified approach where pollinators are concerned. Sub-lethal work is difficult to carry out and time-consuming. However, it is necessary when the landscape is covered with sub-lethal point sources that foragers visit.

“Any stressor should be closely managed and this includes neonicotinoids. With regard to pollinators – the persistence, water solubility and systemic nature of these compounds make them much more difficult to manage than many other pesticides. The connections between multiple stressors, or potential synergies, are difficult to work out.

“We always need controlled studies that conclusively link pollinator health effects with the presence of neonicotinoids to make sure our results are not confounded with something else (land use patterns, weather, pathogens/parasites etc.). This is what regulatory authorities lean on most in making registration decisions.