Neonicotinoid
Neonicotinoids are a class of insecticides which act on the central nervous system of insects with lower toxicity to mammals. Neonicotinoids are among the most widely used insecticides worldwide, but recently the uses of some members of this class have been restricted in some countries due to a possible connection to honey-bee colony collapse disorder.
Mode of action
The mode of action of neonicotinoids is similar to the natural insecticide nicotine, which acts on the central nervous system.[1] In insects, neonicotinoids cause paralysis which leads to death, often within a few hours. However, they are much less toxic to mammals and under the WHO / EPA classification these compounds are placed toxicity class II or class III. Because the neonicotinoids block a specific neural pathway that is more abundant in insects than warm-blooded animals, these insecticides are selectively more toxic to insects than mammals.
They bind at a specific site, the postsynaptic nicotinic acetylcholine receptor, and there are no records of cross-resistance to the carbamate, organophosphate, or synthetic pyrethroid insecticides, thus making them important for management of insecticide resistance. As a group they are effective against sucking insects such as aphids, but also chewing insects such as Coleoptera and some Lepidoptera.
Use
Imidacloprid is possibly the most widely used insecticide, both within the mode of action group and in the worldwide market. It is now applied against soil, seed, timber and animal pests as well as foliar treatments for crops including: cereals, cotton, grain, legumes, potatoes,[2] pome fruits, rice, turf and vegetables. It is systemic with particular efficacy against sucking insects and has a long residual activity. The application rates for neonicotinoid insecticides are much lower than older, traditionally used insecticides.
Thiamethoxam (TMX) is a second generation neonicotinoid insecticide, belonging to the thianicotinyl subclass. Thiamethoxam's chemical structure is slightly different from other neonicotinoid insecticides, making it highly water soluble and therefore readily translocated in plant tissue. TMX is systemic and penetrates into the plant cells where it also triggers various physiological reactions,[3] which induce the expression of specific functional proteins involved in various stress defense mechanisms of the plant allowing it to better cope under tough growing conditions, such as:
- Drought;
- Low pH;
- High soil salinity;
- Free radicals from UV radiation;
- Heat stress leading to protein degradation;
- Toxic levels of aluminum;
- Wounding from pests, wind, hail, etc., and;
- Virus attack.
In 2001, the US Patent and Trademark Office ruled in favor of Syngenta when the company filed suit against Bayer to protect its patent on a class of neonicotinoid insecticides.
Environmental impact
There is controversy over the role of neonicotinoids in relation to pesticide toxicity to bees and imidacloprid effects on bee population. Neonicotinoid use has been strictly limited in France since the 1990s, when neonicotinoids were implicated in a mass die-off of the bee population. It is believed by some to account for worker bees neglecting to provide food for eggs and larvae, and for a breakdown of the bees' navigational abilities and possibly leading to what has become generally known as Colony Collapse Disorder, which is usually associated with the mite pest Varroa destructor.[4][5]
In May 2008, Germany banned seed treatment with neonicotinoids due to negative effects upon bee colonies. Bee keepers suffered a severe decline linked to the use of clothianidin in the Baden-Württemberg region of Germany,[6] allegedly connected to a failure to apply a 'glue' agent that affixes the compound to the coats of seeds. The manufacturer maintains that without the fixative agent, the compound drifted into the environment from sown rapeseed and sweetcorn and then affected the honeybees.
The 2009 documentary Vanishing of the Bees suggests that a link between neonicotinoid pesticides and colony collapse disorder does exist, although the experts interviewed conceded that insufficient data exists in order to make a conclusive case.[7]
Active substances
Available neonicotinoid insecticides include:
References
- ^ Guide to pesticide modes of action http://www.dropdata.org/RPU/pesticides_MoA.htm#neonicatinoids
- ^ http://www.nationalpotatocouncil.org/NPC/p_documents/document_280607084102.pdf
- ^ Thiamethoxam Vigor Effect (Syngenta)
- ^ colony collapse disorder http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2007/04/01/nbees01.xml
- ^ Mysterious Bee Deaths Linked to Pesticides http://www.livescience.com/animals/070615_ap_bee_trouble.html
- ^ Germany bans pesticides linked to bee colony collapse
- ^ 2009 documentary Vanishing of the Bees
External links
- Vanishing of the Bees, a 2009 documentary that explores possible causes of colony collapse disorder