Lettuce residue alarm 

Consumers who regularly eat salad are being exposed to a cocktail of potentially dangerous chemicals. Pesticide residues in lettuce continue to threaten our health, both now and in the future.

Many people in the UK eat lettuce believing it to be a healthy, non-fattening food. As ready-mixed and washed salads are now widely available in supermarkets, they are increasingly popular: we eat on average 59 grams per person per week of 'leafy salads' which contain lettuce, the second most popular green vegetable(1). 
    But Professor Ian Shaw, who presided over the official government residue monitoring programme for six years until 1999(2), remarked, 'It is possible that multiple residues might be present in food. Little is known about the toxicological interactions between pesticides and therefore we must turn our attention to foods more likely to contain multiple residues.'
    The figures published in recent years show a pattern of pesticide use and misuse in lettuce growing. Residues occur regularly, and in multiple mixtures, and the effects of ingesting them over several years are not known. Regulatory anxiety is evident: annual retail surveys have continued since the formation of the Working Party on Pesticide Residues in 1988 (now the Pesticides Residue Committee). The Department of the Environment, Food, and Rural Affairs (formerly the Ministry of Agriculture, Fisheries and Food), has carried out enforcement programmes, collecting samples directly from growers, for five years(3), at increasing cost. Nonetheless, the UK tests fewer lettuce samples for residues than any other country in Europe(4).
    The number of violations of maximum residue limits, and the illegal products used, indicates that growers become desperate to avoid losses to pests and diseases. The crop is extremely vulnerable: one of the most artificially produced crops in horticulture, it inevitably attracts pests and diseases(5), which proliferate in the huge monocultures in which it is often grown.
    It is difficult to grow winter lettuce in the UK where the climate is damp, and there are low light levels in the later seasons. The longer it takes for the lettuces to grow to size, the more likely they are to become infested with fungal diseases, including 'big vein', boytrytis, downy mildew, rhizoctonia, sclerotinia, and ring spot. Insects such as aphids, flea beetles, leaf miners, thrips and whitefly prey on lettuces, and weeds including perennial grasses and polygonums compete with them for nutrients. Slugs, snails, caterpillars, cutworms, leaf miners, mealy bugs and red spider mites, all feed on lettuce, as do mammals, sometimes repelled by growers using sulfonated cod liver oil.
    Intensive pesticide usage, and repeated spray rounds, are necessary to protect the crop. More pesticides are applied to field grown lettuce than any other vegetable crop, with an average 11.7 applications each year(6).
    Residues most commonly detected are of inorganic bromide, a metabolite of the soil sterilant methyl bromide. This is an ozone-depleting chemical, and is subject to an international convention to phase out its use by 2005(7). The use of methyl bromide is also of particular concern in respect of the operators who handle it. 
    Iprodione is one of several fungicides (including the non-approved vinclozolin and chlorothalonil) which have regularly appeared as residues in lettuce. It has been classified as a 'likely' carcinogen by the US Environmental Protection Agency. Manufacturer Rhone Poulenc has voluntarily cancelled all residential uses of iprodione(9).

Rising costs - to whom?
As the table indicates, pesticide residue levels in lettuce have been extraordinarily high for years, peaking at 84% in 1998. Yet despite this, only five growers have been prosecuted by the government for the illegal use of vinclozolin, paying fines ranging from £100 to £2,035. There is still a significant problem, and regulators have responded by increasing resources for the testing of lettuce: in 2000, the monitoring budget was approximately £1.7 million, of which £62,000, 4%, was spent on lettuces. This year, the Department of Food, Environment and Rural Affairs is increasing their spend on the lettuce survey to £75,000, raising the number of samples tested from 115 to 180. A further enforcement programme is planned for winter lettuce(10).

Sustainable lettuces
Organic growers acknowledge that lettuce is a demanding crop(11), but it is possible to produce without the use of synthetic chemicals, so that residues do not occur. This is achieved by selecting varieties which are most resistant to disease, growing on light to medium loam soils, and building soil fertility. Rotational management gives the crop its best chance of survival, and care is taken to avoid environmental stress, and to provide nutritional sustenance in well-composted manure. (AC)

1. Ministry of Agriculture, Fisheries and Food National Food Survey 2000.
2. Annual report of the Working Party on Pesticide Residues 1999, page iii.
3. Report of the Pesticide Residues Committee, First Quarter results January - March 2000.
4. Salad Days, Sustain, 1999.
5. Salad Days, Sustain, 1999, p8.
6. Ibid, p8.
7. Pesticides News 51, p23.
8. Ibid.
9. Iprodione, US EPA RED Facts, Nov 1998.
10. Pesticide Safety Directorate, pers comm.
11. Organic lettuce production, Soil Association technical guides, 1999.
12. Annual reports of the Working Party on Pesticide Residues 1988 to 1999, and the Pesticide Residues Committee 2000, Pesticides Safety Directorate.

[This article first appeared in Pesticides News No. 53, September 2001, page 21]