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Preparing to use pesticides

Learn about the choices you will need to make and understand why spraying at the right time, and in the right weather, matters.

Make the right choices

To get the best control of your pest you need to apply:

  • the right pesticide product at the right dose at
  • the right time using
  • the right pesticide spraying technique with
  • the equipment properly set up and calibrated.

This will help you to get good coverage on the target surfaces.

Once you have made the right choice of pesticide and dose, you are 40% of the way towards getting full control of your pest. Discuss with your local, trusted expert which pesticide to use and at what rate.
The next 20% of control will depend on when you spray: know how the pesticide controls the pest and when the pest is most likely to be controlled.
A further 20% of control will depend on 'fine tuning' your spraying method; key factors are spray mix/water volume, spraying speed and drop size.
The final 20% of control will depend on setting up your sprayer properly.

Choosing the right pesticide is thus only the first step. You will also need to know about timing and to understand about the links between spray mix/water volume, nozzle type, nozzle size, nozzle pressure, flow rate, drop size, spraying speed and the distance of the nozzle from target surface and what that means when setting up your sprayer.

As we work through the knapsack spraying process, this website will help you make the right choices, one step at a time. Once you know the pesticide you will use and have your product label, the first step is to find the right time to spray. Here we explain why that matters.

 

Finding the right time to spray matters

Spraying your pesticide too early can waste spray. If you spray herbicides too soon, you may not control the weeds that have yet to emerge.
Or if you spray insecticides too soon, you may not control the insects that have yet to arrive.
Spraying your pesticide too late can make it harder to control the pest. Growing weeds will compete with the crop for nutrients, sunlight and water; the crop will grow more slowly and there will be less to harvest. Weeds will flower, produce seeds and compete with future crops.
Insects can mature, multiply and spread to other crops. The pests can cause more damage and may be less well controlled by the pesticide.
Delay can also make it harder to spray your pesticide. The crop canopy can become more dense. This makes it difficult to get enough spray past the top leaves of the crop and onto weeds
or onto insects on lower leaves.

Know the growth stage of your crop

Many pesticides only have full effect at a key stage in the life of the pest and/or crop. For example, when:

  • the weed is at an early growth stage; or when
  • the level of disease or the number of insects is low.

You will need to consult your pesticide label to find if there are any key stages to be aware of. If there are, you will need to estimate the dates when your crop and pest will be at that key stage and ready to spray. You will also need to read your pesticide label to find out if there is:

  • a witholding period (when the treated area may not be entered), and/or
  • a harvest interval during which the crop may not be harvested.

These may restrict your 'spraying window' by limiting the dates between which you can spray.

  • Do not spray any plant that is in flower.
  • Do not allow drift to land on any plant that is in flower.

Spraying in the right weather matters

Rain, heat, cold and wind speed can affect how well your pesticide gets to and remains on your target surfaces.

Pesticide that fails to reach the target surface, or runs off, is wasted. It not only wastes your money but it may also pollute the soil or ground or surface water.

Pesticide that drifts on the wind may contaminate you, any people or animals nearby, or other plants of value.

Rain: If you spray pesticide onto plants that are wet, the spray and the rain may pool and run off the leaves.
If it rains after you spray, the rain may wash the pesticide off the plants.
Temperature: Some pests are not active if too hot or too cold but some pesticides need pests to be active in order to work. This means that the temperature has to be within a certain range for the pesticide to control the pest.
Other pesticides may not work at all in very hot or very cold weather and some may evaporate.
Wind: If the wind is too strong, the spray may drift away from target surfaces and out of the treatment zone. It may also disturb the spread of spray drops on the plant.

Assess the wind to avoid spray drift and inversions. Do not spray if the wind speed is too high or if the air is still. Often it is less windy in the early morning and in the evening.

Judge safe wind speeds by using wind socks. The ideal wind speed is 3.2 - 6.5 km/h (1.5 MB .pdf). This is a light breeze which is strong enough to feel on the face or rustle leaves. Do not spray if the wind speed is less than 3 kph: this is calm to light air where leaves are still and any smoke rises vertically. Do not spray if wind speeds are above 20 kph: this is moderate breeze where small branches and leaves move constantly in the wind.

Check your pesticide label for advice on the best weather conditions before, during and after spraying. Check the weather forecast for the best time to spray​.