Misguided Herbicides?

With the arrival of summer, local council employees and contractors are out on the streets spraying against weeds and the telephones at the Pesticides Trust [now PAN UK] and other environmental organisations start ringing with concerned citizens asking whether the chemicals being sprayed are safe. The Pesticide Trust Local Authority Project offers guidance to local authorities on least toxic control methods. Here we ask what councils are using against weeds. Is it safe, and is it necessary?

In 1990, non-agricultural users of pesticides, of which local authorities are by far the biggest group, applied some 55 tonnes of herbicide active ingredients to road verges, footpaths, parks, playing fields and other sites. A high proportion of the herbicides used for these purposes were, in the past, of the triazine group—two of the most widely used were atrazine and simazine. These were popular because they were cheap, and their long term persistence meant that they would continue to kill newly germinating weeds long after the sprayers had gone. This feature, plus the fact that they were often sprayed onto hard surfaces from which they could easily be washed down drains, or directly into waterways, meant that these herbicides were increasingly being found in water.
    Atrazine and simazine have now been banned for non-agricultural uses in the UK, and local authorities, their contractors, and the chemical companies have been forced to find alternatives.
    Of the dozens of herbicide active ingredients and hundreds of products which are approved for non-agricultural weed control, two seem to be coming out on top of the struggle for market position; the non-residual systemic glyphosate and the residual substituted urea herbicide diuron.
    The two chemicals are profiled here from a health and safety and environmental viewpoint.

Glyphosate

The glyphosate based-product Roundup is claimed to be the worlds biggest selling herbicide(1). It is widely thought of by users as being harmless to the environment, and virtually non-toxic to applicators and others who are exposed to it.
    The acute toxicity of glyphosate itself is very low. According to the World Health Organisation, the oral LD50 in the rat of pure glyphosate is 4,230 mg/kg—or 5,600 mg/kg according to Monsanto, the developers of glyphosate(2). The low acute toxicity of glyphosate can be attributed to its biochemical mode of action on a metabolic pathway in plants (the shikimic acid pathway) which does not exist in animals(3).
    There is limited evidence that when glyphosate is administered in high doses over long time periods some test animals developed chronic health effects including birth defects(4). It is extremely unlikely that users of glyphosate or members of the public would ever be exposed to such high doses and the risk of similar health effects occurring in humans is generally considered to be low.

Hidden hazards
However, while glyphosate itself may be relatively harmless, some of the products with which it is formulated have a rather less benign reputation.  Marketed formulations of glyphosate generally contain a surfactant. The purpose of this is to prevent the chemical from forming into droplets and rolling off leaves which are sprayed. Some of these surfactants are serious irritants, toxic to fish, and can themselves contain contaminants which are carcinogenic to humans.
    The most widely used type of surfactants used in glyphosate formulations are known as ethylated amines. POEA (polyoxyethyleneamine) has been frequently mentioned as a surfactant, but in fact it refers to a group of ethylated amine products used in glyphosate formulations. Members of this group of surfactants are significantly more toxic than glyphosate. They are serious irritants of eyes, respiratory tract and skin, and have been found to contain dioxane (not dioxin) contaminants which are suspected of being carcinogenic. Accordingly, the UN FAO has set standards of 1 part per million (ppm) for levels of the contaminant 1,4 dioxane which may be present in POEA surfactants.
    Monsanto states that all surfactants used in its glyphosate formulations fall well within the FAO standard. However, being aware of the irritant and toxic potential of the surfactants in general, the company has now developed new surfactants which have none of these toxic effects. Products containing the new formulants have already been released in France and Denmark, and are due to be released in the UK in June 1994(5).

Irritated public
The Pesticides Trust [now PAN UK] has received a number of complaints from members of the public who have themselves or whose pets have suffered from eye and respiratory tract irritation as a result of exposure to glyphosate-based herbicides being sprayed by council employees or contractors. With the release of new formulations these problems should hopefully lessen. However, since many other manufacturers now formulate glyphosate herbicides, and Monsanto is keeping its new surfactants secret, the problems might be expected to continue for some time, particularly if the older formulations become the cheaper products, and councils and contractors continue in their struggle to keep costs down and keep working in a highly competitive market.

Water Contamination
Two important features of glyphosate have largely precluded concern over its water contamination potential—its relatively low toxicity and its immobility in soil. There are two aspects that attract concern. Firstly, glyphosate in soil is strongly adsorbed to soil particles and hence immobilised. However, this also means that it is carried with soil which is washed off in surface runoff into rivers or lakes. Glyphosate is very stable in water and in plants, but  its residues are not monitored in food or drinking water because of the low level of concern over its toxicity, and because it is notoriously difficult to isolate and detect. Residue monitoring also tends to concentrate on active ingredients, but in the case of glyphosate products, it is the ‘inert’ surfactants which may be of greater concern.

Diuron

Originally developed by Du-Pont, diuron is now marketed for non-agricultural use in the UK in a variety of products by many companies including Rhône-Poulenc and Nomix-Chipman. Its popularity has increased in response to the triazine ban in the UK, which forced councils to find cheap viable alternatives for weed control. Diuron has the advantage over glyphosate in that it has residual activity. This means that while glyphosate alone may have to be sprayed several times in a season, diuron, or a mixture of diuron and glyphosate will remain active for longer, thereby reducing the frequency of treatments required.
    Diuron has a relatively low acute toxicity. The World Health Organisation puts the oral LD50 in the rat of pure diuron at 3,400 mg/kg. It is an irritant to eyes, skin and respiratory tract. Some literature suggests that there may be some evidence of diuron being a teratogen causing musculoskeletal, eye and ear abnormalities in some animals, and of being an “equivocal tumour agent” in mice(7,8).
    No Western government authorities have placed any restrictions on diuron use as a result of its toxicological assessment, but the former Soviet Union prohibited its use “because it is carcinogenic and persistent”(9), and the US Forestry Service has removed diuron from the list of herbicides used because "insufficient information was available for an adequate toxicological evaluation."(10)

Water Contaminant
Diuron is also toxic to fish and aquatic life. Research has shown that when fish are exposed to relatively low concentrations (0.22 and 0.55 mg/litre) over a period of several days or weeks, physiological and behavioural abnormalities occur(11). Such levels of contamination are feasible, particularly in ponds and small lakes where waterside weeds are treated.

Better Solutions?
In the rush to find replacements for the banned triazines, local authorities have provided comfortable market niches for glyphosate and diuron. A major new product actually mixes both herbicides in a single formulation. Many councils wishing to present their ‘green’ credentials have even adopted ‘glyphosate only’ weed control policies. The government has done little to assist in developing sensible strategies, and to a large extent the water companies have been left to fight their battle to keep pesticides out of drinking water alone, and councils have been abandoned to market forces.
    The information suggests that yet again, the replacement of one chemical with another is not a viable and sustainable long term solution. A commitment to sustainable development suggests that resources should be directed at finding alternatives to widespread herbicide use without relying on market forces which are too deeply entrenched in existing infrastructures. (MD)

References:
1.  Advertising supplement, Farmers’ Weekly, 5 June 1992.
2. Monsanto Company, Toxicology of glyphosate and Roundup herbicide, Department of Medicine & Environmental Health, Monsanto, Missouri 5 June 1985.
3. Carlisle SM and JT Trevors, Glyphosate in the Environment Water, Soil and Air Pollution , 39, 409-420, 1988.
4. Agriculture Canada, Pre-harvest use of glyphosate. Discussion Document D91-01, Pesticide Directorate, Food Production & Inspection Branch, Agriculture Canada, Ontario, 27 November 1991.
5. Merritt, Colin, Monsanto UK, Pers. comm., 10 May 1994.
6. EPA, RED Facts—glyphosate, US-EPA, September 1993.
7. NIOSH Database,
8. Antony M, et al, Tumour initiatory activity of a herbicide diuron on mouse skin, Cancer Letters, 48:2, 1989.
9. United Nations, Consolidated list of products whose consumptions and/or sale have been banned, withdrawn, severely restricted or not approved by governments, UN, New York, 1991.
10. Grier, Norma  and Alexandra Foote, Beyond herbicide wars: Trees, weeds, and the US Forest Service in the Pacific Northwest, Journal of Pesticide Reform,12:2, 2-8, 1992.
11. Reffy, DC,  et. al., Changes in erythropoietic activity of sarotherodon mossambicus exposed to sublethal concentrations of the herbicide diuron, Bull. of Environmental Toxicology, 49:730-737, 1992.
12. Montgomery, John H., Agrochemicals Desk Reference Environmental Data, Lewis Publishers, Michigan, 1993.
13. Thames Water Utilities, Use of non-agricultural pesticides in the Thames Water catchment, 1992—A summary, Thames Water, September 1993.

[This article first appeared in Pesticides News No. 24, June 1994, pages 16-17]