As Rachel Carson noted in her seminal book Silent Spring, a quiet landscape can speak volumes. Lately the buzz of bees going about their invaluable work is getting softer and softer…and in some places, it is just about inaudible.
This winter, U.S. commercial beekeepers reported devastating losses of 30 percent to as much as 50 percent of their hives. These losses are on top of annual losses in the 20 percent to 30 percent range since 2007, far exceeding the historical rate, which is approximately 10 to 15 percent. To see bee declines through the eye of a beekeeper, read this story about Steve Ellis in Elbow Lake, Minnesota.
The loss of bees and other pollinators should concern all of us. A third of food crops—among them fruits, vegetables, and nuts— depend on animal pollination. Since 2006, an estimated 10 million bee hives at an approximate value of $200 each have been lost.
Causes of honey bee decline
There are a number of potential causes of bee decline, including loss of flower-rich habitat, infestations with Varroa mites, applications of fungicides and insecticides, use of honey substitutes as bee food, and synergies among pathogens and chemical pesticides.
Much attention has focused on the neonicotinoid, or neonic, neurotoxins now among the world’s most popular insecticides. Startling graphics prepared by the U.S. Geological Survey show how rapidly the neonics imidacloprid and clothianidin have been adopted and how extensively they are now deployed in the United States. In agriculture, neonics are often delivered as seed coatings. The pesticide enters the seed as it germinates and eventually infuses the entire plant, including pollen and nectar.
Europeans have recently announced a two-year precautionary ban on the use of three neonics, clothianidin, imidacloprid and thiametoxam. Despite the evident distress of U.S. bee industry, the U.S. government has yet to take any action.
While it is unlikely that the neonics are solely responsible, it is reasonable to suspect that these widely used, highly toxic insecticides would play a significant role in bee decline and that the government should seriously consider imposing restrictions on their use.
And are these chemicals ever toxic to insects! A study by Christian Krupke and colleagues at Purdue University estimate that the amount of clothianidin coating just one kernel of corn is enough to kill 80,000 honey bees! That same study demonstrated that honey bees can be exposed to neonics by multiple routes, including the clouds of waste talc thrown up by corn seed planters containing very high pesticide concentrations. (The talc is used to keep the treated seeds separate while planting.)
Low levels of neonic exposure
But a central question confronting scientists investigating the causes of bee decline is the impact of the low concentrations of neonics now widespread in the environment that honey bees are likely to encounter.
A new review paper by Henk A. Tennekes and Francisco Sanchez-Bayo in the journal Toxicology (Volume 309, July 5, 2013, pages 39–51; login required) suggests that very low concentrations of neonics can have devastating effects on bees and—here’s the most important part—that conventional risk assessment approaches can miss or underestimate those effects.
According to the paper, neonics are in a group of chemicals, called time-dependent chemicals, whose toxic effects build up during long exposure times. The paper suggests that time-dependent phenomena occur when an insecticide binds very tightly or irreversibly to critical receptors in the target organism. Given a long enough exposure, even very low levels of time-dependent chemicals can kill.
Standard toxicity tests, which focus on the concentration of toxins for relatively short time periods, do not pick up time-dependent effects because they fail to expose target organisms to very low concentrations of a toxin over long enough periods of time.
Tennekes and Sanchez-Bayo use imidacloprid as a test case to demonstrate how standard risk assessment protocols can miss the harmful effects low levels of the chemicals can have on honey bees.
The paper assessed the impact of imidacloprid on honey bees by determining the time it took for 50 percent of the bees to die (t50) when exposed for varying time intervals to low doses of the chemical. It then related the exposure data to the pesticide concentrations typically found as plant residues under field conditions and calculated that 50 percent of worker bees would die within seven to ten days if they fed on a such a field. By contrast, the authors assert that standard risk assessments suggest field concentrations of imidacloprid pose no risks at all to honey bees.
Tennekes and Sanchez-Bayo propose a new risk assessment protocol based on t50s to evaluate the effects of time-dependent chemicals and recommend that going forward regulatory agencies employ such protocols to assess the harmful effects of neonics.
Regulators should consider these recommendations. Pollinators are too important to agriculture and other ecosystems, and neonics too widely used, for regulators to be ignorant of the threats low levels of these pesticides pose.
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