neonicotinoid

neonicotinoid, any of a class of synthetic systemic insecticides that are chemically related to nicotine. Neonicotinoids are used especially in agriculture to control destructive pests such as aphids and mites. Since their introduction in the late 1980s, the chemicals have become the most widely used pesticides in the world. The prophylactic use of neonicotinoids has had a negative impact on a broad range of nontarget organisms, including pollinators and soil and aquatic invertebrates, raising environmental concerns. Consequently, their use has been heavily restricted in some countries.

Mechanism of action and uses

Neonicotinoids are water-soluble, which allows them to be absorbed easily by roots or leaves. Unlike contact pesticides, which remain on the surface of the treated leaves or other plant parts and can be washed off, systemic pesticides like neonicotinoids are taken up by a plant and transported throughout its body. The chemicals can thus be found in every part of the plant, including the leaves, roots, stems, fruits, flowers, and even the pollen and nectar. Systemic protection is longer-lasting than contact sprays and is particularly useful against root-feeding insects and boring insects that typically evade foliar pesticides. The chemicals are effective at low concentrations and can kill insects directly on contact or through the ingestion of treated plant tissue. Neonicotinoids selectively bind to the postsynaptic nicotinic receptors of insects, where they affect the central nervous system to produce paralysis and death. The chemicals do not bind strongly to the equivalent receptors in vertebrates, so they are selectively more toxic to insects.

The major neonicotinoids in commercial pesticides are acetamiprid, clothianidin, dinotefuran, imidacloprid, thiacloprid, and thiamethoxam. These chemicals are most commonly used as seed dressings for a broad variety of crops, including sunflowers, cereal grains, beets, potatoes, and rapeseed. Because crops grown from treated seeds are protected from pests for several months following sowing, such seed dressings serve as an easy and efficient prophylactic insecticide for annual crops. The chemicals can also be applied as foliar sprays for certain horticultural and arable crops, and they are commonly used in home gardens on vegetables and ornamental flowering plants. In addition, they are used in bait formulations against household cockroaches and ants and also as granular formulations for the control of soil-dwelling insect pests in pastures and other grasslands. Treated irrigation water and soil drenches are used on certain perennial crops, and neonicotinoids can be injected into live trees or timber products. Moreover, they are used topically on dogs and cats for the treatment of external parasites.

Environmental concerns

Neonicotinoids have been shown to have lethal or sublethal effects on a wide range of beneficial and nontarget insects. The systemic nature of the chemicals means that, even though a neonicotinoid may have been applied to a plant before it was blooming, its nectar and pollen are likely to contain the insecticide, which poses a hazard to foraging pollinators. This holds true for many agricultural crops that rely on insect pollination as well as for many nursery plants, which may be retailed to well-meaning customers for use in pollinator gardens. As with any insecticide, neonicotinoids can contaminate plants unintentionally, whether as stray airborne droplets from the application of foliar sprays or as stray airborne dust from the sowing of treated seeds. Depending on the amount of neonicotinoid absorbed, wildflowers and other food sources can become deadly or harmful for bees, butterflies, and other insects. Such chemicals have been implicated in the decline of honeybee populations and are suspected of causing or contributing to colony collapse disorder, in which a hive suddenly dies off. Although no causal link between neonicotinoids and population decline has been established, the chemicals have been associated with atypical honeybee behaviour, such as a decreased ability to locate food sources, and with weakened immune function, which may allow a preexisting pathogen infection to worsen.

In addition to poisoning floral resources, neonicotinoids can contaminate the leaves of larval host plants for moths and butterflies at lethal or sublethal levels. For example, monarch caterpillars feeding on neonicotinoid-exposed milkweeds have reduced survival rates and lower reproductive success. The neonicotinoids imidacloprid and clothianidin have been specifically correlated with the diminishment of endangered migratory monarch populations in North America. Beneficial predatory insects, such as ladybugs, lacewings, mantids, and parasitic wasps, can become lethally or sublethally contaminated by neonicotinoids when they consume aphids or other prey that have been feeding on treated plants or when they emerge from their eggs on such plants. Beyond their importance as pollinators and agents of biological control, these and other insects are vital food for a myriad of other animals, so their decline has ramifications across the food web.

Highly persistent in the environment, neonicotinoids have been found in soil, sediments, ground and surface water, and wetland vegetation. Given that they are water-soluble, neonicotinoids that are not absorbed by target plants move easily through the environment with rainfall and accumulate in soils and aquatic systems, posing a serious threat to invertebrate animals. Soil-dwelling invertebrates, including ants, beetles, and earthworms, can be affected by neonicotinoid pollution and may suffer long-term exposure to these toxic chemicals. Aquatic invertebrates, which serve as decomposers, parasites, predators, and an essential food source for other organisms in aquatic ecosystems, are particularly susceptible to neonicotinoids, as they cannot usually move to uncontaminated environments. Freshwater environments are commonly contaminated, but lethal and sublethal effects have also been documented in a number of marine invertebrates, including crabs, shrimps, and other crustaceans as well as certain mussels, oysters, and gastropods.

Although the chemicals have been touted as being safe for vertebrate animals, neonicotinoids have been correlated with declines in bird populations. In areas with high insecticide use, the number of food resources available to insectivorous birds decreases—an effect obviously not limited to neonicotinoids. However, annual declines in at least 14 European bird species have been noted specifically in areas where the surface water contains high levels of the neonicotinoid imidacloprid. In North America migratory songbirds that consume even a tiny amount of neonicotioid-laced seeds have been shown to become lethargic and lose weight. The birds can recover from this exposure, though these sublethal impacts likely affect survival and subsequent reproduction, especially since the sowing of neonicotinoid-treated seeds often coincides with spring migrations.

In the early 21st century the outdoor use of neonicotinoids, particularly imidacloprid, clothianidin, and thiamethoxam, was highly restricted in some countries, including all those in the European Union. In the United States a number of states have restricted neonicotinoids for general use, and certain major retailers have voluntarily limited their use of the chemicals on nursery plants.