Neonicotinoids: Nerve Agents in Our Food Chain

It is now widely accepted that bee populations can be threatened by compounds called neonicotinoids, which are the most commonly used insecticides in the world. Neonicotinoids can also enter the human food chain as the chemicals persist in the environment for several years after use. Because of this, methods that can detect even trace levels of these compounds in our foodstuffs are necessary to help ensure a safe food supply. These methods are also critical for facilitating compliance and monitoring in countries where the use of neonicotinoid pesticides is strictly controlled or banned.

What are neonicotinoids and how do they work?

Neonicotinoids are biologically active substances that are chemically similar to nicotine and are used in agriculture for pest control to protect crop plants. They are particularly useful because their toxicity is much higher against insects than against other species like humans or other mammals. That is because nicotinoids are better able to bind to receptors in the insect nervous system than in mammals (including humans).

Neonicotinoids work by binding onto the nicotinic acetylcholine receptor, the same receptor in the nervous system that nicotine binds to. This receptor naturally binds to acetylcholine to stimulate the nervous system. The binding of acetylcholine and nicotine is temporary, but the binding of neonicotinoids is permanent and irreversible. This means that the receptor becomes permanently blocked and unable to bind to its natural partner, acetylcholine. Moreover, the constant binding results in nonstop activation of the receptor. The permanent blockage and overstimulation of the nicotinic receptors result in impaired movement, paralysis and eventually, death.

Neonicotinoids are applied either by coating seeds, by applying as a “drench” to the roots of plants, or by spraying onto foliage. The water-soluble pesticide is taken up systemically by the plants, thereby protecting them from insect pests, especially those that chew on or suck the sap of plants. This protection usually lasts throughout a plant’s lifetime, as neonicotinoids are relatively persistent chemicals. And unlike contact pesticides, which are applied to the surface of plants, neonicotinoids cannot be washed off.

Effects on bees and food chains

The water solubility and persistence of neonicotinoids can become an issue when it contaminates the environment, by leaching into subsurface water and groundwater, or by entering the air as dust from coated seeds or drift from sprays, for example. Such contamination can accumulate, and neonicotinoids can persist for years, thereby exposing non-target organisms such as bees. Chronic exposure to sub-lethal doses of neonicotinoids has been reported to impair bee health, disturb important bee behaviors and potentially contribute to colony collapse disorder—a serious threat to bee populations worldwide and to our own food security.

The reduction of bee numbers, and of other non-target insect populations, are having knock-on effects on aquatic and bird species that feed on insects. The diversity of water insects and bird species have been reported to be significantly lower around agricultural sites than around land used for other purposes. The impact of neonicotinoids is also being reported in the human food chain, with neonicotinoids being detected in many foods for human consumption, such as honey. While the toxicity of neonicotinoids is relatively low for humans, with most people scarcely being exposed, neonicotinoids are not a healthy substance.

Detecting neonicotinoids in food

It is now routine that food is tested for neonicotinoids and their metabolites, using highly sensitive, selective and precise methods such as liquid chromatography tandem mass spectrometry (LC-MS/MS). An LC-MS/MS method for the high-throughput detection and quantification of multiple neonicotinoid analytes at the same time has been developed, which takes advantage of data-independent SWATH acquisition and sample preparation using QuEChERS (quick, easy, cheap, effective, rugged, and safe). SWATH acquisition allows for the continuous acquisition of product ion spectra for all ions across the entire mass range, providing high resolution data for screening, qualifying and quantifying, not only neonicotinoids, but also other pesticides and their metabolites (see Figure 2).

Figure 2: Example data from the method analyzing 190 pesticides via SWATH acquisition. With SWATH acquisition, a digital data archive is made of each sample, such that it can be retrospectively investigated for additional contaminants in the future. Credit: SCIEX

This method has been demonstrated to detect 190 types of pesticide residue in various vegetables and fruit, and an updated version has been included in the Single Residue Methods of the EU Reference Laboratories for Residues of Pesticides for the analysis of various pesticide metabolites and some acidic pesticides in products of animal origin (such as muscle, liver, kidney and milk). Similar LC-MS/MS methods have also been employed to screen human urine samples to monitor human exposure and to measure excretion of ingested neonicotinoids.

As the risks to humans of chronic exposure to specific neonicotinoids, and the possible effects of exposure to combinations of neonicotinoids, are as yet largely unknown, it would be prudent to continue monitoring neonicotinoid levels in food as well as in our environment—and even in people who might be exposed to higher levels than average due to their occupation. 

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