Written by K. Jean Cottam, PhD Wednesday, 17 August 2005

Grubs of several beetle species eat grass roots, sometimes actually killing patches of turf that can be rolled back. Birds, raccoons and skunks feast on these spring-time delicacies. The surviving grubs eventually pupate and emerge as beetles, to mate and lay eggs back in the turf, completing the ageless cycle. Homeowners may be alarmed by these temporary foragers, and use toxic chemicals to “protect their lawn”. This author encountered some grub infestations on her front lawn on two occasions, but did absolutely nothing to get rid of them. This turf was essentially healthy and tight, and healed itself without any kind of intervention on my part.

Many chemicals previously used to kills grubs were found to be too toxic, and are being discontinued. The replacement is imidacloprid – “ Merit 0.5 G”, a version of this systemic insecticide. Imidacloprid is a systemic nitroguinidine insecticide that belongs to neonicotionid family of chemicals. Its action is similar to the tobacco toxin nicotine and it is used, among other things to kill grubs (the larvae of European Chafer, Japanese Beetle and Black Turfgrass Ataenius Beetle in Turfgrass). This article is based on data supplied by Bayer CropScience, Inc., the manufacturers of “Merit 0.5.G,” to Health Canada’s Pest Management Regulatory Agency (PMRA), as well as on several independent studies.

Introduced in Canada in 1999, Merit 0.5 G was temporarily registered in Ontario, Quebec and Halifax in 2001. The registration was finalized nationally at the end of March 2005. Although extensive information was published supporting registration of other pesticides such as the herbicide 2,4-D, nothing is to be published for public review regarding imidacloprid.

Imidacloprid is not as toxic as older pesticides were upon skin contact, but is quite toxic when swallowed. Thus, small children who tend to put their hands in their mouths are more at risk. Nicotine is known to irreversibly impair brain development in the very young, so what will this chemical, with similar action, do to our grandchildren?

Imidacloprid is persistent. Its half-life (the amount of time required for half of an applied pesticide to break down or move away from the test site) is highly variable, and may range from 27 to 730 days, depending also on soil type and the presence or absence of ground water (Bacey, Soil; PMRA). Imidacloprid was proven persistent in a field test in Minnesota, where the concentration didn’t decrease for a year following treatment. Other studies showed that over half of the applied insecticide was still present one year after application (Cox, 15,20).

The development of resistance in pest species to imidacloprid within a few years is of serious concern (Buffin, Resistance). While the label of imidacloprid warns against its use more than once per season, we have noted that it takes over two years, according to the PMRA, for half of the application to degrade to its first degradation product. Thus the soil is never rid of this toxin. As with antibiotics, when a chemical is prevalent, organisms adapt to it quickly, and insect resistance to imidacloprid has arisen in as little as two years.

Allowing annual application of imidacloprid, when its half-life may be greater than two years, leads to accumulation in the environment of a chemical that is known to be toxic to a broad range of beneficial organisms. Organisms that reproduce rapidly may develop resistance to an insecticide, but others may simply sicken and die.

Imidacloprid is translocated throughout the plant--otherwise the use of imidacloprid soil drench for treatment of trees would not work--and bees die as a result of eating the poisoned pollen.

The PMRA insists that “there is not unequivocal evidence” that Canadian bees are susceptible to imidacloprid, but the chemical was severely restricted in France when it was connected to plummeting bee populations. As is well known, bees are essential in spreading the pollen and thus in fertilization of various plants–-the basis of our food system.

Imidacloprid is also acutely toxic to earthworms, which constitute about 80% of the beneficial soil organisms by weight and make an important contribution to soil fertility as well as to the breakdown of organic material (Cox, pp. 18-19). It also kills ground beetles and other predators of grubs.

The United States Environmental Protection Agency (EPA) considers imidacloprid a potential water contaminant due to its mobility in soil (Cox, 15). Imidacloprid may contaminate streams and ponds via drift during application or in runoff water (Bacey, Environmental Fate). Even though the United States EPA is aware that imidacloprid (approved in 1994) has the potential to leach to ground water, the EPA has failed to restrict its use. Thus the chemical could be sold to both homeowners and lawn care companies. (Cox, 19-20.)

Imidacloprid’s high persistence in the environment and broad spectrum of non-target toxicity should be sufficient in this day and age to preclude the registration of this pesticide.

If it takes years for imidacloprid to take the first breakdown step, what happens after that? A diagram of the imidacloprid’s breakdown provided by the manufacturer, Bayer, reveals that after a number of transformations the chemical degrades slowly into 6-chloronicotinic acid. This then breaks down into carbon dioxide and unidentified residues, the latter supposedly bound to the clay mineral fraction of the soil. Evidently, according to the manufacturer, there is virtually no potential for exposing vulnerable beneficial organisms to the “bound residues.”

Dr. Meg Sears questions this, insisting that the complete environmental fate of the chemical should be tracked. The next logical chemical step would be degradation into 2-chloropyridine, a product that is toxic, soluble and persistent in the environment. Conveniently for Bayer, the PMRA avoids discussing the likely presence of 2-chloropyridine. This is similar to their reluctance to investigate or disclose the presence of toxic dioxins in the herbicide 2,4-D.

I am citing here Dr. Sears’ comment in an e-mail addressed to me and dated 13 July 2005: “I have no idea of the identity of the ‘bound residues’, and neither does the PMRA. In fact, they gave no information as to how it was determined that anything was remaining in the soil matrix. That is part of the secrecy. They gave that convoluted hypothetical discussion of possible degradation routes, but didn’t say what actually happens.”

Dr. Sears explained further that, according to PMRA policy, the investigation of chemical breakdown is usually done using chemicals that are “labelled” with radioactive carbon atoms. Apparently, the PMRA’s open literature on imidacloprid doesn’t apply the “labelling” process to the portion of the molecule that would subsequently form 2-chloropyridine.

When asked what solvents are used during the Soxhlet method of extraction of “bound residues” and what precautions are taken to avoid degradation or volatilization of 2-chloropyridine, or to ensure entrapment if volatilized, the spokesman for the PMRA responded that this was Confidential Business Information under the Pest Control Products Act and therefore could not be released without the express consent of Bayer, the registrant of the pest control product. Such secrecy is highly suspect.

It is noteworthy that commercial imidacloprid insecticides, including Merit 0.5 G, like nearly all pesticides, contain a multitude of “inert” or “other” little known ingredients, called formulants, which make the given product more potent or easier to apply. The crystalline quartz silica in Merit 0.5 G formulant was classified in 1997 by the International Agency for Research on Cancer (IARC) as “carcinogenic to humans” and “as known to be a human carcinogen” by the United States Department of Health and Human Services, Public Health Service, National Toxicology Program (2000), because-–similarly to smoking--it has been linked to lung cancer and emphysema (Cox, 16, 20; Buffin, Inert ingredients).

To sum up, it is impossible to ameliorate this situation by simply following instructions on the label. Among other things, bees cannot read and will repeatedly encounter the pesticide. So will everyone else, since the pesticide lingers years after the signs are gone. Sustainable landscaping practices, with at least eight inches of topsoil and compost, a sturdy grass mixture and possibly application of nematodes is recommended instead. (Note that they must be employed at specific times of the year and strictly according to instructions.) Also recommended is use of endophytic grasses which are resistant to grubs.

Hence, the licensing of imidacloprid is not justified. This chemical is persistent and harmful to non-target species, with target species developing resistance quickly. Also, there is a concern that with repeated use soil could become more and more toxic with time. The industry spokesmen tend to dismiss the use of organic nematodes as ineffective. The nematodes are living organisms that must be treated with proper care; otherwise, they will die prematurely, without accomplishing their task. For example, they must not be placed in soil contaminated by an insecticide such as Merit 0.5 G, where toxicity--as we have seen--may last for several years.

The term “pesticides” embraces herbicides, insecticides, rodenticides and fungicides. In light of growing evidence that lawn chemicals are dangerous to human health, it is imperative that all use of pesticides by the City of Ottawa, including Merit 0.5 G insecticide, should be severely restricted. One possibility would be to require a permit from the Medical Officer of Health. Whether on City or private property, the best defence against grubs is a healthy turf.


PMRA Labels and Regulatory Note, using Bayer CropScience, Inc. data. Dr. Meg Sears’ correspondence with spokesmen for the PMRA.

Bacey, Juanita. Environmental Fate of Imidacloprid. Environmental Monitoring and Pest Management Branch, Department of Pesticide Regulation, 830 K Street, Sacramento, CA 95814.

Buffin, David. Imidacloprid. Pesticides News, No. 62, December 2003, pp. 22-23. Web site study:

Cox, Caroline. Insecticide Fact Sheet: Imidacloprid. Journal of Pesticide Reform. Spring 2003. Vol. 21, No. 1.

K. Jean Cottam

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