Neonicotinoids
Neonicotinoids (neonics) have been the number one insecticide group since the mid 1990s. They have been widely used throughout the United States and they quickly spread to Europe. Neonicotinoids are a neurotoxicant that alter the acetylcholine receptors (integral to nerve impulses) of common pests such as aphids, thrips, and flies. They are particularly effective against biting, sucking, chewing, and burrowing insects; hence their broad spectrum applicability in agriculture, aquaculture, and as a vector control for livestock. Patented in 1991, they have been widely accepted by U.S. farmers and have been used by farmers around the world prior to the EU ban. Over the last 25 years, there has been a concurrent phenomenon between neonicotinoid use and pollinator decline. Subsequently, there have been studies trying to link the two issues.
Most research has been conducted in the U.S. and EU, to explore the possibility of a correlation. In 2018, the EU banned the use of the three most common neonics: Imidacloprid (IMI), Thiamethoxam (THX), and Clothianidin (CLO). They believed that even though there was no concrete evidence of comorbidity, there was enough to make a decision to ban their use.
There are three main pollinators used in commercial and small scale agriculture: Apis Mellifera. Bombus Terrestris, and the Solitary Bee. One species, (Apis) is solely responsible for the pollination of £651 million in the UK and €180 billion of agricultural crops around the world. During peak foraging season, worker bees will average 10 flights per day, meaning that their exposure to neonicotinoids may increase exponentially. The U.S. sowing season for maize and soybeans, overlaps with the developmental stage of Apis (February through May) and this co-occurrence may lead to heightened levels of exposure, resulting not only in worker bee death, but diminished queen fecundity, brood strength, and may lead to early swarming (Krupke et al., 2017).
In commercial agriculture, neonics are used in two ways: as a foliar (applied to the leaves) spray and in seed coating. Seed coating is believed to be a preventative measure against pesticide drift (a common occurrence when spraying fields) and is predominantly used on seeds for maize and soybean crops. However the seed coating does not take into account the vascular differences between plants. Each plant may have a different rate of uptake and therefore have a different level of neonic toxicity. In addition, studies have shown that the dust from sowing maize seeds have resulted in a direct increase of neonics in the surrounding fields.
The USDA allowed the wide-spread use of neonics because it was believed that they could not pass the mammalian blood brain barrier. However, studies have shown that IMI has been able to pass this barrier in mice causing in vivo and in vitro effects. Specifically, in the Thalamus where there is a high density of acetylcholine receptors, this disruption is known to have neurological effects (e.g. dementia and depression) as well as effects on reproduction (e.g. limited motility of sperm and a reduction in viable pregnancies). Additionally, the metabolites of IMI, Thiamethoxam (THX), and Acetamiprid (DMAP) have adverse effects on humans in vitro leading to congenital heart defects, neural tube defects, and a possibility of Autism Spectrum Disorder (Cimino et al. 2017).
The rate of neonicotinoid use is pervasive, 90% of maize seeds and 44-50% of soybean seeds used in commercial agriculture are treated with IMI and Clothianidin (CLO). The United States Environmental Protection Agency (EPA) has regulated limits of neonics fit for human consumption between 50-200 ppb in honey and other apiculture products. However studies conducted in Europe, the UK, and the U.S. tested honey, fruit, and vegetables on the market and found that 90% of honey contained at least one neonic, 72% of fruit and 42% of vegetables tested positive for at least two neonics, and IMI had the most consistent detection rate of 70%, at levels just below 50 ppb (Chen et al., 2014). The neonicotinoid Thiacloprid, was found in apple juice and applesauce during the years 2004 to 2010 at rates of 4.7% to 12.6% (well above the recommended dosage set by the EPA) (Chen et al., 2014). These rates are important because as neonic usage increases, there is a possibility of contamination exceeding 200 ppb in the coming years. If the UK is relying on 3rd country parties for trade, setting residue limits will be paramount for agricultural imports.
Additionally, neonics are highly persistent and have been found in groundwater, soil, non-target plants and vertebrates, and market products. In a 2017 study, 121 agrochemicals (and their metabolites) were found in pollen products from the U.S. and UK. Since 1996 there have been 330 papers that suggest a rise in resistance amongst pests such as aphids and thrips (the most economically destructive), and a lack of verifiable yield increase of maize crops planted in Indiana, U.S.
Overall, it has been difficult to prove whether neonics are lethal, but they are known to cause sublethal effects such as behavior change, diminished homing capabilities, loss of appetite, and immune suppression; all of which leave colonies vulnerable to infection and disease (Kammoun et a/., 2019). In lab controlled tests, Bombus exposed to IMI at a dose rate of 6ng/g (slightly higher than would be found via field exposure), grew more slowly, had a diminished appetite, and produced 85% fewer queens Lentola et al., 2017). This rate of CLO is similar to the rates of residual CLO found in flowering plants and soil in the UK, suggesting this may be a contributing factor to the overall decline of Apis health in the effects of neonicotinoids (Woodcock et al., UK (Woodcock et al., 2019).