Fate of insecticides in air soil and water

The fate of insecticides depends on their physiochemical properties and the medium to which they are applied and ultimately adsorbed.

The presence of insecticides in air can be caused by: spray drift and volatization from the treated surface; the extent of drift is dependent on: droplet size and wind speed; and volatization is dependent on: time after treatment, the surface on which the insecticide settles, the temperature, humidity and wind speed and the vapour pressure of the active ingredient. Generally it may be expected that the ecological risks

Table 4.9. Vertebrate incidents involving carbamate and organophosphate insectides (UK Data 1990-94) excluding abuse (from Hunter, 1995).

Source Animals affected

Table 4.9. Vertebrate incidents involving carbamate and organophosphate insectides (UK Data 1990-94) excluding abuse (from Hunter, 1995).

Source Animals affected

Granular applications

Blackbird, sparrow, pheasants, gulls

Secondary poisoning

Buzzard

Seed treatments

Corvids, duck, dog, geese, pigeons

Improper disposal

Gulls, dog, cattle

Poor storage

Dog

Veterinary medicines

Buzzard, corvid, parrot, poultry, dog

associated with insecticides in the atmosphere are probably very small when viewed in the context of atmospheric pollution in general (Kips, 1985) and certainly the impacts are negligible compared with that from direct agricultural applications. For most compounds aerial deposition in remote, natural or semi-natural ecosystems also results in negligible environmental exposure, although for some highly volatile, persistent compounds there appears to be the potential for prolonged exposure, albeit at low concentrations (Dubus et al., 1998).

The sources of insecticides in aquatic systems can be quite diverse, from the chemicals being applied directly to control various pests, from runoff from treated agricultural land, from the atmosphere during precipitation, leaks, spillages and improper use of pesticides. Drinking water supplies in the EU should contain no more than 0.1 |ig l-1 of a single pesticide and 0.5 Hg l-1 total pesticides while in the USA and Canada maximum limits for pesticides are based on toxicological assessments (Carter and Heather, 1995). Pesticides are present in water which is treated to produce drinking water at concentrations above regulatory limits and conventional water treatment processes have a limited capacity for pesticide removal (Croll, 1995). Soluble insecticides tend to be washed through aquatic systems and cause little harm but less soluble chemicals can bind to suspended particles and accumulate in sediments and become a source of possible future contamination of water (Edwards, 1973, 1987).

Edwards (1987) placed the various factors that influence the persistence of insecticides in soil in the following order of importance:

1. The chemical structure of the insecticide; the less volatile an insecticide the longer it persists. Granules persist longer than emulsions which persist longer than miscible liquids. Wettable powders and dusts have a very low persistence.

2. The type of soil to which the insecti cide is applied; the organic content is the single most important factor influencing persistence. Agricultural soil contains bacteria, fungi, actinomycetes, algae and protozoa and the total and relative numbers are influenced by type of soil, seasonal changes, tillage, crop type, fertilization levels and aeration (Kips, 1985). The amount of soil moisture and rainfall is also important.

3. The microbial population of the treated soil, depth of cultivation of the insecticide, mean temperature of soil (increased temperatures increase degradation).

4. The mineral content and acidity of the soil, clay soils retaining insecticides longer than sandy soils.

5. The amount of plant cover and formulation of insecticide and its concentration.

Pesticide persistence in the soil is also influenced by the phenomenon of enhanced biodegradation following repeated application at the same site. Biodegradation, i.e. primarily decomposition by micro-organisms, is by far the most important mechanism preventing the accumulation of some insecticides in the environment (Rombke and Moltman, 1996). Biodegradation has been shown to be responsible for the failure of a number of soil insecticides (e.g. Suscon Blue).

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