(Beyond Pesticides, August 3, 2010) A new study shows that due to a flaw in standard risk assessments, which consider toxic effects at fixed exposure times, the risks posed by the neonicotinoid pesticides imidacloprid and thiacloprid are likely to be underestimated. The authors believe that minute quantities of imidicloprid may be playing a much larger role in killing bees over extended periods of time than previously thought. The study, â€śThe significance of the Druckreyâ€“KĂĽpfmĂĽller equation for risk assessmentâ€”The toxicity of neonicotinoid insecticides to arthropods is reinforced by exposure time,â€ť was published online July 23, 2010 in the journal Toxicology.
The authors believe that standard risk assessment calculations underestimate toxicity because they do not accurately account for the interplay of time and level of exposure. According the study:
The essence of the Druckreyâ€“KĂĽpfmĂĽller equation states that the total dose required to produce the same effect decreases with decreasing exposure levels, even though the exposure times required to produce the same effect increase with decreasing exposure levels. Druckrey and KĂĽpfmĂĽller inferred that if both receptor binding and the effect are irreversible, exposure time would reinforce the effect. The Druckreyâ€“KĂĽpfmĂĽller equation explains why toxicity may occur after prolonged exposure to very low toxicant levels. . .Traditional approaches that consider toxic effects at fixed exposure times are unable to allow extrapolation from measured endpoints to effects that may occur at other times of exposure.
The Druckreyâ€“KĂĽpfmĂĽller equation was developed in the 1960â€™s to estimate cancer risks, but the study authors say the principle is relevant to environmental toxicology as well. They say that similar doseâ€“response characteristics have recently been established for the toxicity of the neonicotinoid insecticides imidacloprid and thiacloprid to arthropods that confirm the theorem of Druckrey and KĂĽpfmĂĽller.
â€śThe risks of the neonicotinoid insecticides imidacloprid and thiacloprid to arthropods in water and soil may be seriously underestimated,â€ť said Henk Tennekes, PhD, lead author of the study. â€śThe acceptable limits are based mainly on short-term tests. If long-term studies were to be carried out, far lower concentrations may turn out to be hazardous. This explains why minute quantities of imidacloprid may induce bee decline in the long run.â€ť Because of their high persistence, significant quantities of neonicotinoids may remain in the soil for several years. Consequently, untreated plants growing on soil previously exposed to imidacloprid may take up the substance via their roots and become hazardous for bees.
Neonicotinoids are a class of chemicals that target nerve cells in a similar way as nicotine, acting as neurotoxins to insects. One of the most commonly used neonicotinoid is the insecticide imidacloprid, manufactured by Bayer Crop Science and used in agriculture to control aphids, beetles, and other sucking insects. The use of imidacloprid was banned in France after it was suspected to be responsible for the decline of honeybee populations in the late 1990s.
Imidacloprid has been linked to neural effects in honeybees, including disruptions in mobility, navigation, and feeding behavior – similar behaviors that are being displayed by bees suffering from Colony Collapse Disorder (CCD). In CCD, bees are flying off in search of pollen and nectar and simply never returning to their colonies. Researchers say the bees are presumably dying in the fields, perhaps becoming exhausted or simply disoriented and eventually falling victim to the cold.
Research is ongoing as to the cause of the CCD phenomenon, but pesticides, especially neonictinoids, such as imidacloprid, have been implicated. CCD can be especially devastating since honeybees are essential pollinators of crops that constitute over one third of the U.S. food supply or $15 billion worth of food. For more information on pollinators and CCD, read our factsheet: Pollinators and Pesticides: Escalating crisis demands action.
Beyond Pesticides believes that pesticides are likely to be a part of the CCD equation and a precautionary approach must be taken. Solutions to the loss of bees and human productivity are clearly within our reach if we engage our communities and governmental bodies. We know how to live in harmony with the ecosystem through the adoption of sustainable practices that simply do not allow toxic pesticide use. Because our survival depends on healthy pollinators, we must do everything in our power to solve this problem.
Read other Daily News Blog postings on pollinators and pesticides.