Further review of certain persistent organochlorine pesticides used in Great Britain.

  • Great Britain. Advisory Committee on Pesticides and Other Toxic Chemicals
Date:
1969
Catalogue details

Licence: Public Domain Mark

Credit: Further review of certain persistent organochlorine pesticides used in Great Britain. Source: Wellcome Collection.

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    DDT EFFECTS ON SOIL AND SOIL ORGANISMS 37. There is evidence that DDT residues persist and remain active in the soil longer than those of any other organochlorine insecticide in common use. Persistence can vary widely depending on conditions; for example, DDT can be lost much more rapidly when left undisturbed on the soil surface than when incorporated into the top soil. In the latter case, it may take 3-6 years for residues of DDT to decline by 50 per cent, but the rate of loss itself diminishes with time so that long-standing residues decline by only a few per cent per year and in one prolonged study, about 35 per cent of the DDT originally applied still remained in the soil after ten years. Commonly used rates of application of DDT have marked effects on soil invertebrates, but most workers agree that the amounts of DDT found in soils in Great Britain at present do not influence significantly the total number of soil micro-organisms or the beneficial processes such as organic turnover, nitrification and ammo-nitrification for which they are responsible. 38. Control of orchard pests accounts for much of the DDT used in agriculture in the United Kingdom and orchard soils can contain large DDT residues as a result of spray run-off [Appendix V (v)]. Earthworms ingest large quantities of soil and they form an appreciable item in the diet of some birds. From the few quantitative data available, it seems that applications of less than 25 lb DDT/acre are unlikely to have any effect on the survival of field populations of earthworms. In Great Britain, fruit growers applied an average of 2 lb DDT/acre in 1966, with a range from 0-5 to 5 lb, depending on individual spray programmes. Residues have been found in earthworms taken from orchard soils where repeated applications of DDT have been made. The actual residue levels found are related to some extent to the species of earthworms, being largest in those which live mainly in the top soil. Earthworms living close to the trunks of sprayed trees in an orchard that had received a total of 15-25 lb DDT/acre over a period of 10 years contained 13-6 p.p.m. DDT; those living 6 ft away from the trees contained 6-1 p.p.m. and those 12-15 ft away from the trees contained 3-3 p.p.m. In another location, earthworms from an orchard that had received 4 lb DDT/acre per annum applied in 2-3 doses for several years contained 16 p.p.m. DDT; total residues of about 40 p.p.m. were found in another survey [Appendix V (i)]. EFFECTS ON BENEFICIAL INSECTS 39. Soon after the general introduction of DDT it became clear that its use in some situations, especially in orchards, was leading to a resurgence of pests and to the development of secondary pests, particularly mites. 40. The effects of DDT on predators and parasite populations are probably associated both with its persistence and its wide spectrum of activity. Many parasites and predators are very susceptible to DDT and there is much variation both within and between species. In general, the use of DDT reduces both the numbers and the range of species of beneficial arthropods. Nevertheless, where it has been used over a period of time with evidence of serious effects, the ‘status quo’ above ground has apparently been restored a few years after the cessation of use. 41. Several investigators have considered the possibility of indirect and non- lethal effects of DDT on pest populations. It is very difficult to make conclusive Sg pp—2*
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