Carbofuran – 2,3-dihydro-2,2-dimethyl-7-benzofuranyl N-methylcarbamate – is a broad spectrum pesticide. A member of the carbamate family, it inhibits the enzyme cholinesterase, which is involved in virtually all physiological responses in insects, mammals and birds. It was developed by the US manufacturer FMC in the 1960s – marketed as Furadan – to replace the highly persistent organochlorine pesticides. The patent expired in the mid- to late-1980s, and carbofuran is now manufactured in a number of countries.
The product is available in liquid, silica-based and corncob granular formulations. Sand or granulated corncob formulations were designed to allow the active ingredient to be released more slowly into the roots of a developing plant. Carbofuran has also been used as a seed treatment insecticide. Once absorbed, it is transported systemically by the sap, whereupon insects or other pests that feed on the plant become poisoned.
Plant protection products containing carbofuran as the active ingredient have been used worldwide to control insect pests in sugarcane, sugar beet, maize, sorghum, coffee, cereal and rice crops, among others.
There is now strong evidence that, under the right conditions, carbamates can also persist in the environment. In soil, chemical processes are influenced by factors such as pH, temperature, clay content, organic matter content, moisture content, the presence of microorganisms, and the types of functional groups that are attached to the pesticide molecule.
Carbofuran tends to be rapidly hydrolysed in alkaline soils but is generally more stable and even persistent in neutral or acidic soils. Moreover, external environmental factors, such as wind, humidity, soil and air temperature, as well as rainfall, also influence the degradation and dissipation of pesticides within soil.
Carbofuran is also relatively soluble in water and has the potential to contaminate a variety of aquatic resources, including groundwater, a concern raised by the US Environmental Protection Agency (EPA). Field flooding, following adverse weather or as an agricultural practice, has resulted in extensive carbofuran-related deaths of non-target organisms, particularly waterfowl. Surface water can also be compromised as a result of improper disposal, accidental spillages and direct contamination. For all these reasons, the behaviour and fate of carbofuran is likely to differ between temperate and tropical environments and so potential risks need to be assessed accordingly.
By the early 1970s, it became apparent that carbofuran was poisoning a wide range of non-target species, especially birds. Besides being more visible than other non-target species, birds are very poor at metabolising carbamate, or organophosphorus, compounds before succumbing to their toxic effects.
Between 1973 and 1975, field investigators in Canada documented at least four separate incidents in British Columbia (BC) involving more than 1300 bird deaths. As a consequence, in 1976, the granular formulation of the pesticide was withdrawn in Canada. However, 10 years later, the product was back on the market, as a result of pressure from the agricultural authorities. Ironically, in the same year over 1000 song birds, as well as waterfowl and scavengers, fell foul of the pesticide.
Legislation and regulation
Manufacturers argue that the product’s safety to wildlife depends in the main on proper use and diligently following label instructions, putting the onus on growers and farmers. While proper usage and precautions are indeed integral to ensuring the health and safety of applicators, researchers and conservationists believe that carbofuran is fundamentally unsafe to wildlife. Given that many label instructions rely on the assumption that wildlife are not present in agricultural areas, many of these instructions are simply untenable.
Manufacturers have also long maintained that the use of carbofuran is fundamental to crop success and to securing global food resources. But in 2006, the EPA concluded in its Interim Re-registration Eligibility Decision that the risks posed to people and the environment far outweighed any of the benefits offered. As a result, no uses of carbofuran were deemed eligible for re-registration in the US. The EPA planned to terminate all remaining uses of the product through its cancellation process.
In response, FMC and several food industry lobby groups challenged these decisions, successfully convincing a federal court to allow certain imports containing carbofuran to continue entering the country. The same group then made an appeal to the US Supreme Court to reinstate domestic food tolerances for carbofuran.
In July 2010, the EPA’s decision to ban residues of carbofuran on domestically sourced foodstuffs was upheld, cancelling, as of November 2010, the registration or re-registration of any formulation or use of the product within the US.
On 30 March 2011, FMC announced that it, together with the National Corn Growers Association among others, intended to appeal to the US Supreme Court to overturn the decision. On 31 May 2011, the Supreme Court refused to hear the case, allowing the US EPA’s decision to stand.
In keeping with EPA proceedings, Canada’s regulatory authority, the Pest Management Regulatory Agency (PMRA), also recommended the cancellation of all remaining uses of carbofuran, citing unacceptable risks to human health and to the environment. Again, despite protests from the manufacturers, the decision was upheld and carbofuran was banned for use in all domestic-sources foodstuffs in Canada.
Colourless, odourless, and relatively inexpensive, carbofuran has also lent itself well to several uses for which it was never intended. In 2009, carbofuran gained public notoriety when the popular US television programme 60 Minutes documented its deliberate use to poison lions in Kenya. In response to the programme, which specifically linked Furadan products to the deaths, FMC promptly announced a buyback of all Furadan products in Kenya. This buyback appears to have been successful since it is now reportedly very difficult, though still not impossible, to buy carbofuran in Kenya.
The 60 Minutes report also indicated that the poisonings were indirectly killing scavengers, such as vultures and hyenas, integral to the functioning of a healthy ecosystem.
Scavenging species in Kenya are undergoing a dramatic decline and there is concern that if the current population declines go unabated, Kenya’s lions may become extinct within the next five years. There are also concerns about repercussions to Kenya’s vital beneficial insect/pollinator population.
Equally concerning is the practice of luring local and migratory birds with carbofuran-laced baits on rice fields and sprinkling the product on lakes to catch fish, both for human consumption. The repercussions to poachers, who handle and mix the product in bait, and to consumers of bush meat, have not yet been investigated.
The pesticide has also been linked to the deaths of elephants and leopards in India, and birds of prey on game estates in the UK – in the latter example, ostensibly to protect grouse and other birds whose populations are managed for the shooting industry. In North America, carbofuran has been implicated in the deaths of domestic dogs, coyotes and a number of bird of prey species. In parts of Europe, it has been used to poison otters, birds of prey and predators such as foxes, thought to compete with hunters. Disparate regulations across territories and a thriving black market have aided in its use as a poison.
The way forward
Broad spectrum pesticides are slowly being phased out and replaced by new products that offer, at comparable costs, some discrimination between target insects and non-target organisms or wildlife. However, such products may still pose significant repercussions to beneficial non-target insects – the fact that the nicotinic systemic insecticide imidacloprid also kills honeybees is an example.
Pest species that threaten crops can also be managed by means other than pesticide applications. Indeed a range of agricultural options are available to farmers and growers, such as integrated pest management and pre-testing soil to determine the presence or absence of nematodes. These measures can save growers a considerable amount of money and potentially reduce wildlife mortality.
Some of the issues that have emerged following use of carbofuran have also highlighted the fact that applicators do not always wear adequate protection and are not always able to understand and follow label instructions. This is an area that must be addressed. There is also urgent need to conduct studies that will examine the risks posed to consumers of all pesticide-hunted and fished meat.
Regardless of the issues related to its lack of fundamental safety to wildlife, carbofuran is only one of many products used to poison wildlife and there are many lessons to be learned from its use. While banning this product removes a very real threat, it is also true that such a measure does not address the root causes of human–wildlife conflict, which are underpinned by factors like the increasing population and the inequitable distribution of wealth and demand for resources. Until the issues that trigger such conflict are resolved, poisonings will continue.
Perhaps most fundamentally of all, the regulatory battle that has been waged over the fate of carbofuran provides an important glimpse into the realm of chemistry stewardship at the international level. We must ask ourselves why, even in ‘developed’ nations, it has proven to be so difficult to regulate a product shown to be consistently harmful to wildlife and known to have the potential to infiltrate and contaminate groundwater resources. We must also ask why a product can be deemed unsafe for use in some parts of our world but acceptable in others.
1 N. L. Richards (ed), Carbofuran and wildlife: global perspectives and forensic approaches. Chichester, UK: John Wiley & Sons, 2011.
2 Details of the US EPA proceedings can be found at: http://www.epa.gov/opp00001/reregistration/carbofuran/carbofuran_noic.htm
3 60 Minutes video: www.cbsnews.com/video/watch/?id=5189491n&tag=related;photovideo.