John Unsworth

Agrochemical spray drift; assessment and mitigation—A review*

During application of agrochemicals spray droplets can drift beyond the intended target to non-target receptors, including water, plants and animals. Factors affecting this spray drift include mode of application, droplet size, which can be modified by the nozzle types, formulation adjuvants, wind direction, wind speed, air stability, relative humidity, temperature and height of released spray relative to the crop canopy. The rate of fall of spray droplets depends upon the size of the droplets but is modified by entrainment in a mobile air mass and is also influenced by the rate of evaporation of the liquid constituting the aerosol. The longer the aerosol remains in the air before falling to the ground (or alternatively striking an object above ground) the greater the opportunity for it to be carried away from its intended target. In general, all size classes of droplets are capable of movement off target, but the smallest are likely to move the farthest before depositing on the ground or a non-target receptor. It is not possible to avoid spray drift completely but it can be minimized by using best-management practices. These include using appropriate nozzle types, shields, spray pressure, volumes per area sprayed, tractor speed and only spraying when climatic conditions are suitable. Field layout can also influence spray drift, whilst crop-free and spray-free buffer zones and windbreak crops can also have a mitigating effect. Various models are available to estimate the environmental exposure from spray drift at the time of application.

SIGNIFICANCE OF THE LONG RANGE TRANSPORT OF PESTICIDES IN THE ATMOSPHERE

Synopsis:Since the 1960’s there has been a growing body of data regarding the presence of pesticides in the atmosphere. The monitoring results obtained show that traces of pesticides may undergo long range transport and be deposited considerable distances away from the treatment areas, including remote areas such as the Arctic and Antarctic regions. Pesticides have been found in air, rain, cloud water, fog and snow. The appearance and subsequent behaviour of pesticides in the atmosphere are complex processes and the concentrations found depend on several variables such as their volatility, photostability, method of application and extent of use. Whilst volatility of pesticides can be linked to their Henry’s Law constant this is very much a simplification since it is also influenced by the surfaces treated, e.g. soil or leaves, and by the extent to which aerosols are formed during the application. The disappearance of pesticides from the atmosphere is due to hydrolysis, indirect photolysis via OH. radicals and to deposition in rain. Pesticides which are resistant to hydrolysis and photolysis can be transported over great distances, for example, organochlorine insecticides have been detected in the Arctic regions. In general, concentrations in rainwater are, when detected, in the low or sub mg/l range and highest concentrations are found during the time of application. The use of fugacity models has been shown to be a useful approach to predict concentrations in air. Under most conditions the presence of pesticides in air, or rainwater, has no significant effects on non-target systems, including direct and indirect effects. Exceptions to this are damage by auxin-type herbicides to sensitive plants which has resulted on restrictions in their use in certain areas and transient chlorotic spotting thought to be caused by drift of aerosols from application of low rate sulfonyl urea herbicides. For animal species one possible exception has been postulated. This is for persistent organochlorine pesticides in Arctic regions where, due to the very oligotrophic nature of the Arctic ocean, they are more liable to bioaccumulate and be transported in the food web giving enhanced levels in mothers’ milk.

The impact of altered herbicide residues in transgenic herbicide-resistant crops on standard setting for herbicide residues

The global area covered with transgenic (genetically modified) crops has rapidly increased since their introduction in the mid-1990s. Most of these crops have been rendered herbicide resistant, for which it can be envisaged that the modification has an impact on the profile and level of herbicide residues within these crops. In this article, the four main categories of herbicide resistance, including resistance to acetolactate-synthase inhibitors, bromoxynil, glufosinate and glyphosate, are reviewed. The topics considered are the molecular mechanism underlying the herbicide resistance, the nature and levels of the residues formed and their impact on the residue definition and maximum residue limits (MRLs) defined by the Codex Alimentarius Commission and national authorities. No general conclusions can be drawn concerning the nature and level of residues, which has to be done on a case-by-case basis. International residue definitions and MRLs are still lacking for some herbicide-crop combinations, and harmonisation is therefore recommended.