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= Neonicotinoid Resistance = Neonicotinoid resistance refers to the increased tolerance of neonicotinoid insecticides in pests. As pests become less susceptible to neonicotinoids, mortality rates drop. Populations of insects may be partially or completely resistant depending on location, type of pest, and which neonicotinoids is used.

Neonicotinoids were first released for commercial use in 1991 in the form of ‘imidacloprid’, one of the seven major neonicotinoids currently in use. By 1996 the first report of resistance to imidacloprid was released. Developing resistance to neonicotinoids is generally attributed to increased usage. By exposing pests to large amounts of neonicotinoids we apply selective pressure, meaning only those organisms predisposed to neonicotinoid resistance live to breed and produce offspring.

Since the first noted case of neonicotinoid resistance in 1996 hundreds of cases have been reported, with more cases reported every year. Some pests seem to be more prone to neonicotinoid resistance than others. Over 200 cases of resistance to neonicotinoids have been reported in Tobacco Whitefly populations, with Green Peach Aphids having the second highest number of resistance reports.

Mechanisms of Neonicotinoid Resistance
Mechanisms of neonicotinoid resistance generally fall into two categories; changes in gene expression that allow for higher levels of detoxification, or mutations of the nicotinic acetylcholine receptor (nAChR) that neonicotinoids target.

Gene Overexpression
Overexpression of genes coding for cytochrome P450s has been shown to correlate with neonicotinoid resistance in many insect populations. Cytochrome P450s are a family of haem-containing monooxygenases found in both vertebrates and invertebrates. The enzymes are used in a variety of metabolic pathways and use their haem group to oxidize substrates. In insects these enzymes can oxidize neonicotinoids, allowing for them to be metabolized into less toxic metabolites. Some pest species that are known to feed on nicotine-producing plants naturally show increased expression of P450s. Silverleaf Whiteflies (Bemisia tabaci) have shown high levels of resistance to neonicotinoid insecticides with resistant individuals showing overexpression of CYP6CM1, a gene coding for cytochrome P4505.

Point Mutations
Neonicotinoids act by targeting the invertebrate nicotinic acetylcholine receptor (nAChR). When neonicotinoids bind to the receptor the nAChR channel opens, causing neurons to fire continuously. The end result of the binding is paralysis and death. Point mutations leading to amino acid substitutions in the receptor protein can lower the affinity for neonicotinoids, leading to decreased susceptibility. Some green peach aphid (Myzus persicae) populations in the South of France and Northern Spain have developed a point mutation on the binding site of the receptor that makes neonicotinoids much less effective. The mutation leads to an amino acid substitution that weakens the interactions between the neonicotinoid and the receptor, making it harder for the neonicotinoid to bind.

Consequences of Neonicotinoid Resistance
As cases of neonicotinoid resistance grow, the amount of neonicotinoids used increases. Neonicotinoid use itself increases levels of resistance, which leads to a greater amount of neonicotinoids being used in an attempt to keep pest populations under control. This higher use of neonicotinoids in turn leads to higher levels of resistance. This cycle leads to an excess of neonicotinoid use which has wide reaching consequences.

Economic Consequences
The primary use of neonicotinoids world-wide is in agriculture. Farmers use neonicotinoids to prevent feeding and damage by pests. As pests become resistant to neonicotinoids farmers are unable to prevent damage to crops leading to lower crop yields and potential crop failure. This presents risk not only to the food supply, but to the economy of regions that rely heavily on agriculture.

Environmental Consequences
One of the critical environmental consequences of neonicotinoid resistance is a growing risk to pollinators. During the seed-drilling process neonicotinoid-treated seeds let off dust containing high levels of neonicotinoids. This dust carries a high risk for pollinators such as honey bees which may be exposed. Neonicotinoids may also  accumulate in the soil and in groundwater, further exposing pollinators and other non-target organisms.

Public Health Consequences
Although the primary use of neonicotinoids is in agriculture, neonicotinoids have recently begun to be used to control pests in a public health setting. Most notably in malarial vector control and bed bug extermination.

Malarial Vector Control
Resistance to pyrethroids — the most commonly used pesticide against mosquitos — has led to treatments that use a combination of pyrethroids and neonicotinoids. Although this may work temporarily, there is growing evidence of neonicotinoid resistance developing in mosquitos. Researchers in Côte d’ivoire found that mosquitos bred in agricultural areas where neonicotinoids were in use showed resistance and had much lower mortality levels when compared to mosquitos bred in urban areas where neonicotinoid use was less common.

Bed Bug Infestation control
Bed bugs, which feed mainly off human blood, are not known to spread infectious disease but cause an itch that can lead to scratching and secondary skin infections. Neonicotinoids are often used in combination with pyrethroids to counter bed bug infestations. Although the use of neonicotinoids against bed bugs is a relatively new development, some populations have already begun to show resistance. This resistance is likely due to the overexpression of genes coding for cytochrome p450s.

Cross Resistance
Cross-resistance refers to resistance to one class of pesticide that conveys resistance to another. Generally this occurs when the mechanism of action of the two pesticides are similar. A pest that displays resistance to neonicotinoids may also be resistant to other classes of pesticides that use similar binding target sites, or are detoxified in a similar way.

Many pests that are resistant to neonicotinoids owe their resistance to overexpression of genes coding for cytochrome P450, which allows for enhanced detoxification. High levels of P450 expression may also lead to corresponding resistance to other classes of insecticides. Researchers found in 2010 that Silverleaf Whiteflies who over-expressed genes encoding for P450 showed resistance to multiple neonicotinoids, and were also likely to be resistant to pymetrozine, an insecticide used against aphids and whiteflies.

Bed bugs have also been known to show cross-resistance. Bed bugs originating from Jersey City that had been bred in a lab for multiple generations showed resistance to neonicotinoids despite never having been exposed to them. It is likely that this is due to cross-resistance originating from the Jersey City line’s previous exposure to an alternative class of pesticide.

Management
The main suggestion for the management of neonicotinoid resistance, is decreasing the use of neonicotinoids. Currently, neonicotinoids are often used prophylactically to protect seeds and seedlings by treating the seeds with neonicotinoids before they’re planted. Seed-treating with neonicotinoids is so common that seed providers will often treat seeds with neonicotinoids before distributing seeds to farmers, meaning that in regions such as the United States there’s a low availability of untreated seeds for some crops such as soybeans and maize. The evidence for neonicotinoids as a prophylactic treatment against pests is varied, but research suggests often the seed treatments do not lead to an increase in crop yield.

Additionally, in areas with low literacy neonicotinoids are often used at much higher concentrations than necessary which can lead to an increase in resistance.

By decreasing prophylactic use of neonicotinoids and limiting neonicotinoid use to an as-needed basis the development of resistance to neonicotinoids can be slowed.