Neonicotinoids : 7 A THREAT TO HONEY BEE SURVIVAL? Neonicotinoids Falko Drijfhout, PhD, MRSC, FRES, FHEA, Keele University I
n the past three years several discussions and meetings have taken place to consider the possible threat of neonicotinoids to honey bees, resulting in restriction on the use of three neonicotinoids by the European Union (EU) for two years, from 1 December 2013. Earlier that year (January 2013) a conference1
was held in London to focus
on the scientific evidence of the risks of pesticides, in particular the neonicotinoids, to both honey bees and bumblebees. Evidence was presented by research laboratories from academia, government and industry to discuss the current scientific evidence, knowledge gaps and future directions. From this conference, especially the final day of open discussion, it was clear that there is a wide variety of opinions regarding the impact of neonicotinoids. In this article the background to these discussions will be given as well as what impact these pesticides may have on bees. The honey bee will be the main focus in this article, although several studies have included bumblebees and other solitary bees.
Neonicotinoids: What Are They? Neonicotinoid pesticides were first
registered for use in the mid-1990s. Figure 1 shows the structure of the five neonicotinoids commonly used not only on agricultural crops such as maize, sunflower and oilseed rape, but also on garden plants via plant protection products on the shelves of garden centres. These neonicotinoids are classified as systemic insecticides. This means that they are not primarily used as a foliar spray, but the seeds are treated with these insecticides and, therefore, they are
May 2015 Vol 97 No 5 Figure 1. Structures of five commonly used neonicotinoids
absorbed and transported throughout the treated plants. In this manner the plant is protected from sap-sucking insects and those that chew on the plant, but other insects flying around the plants are not affected, unlike when insecticides are sprayed on crops.
The neonicotinoids have been described as a milestone in pesticide research due to their relatively low risk for non-target organisms and the environment, and high target specificity (relatively non-toxic to mammals and birds). Due to this high target specificity, neonicotinoids are seen as an important class of treatments to be maintained for integrated pest management (IPM) strategies. Others see this approach as a negation of the principles of IPM because, being systemic, these pesticides are used before their need is demonstrated. These insecticides have begun to replace pyrethroids, chlorinated hydrocarbons, organophosphates, carbamates and several other classes of insecticide products.
Exposure to Honey Bees
Because neonicotinoids are absorbed into the plant, they can potentially be present in pollen and nectar, creating possible toxicity
for pollinators. Several additional routes for exposure have been documented including soil, seed coatings during planting (seed drilling) and direct contact with foliar sprays or foliar residues on plant surfaces.
Bee Problems
Of the commonly used neonicotinoids, only thiacloprid is mildly toxic. Imidacloprid, thiamethoxam and clothianidin are all highly toxic to bees with contact LD50
to kill 50% of a test population) ranging from 0.0179 µg/bee to 0.024 µg/bee. Other studies have used a slightly different parameter (LC50
to kill 50% of a test population). LC50
(dose needed
, the concentration needed for
imidacloprid is 185 ppb (parts per billion). These levels have an acute toxic effect and are known as lethal levels, but research has shown (see below) that it is highly unlikely that bees will encounter this level from residues in pollen and nectar. Unless neonicotinoids are used as a foliar spray, or excessive dust is caused by seed drilling, bees are unlikely to encounter these levels in an agricultural setting. In contrast (to acute lethal exposure), bees are much more likely to be exposed to sub-lethal doses, which are caused by bees consuming pollen
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