Imidacloprid is a widely used insecticide with relatively low human toxicity. It has raised concerns because of its possible impact on bee populations, ability to cause eggshell thinning in birds, and reduced egg production and hatching success. By David Buffin.

Imidacloprid is a relatively new insecticide, having first been registered for use in the UK in 19931 and in the United States in 19942. It is a systemic insecticide, chemically related to the tobacco toxin, nicotine. It works by blocking the elements of the insect nervous system which are more susceptible to the toxic effects of imidacloprid than those of warm blooded animals3.
    Imidacloprid is manufactured by Bayer CropScience. Since its launch in 1991, products containing imidacloprid have gained registrations in about 120 countries and are marketed for use on over 140 agricultural crops. With annual sales of more than 600 million Euro (2001), imidacloprid is one of the top selling products of Bayer CropScience4. It is marketed under a variety of names including Gaucho, Admire, Confidor and Winner.
    Imidacloprid has a wide range of uses – soil, seed and foliar. It is used to control sucking insects such as rice-, leaf- and plant hoppers, aphids, thrips and whitefly. It is also effective against soil insects, termites and some species of biting insects, such as rice water weevil and Colorado beetle but has no effect on nematodes or spider mites. It can be used as seed dressing, as soil treatment and as foliar treatment in different crops including rice, cotton, cereals, maize, sugar beet, potatoes, vegetables, citrus fruit, apples and pears, and stone fruit5. In European countries such as France, UK, and Holland, imidacloprid is widely used as an insecticide in sugar beet crops6.
    Imidacloprid can be phytotoxic (toxic to plants) if not used according to manufacturers instructions7, and it has a tendency to reduce seedling emergence and crop vigour8.

Inert ingredients
All pesticides contain ingredients other than the active ingredient called ‘inert’ or ‘other’ ingredients, which assist in the physical delivery of the commercial product. The inert ingredient, crystalline quartz silica, found in the imidacloprid product Merit 0.5 G, is ‘carcinogenic to humans’ according to the International Agency for Cancer, and is ‘known to be a human carcinogen’ by the US National Toxicology Program (NTP). Another inert, naphthalene, found in the product Leverage 2.7, has been classified by NTP as having ‘clear evidence of carcinogenic activity’9.

Acute toxicity
The imidacloprid active ingredient is considered by the World Health Organisation to be moderately toxic. In laboratory animals, symptoms of acute (short term) oral exposure to imidacloprid included apathy and laboured breathing which lasted for five days. The LD50 for imidacloprid (an oral dose that results in mortality to half of the test animals) is 450 mg/kg body weight in rats and 131 mg/kg in mice. The 24-hour dermal LD50 in rats is >5,000mg/kg. It is considered non-irritating to eyes and skin from tests on rabbits10.
    Symptoms following acute exposure to the agricultural imidacloprid formulation (imidacloprid and inert product) included reduced activity, lack of coordination, tremors, diarrhoea and weight loss. Some symptoms lasted up to 12 days after exposure, twice as long as the symptoms of exposure to the active ingredient imidacloprid alone11.

Chronic toxicity
Chronic feeding studies with rats showed that the thyroid is especially sensitive to imidacloprid. Thyroid lesions were caused by doses of 17 mg/kg of body weight per day in males. Slightly higher doses of 25 mg/kg per day reduced weight gain in females. At still higher doses such as 100mg/kg per day, effects included atrophy of the retina in females12

Mutagenic effects
Imidacloprid may be weakly mutagenic13. In tests of the ability of imidacloprid to cause genetic damage submitted to the EPA as a part of the registration process, no evidence of genetic damage was found, or evidence only at high exposures. However, a new technique that looks at the ability of a chemical to cause genetic damage by chemically binding to DNA found that the imidacloprid insecticide Admire, increased the frequency of this kind of damage. DNA adducts (the binding of a chemical to DNA) were five times more common in calf thymus cells exposed to Admire than in unexposed cells14.

Reproductive effects
Laboratory studies on imidacloprid have shown it can have an impact on reproduction. Imidacloprid fed to pregnant rabbits between the sixth and eighteenth days of pregnancy caused an increase in the frequency of miscarriages and an increase in the number of offspring with abnormal skeletons. These effects were observed at a dose of 72mg/kg per day. In rats, a two-generation feeding study found that rats fed imidacloprid gave birth to smaller offspring; their weight was reduced at a dose of 19 mg/kg per day15.

Ecological effects
The acute toxicity of imidacloprid varies widely among bird species. It is ‘highly toxic’ to certain species including house sparrow, Japanese quails, canaries and pigeons. Based on these tests, the ecological effects branch of the US Environmental Protection Agency (EPA) concluded that their ‘levels of concern’ were exceeded for both non-endangered and endangered songbirds. Imidacloprid causes abnormal behaviour such as lack of coordination, lack of responsiveness and an inability to fly, even in birds for which it is not highly toxic, such as mallards. Other problems include eggshell thinning (at exposures of 61mg/kg), decreased weight (at exposures of 150 parts per million (ppm) in food) and reduced egg production and hatching success (at exposure of 234 ppm in food.)16 In studies with red-winged blackbirds and brown-headed cowbirds, the birds learned to avoid imidacloprid treated seeds after experiencing transitory retching and loss of co-ordination17

Aquatic species
Imidacloprid is acutely toxic to adult fish at relatively high concentrations (over 80 ppm) with juvenile fish being considerably more susceptible. It is however extremely toxic at low concentrations to some species of aquatic animals, including the freshwater crustacean Hyalella aztecais, and the estuary crustacean Mysidopsis bahia18

Beneficial insects
The application of imidacloprid by foliar spraying, is highly toxic to honey bees. 
    Imidacloprid is acutely toxic to earthworms, for example the LC50 (the lethal concentration required to kill 50% of a test population) of the species Eisenia fetida is between 2 and 4 ppm in the soil. At lower concentrations, the activity of the enzyme cellulase in the soil, that allows the break down of plant litter is reduced by imidacloprid concentrations of 0.2 ppm.
    Soil application of granular imidacloprid is the most common. This is less harmful than the foliar spray because it is less likely to come into direct contact with non-target insects. However many natural enemies supplement their diet by feeding on plant material. As imidacloprid is systemic it can be translocated to the surface of the plant, increasing the chances of direct contact with insects on the plants. Laboratory tests have also shown that imidacloprid is acutely toxic to a variety of predatory insects including mirid bugs, ladybirds and lacewings19.

Food contamination
Little information about monitoring of imidacloprid in food crops is publicly available. The US Department of Agriculture and the Food and Drug Administration (FDA) do not include imidacloprid in their food monitoring programs. However, two published studies from Spain have found, in one case, that all the greenhouse vegetables tested one week after treatment contained residues and, in another case found imidacloprid in tomatoes, potatoes, peppers, carrots, eggplants, pears and melons. Twenty-one percent of the samples were contaminated.

Water contamination
According to the US EPA, imidacloprid, has the potential to leach to ground water. In addition, high solubility and mobility are concerns for transport to surface water by dissolved runoff.
    The half life (the amount of time required for half of an applied pesticide to break down or move away from the test site) of imidacloprid in water was much greater than 31 days at pH 5, 7, and 9.
    Its ability to move through soil has been tested, along with other widespread water contaminants and it was found to be the quickest. Nevertheless, the EPA did not classify imidacloprid as a restricted use product, probably for economic reasons.

The development of resistance to imidacloprid in pest species is of serious concern. Tests by the University of Missouri have found cross-resistance of pests to other insecticides. Thrips selected for resistance to the organophosphate insecticide diazinon were also found to be resistant to imidacloprid20.

Regulatory status
There have been restrictions on the use of Gaucho (imidacloprid) in France since the 1990s because of concerns over the product’s toxicity to bees. In January 1999 the government suspended Gaucho for use on sunflowers. The insecticide retained its authorisation for use on cereals, sugar beet, and maize. The Ministry of Agriculture decision ruled that in the absence of sufficient technical and scientific evidence linking the use of Gaucho to the decline in the bee population, a temporary suspension of the product nationwide would help ‘limit the risks of exposing bees to the potentially detrimental effects of Gaucho’21
    Early in 2003, the Ministry decided to prolong Gaucho’s ban on sunflowers by three years as ‘no distinctive scientific factor has emerged reversing this decision.’ Bayer has expressed regret that the ban on sunflowers has not been lifted as ‘no improvement has been observed’ in French bee populations since the product was suspended in 199922. It is now thought that the replacement insecticide fipronil, is also having an impact on bee populations23
    There have been similar concerns raised in Canada. Prince Edward Island beekeepers have reported serious losses of bees which they believe since 1995 is linked to residues from imidacloprid. Potatoes on the island have been treated with soil applications of Admire (imidacloprid) to prevent Colorado potato beetle. It is believed that the rotational clover and canola crops have sublethal residues of imidacloprid in the pollen and nectar which cause slow death of bees in the colony. It may be that bees on the island may be affected by the cumulative effects of applications of Admire. Beekeepers have experienced high colony losses of 50-80% since 1999. New Brunswick has been increasing the use of Admire since 1998. Beekeepers in potato areas have reported large losses of 50-60% in 2001 and 2002 which may be attributable to the use of Admire. Similar figures are reported in potato areas of Ontario.

1. Agrow, No.188, July 23 1993, p18.
2. Caroline Cox, Imidacloprid, Journal of Pesticide Reform, Spring 2001,, National Coalition for Alternatives to Pesticides, PO Box 1393, Eugene, OR, US, 
3. Extoxnet, Extension Toxicology Network, A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University and University of California at Davis,
4. Bayer website, 9.10.2003, www.bayercropscience/com/bayer/cropscience/cscms.nsf/id/F86E6F8C873F9E6FC1256CB60038881C
5. CDS Tomlin, The Pesticide Manual, British Crop Protection Council,,
6. Bayer launches Gaucho in UK, Agrow, PJB Publications Ltd., No. 188, 23 July 1993.
7. Extoxnet, Op. cit 3.
8. Agrow, Op. cit. 6.
9. Cox, Op. cit. 2.
10. Extoxnet, Op. cit 3.
11. Cox, Op. cit. 2.
12. Cox, Op. cit. 2.
13. Extoxnet, Op. cit 3.
14. Cox, Op. cit. 2.
15. Cox, Op. cit. 2.
16. Cox, Op. cit. 2.
17. Extoxnet, Op. cit 3.
18. Cox, Op. cit. 2.
19. Cox, Op. cit. 2.
20. Cox, Op. cit. 2.
21. Gaucho suspended for use on sunflowers in France, Agrow, PJB publications, No. 321, 29 Jan 2003.
22. Gaucho spared on Maize, Agrow, PJB Publications, No. 417, 31 Jan 2003.
23. Gaucho/Admire overview, The Honey Council,

[This article first appeared in Pesticides News No. 62, December 2003, pages 22-23]