Weather essentials for pesticide application

Typically, there is a short window of opportunity to safely and efficiently control pests and diseases in agricultural crops. The opportunity may be shortened by unsuitable weather conditions. It is therefore essential that spray applicators are able to identify and react to weather conditions at a local scale.

Weather Essentials for Pesticide Application aims to help those applying pesticides to understand, observe and interpret weather conditions at the spray target, especially those driven by local microclimates. This knowledge will support the adoption of effective application methods that avoid spray drift.

Particles far smaller than can be seen by the human eye can drift and cause damage many kilometres away from the site of application.

The weather factors that are important to the application of pesticides can be significantly and critically different to conditions indicated by forecasts, maps and off-site weather observations. This is especially true for conditions overnight and into mid-morning when surface inversions are likely to exist and local winds develop. Local winds can transport concentrated volumes of pesticides long distances from the target.

The weather factors that need to be considered before and while applying pesticides are:

  • atmospheric stability (including up and down air currents);
  • general wind speed, direction and turbulence;
  • local wind flows, such as drainage winds caused when air over sloped terrain is cooled by conduction, becomes dense and drains to lower levels, sea breezes and land breezes;
  • temperature of the air and the surface; and
  • humidity.

Intuitively, the worst weather conditions for spraying are strong winds, high temperature and low humidity. Research has found that spray particles are likely to drift further and in higher concentrations when the atmosphere is stable and where there is a surface temperature inversion.

During the night there is high potential for pesticides to drift at damaging concentrations for long distances in light winds (less than about 12km/h) and gentle breezes (less than about 18km/h) when the temperature is cool, humidity is very high and temperature inversions exist.

Field tests indicate that the greatest drift deposits occurred with relatively high wind speeds, coupled with a temperature inversion and spray in the small droplet spectra (about 200μm) (Bird 1995).

Applicators and advisers need to be able to recognise and respond to all weather factors so that they can make timely adjustments to spray practices to minimise the risk of spray drift and maximise the potential for the pesticide to reach the appropriate target.

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