While public attention has recently focused on the threat to honey bees and bumble bees from neonicotinoid (neonic) pesticides, there is growing evidence that another species may be a risk from these pervasive chemicals—humans. Many scientists now say that exposure to neonics may pose a risk to human health. Laboratory tests with cell cultures and rodents led the European Food Safety Authority (EFSA) to categorize two neonics—imidacloprid and acetamiprid—as possibly impairing the developing human nervous system.
Neonicotinoids—first introduced in the mid-1990s—are the fastest growing and most heavily used class of insecticides in the United States. See USGS Pesticide Use Maps for the most current information, but note USGS's disclaimer that beginning in 2015 the data reports no longer include seed treatment uses of pesticides; for the neonics this represents somewhere around 90% of total pounds of neonic pesticides used in agriculture—a very serious under-reporting. See below for imidacloprid (note the drop-off since 2015 due to failure to include seed treatments)
Due to their systemic nature, neonicotinoids are taken up by the roots or leaves and trans-located to all parts of the plant, making the treated plant toxic to insects, as well as causing lethal and sublethal adverse impacts on other invertebrates and some vertebrates. (for ecosystem and biodiversity impacts see van der Sluijs et al 2014; Pisa et al 2014).
There is disturbing evidence that these neonics are making their way into our food and water supply. A study by the U.S. Geological Survey found that neonics are widespread contaminants of surface and groundwater that could be a source of drinking water. In nine rivers monitored in the Midwest, where neonics are most heavily used, the study found clothianidin in about three-quarters of monitored sites, thiamethoxam in about one-half, and imidacloprid in about one-quarter. Limited testing from the U.S. Department of Agriculture has found neonics in fruits and vegetables, where the pesticide’s systemic nature means it cannot be washed off the surface of these foods.
The presence of neonics in food and water raises concerns about their potential health effects on people. Studies in animals are not reassuring. Animal studies report neurobehavioral impairments in rodents that were exposed to imidacloprid prenatally, from a single high-dose injection of the pesticide to the pregnant rat (337 mg/kg at day 9 of pregnancy) (Abou-Donia et al 2008). In fact, a study by the German chemical giant Bayer, a major neonic maker, reported similar findings in rats born to mothers that received daily doses of imidacloprid in their food throughout pregnancy and lactation (55-58 mg/kg/day). EPA reviewers concluded that the treatment produced persistent changes in brain structures and poor performance on some behavioral tests.
Even more alarming, a study by NIH-funded researchers from UNC Chapel Hill and UC Davis reported that frequent exposure (self-reported by parents) to imidacloprid applied as flea and tick treatments for pets (Advantage by Bayer) during pregnancy was associated with Autism spectrum disorder (OR 2.0, 95% CI 1.0-3.9) in prenatally-exposed children (Keil et al 2014). While the study is limited by the potential for exposure misclassification and by small numbers of cases that were exposed, it is consistent with effects reported for other related pesticides, and highlights the need for more study (see Roberts et al 2007and Eskenazi et al 2007 for health impacts from organophosphate pesticides, which share a similar mechanism of toxicity to the neonics).
The potential for neonic pesticides to interfere with brain structure and nerve function is not surprising. The pesticides are designed to target the insect nervous system—this is how they are effective insecticides. The neonics bind to a receptor on nerve cells which normally binds to acetylcholine (the nicotinic acetylcholine receptor, nAChR). However, unlike acetylcholine, which is rapidly degraded by another protein after it has performed its intended function, the neonics are degraded very slowly, causing havoc with the affected nerve cell and ultimately killing a target insect within hours or days. In people, the acetylcholine receptor has many functions, including proper formation of the correct architecture of the brain during fetal growth, muscle contraction, learning, and the ability to pay attention.
The emerging science suggesting that neonic pesticides pose a health risk to people, coupled with its contamination of waterways and food, raises an alarm beyond the already significant risk to bees. While more study is needed on the potential human health risks, the widespread use of these agrochemicals is certainly imprudent and potentially disastrous.