Chemical control

When confronted with an above-threshold pest population, chemical intervention is often needed, however unnecessary spraying or choosing the wrong pesticides can:

  • Flare secondary pests
  • Hasten the development of pesticide resistance
  • Contaminate the harvested product
  • Increase operating costs
  • Reduce profitability.

Both short and long term impacts must be considered when selecting an insecticide, even in a relatively short duration crop.

Things to consider when choosing an insecticide

Is the product registered?

  • The use of insecticides not registered in a particular crop is illegal and can jeopardise domestic and overseas markets

Efficacy against the target pest

  • Ideally insecticides should provide the desired level of control with  minimal negative side effects (e.g. development of resistance or flaring secondary pests)
  • Efficacy is often judged by the percentage kill and speed of kill. Contact insecticides have a rapid knockdown effect, but insecticides that rely on ingestion by the pest may take days before maximum kill is achieved (e.g. biopesticides such as Bt or NPV)
  • The stage or size of insects  influences efficacy – larger insects are generally more difficult to control
  • Level of residual control varies; from very little to weeks (e.g bifenthrin for RLEM control) under favourable conditions
  • Application affects efficacy e.g. onto the insect or the plant surface.

Susceptibility of the crop to pest damage

  • If the crop stage tolerates considerable damage, there is no need for 100% pesticide efficacy. In such a scenario, a 70% efficacy product such as NPV (for Helicoverpa) would suffice. This approach ensures that beneficial insects are conserved, and more effective products are reserved for later crop stages more susceptible to pest attack.

Impact on natural enemies

Insecticide Resistance Management Strategies (IRMS)

  • Developed for regional areas or can be noted on the label. They usually include restrictions on the maximum number of sprays per crop, per season for some insecticides, rather than on the timing of applications.

Ability to control multiple target species

  • Selecting spray mixtures or products with dual or multi-pest activity may be important where more than one pest species requires control
  • Before mixing pesticides, always check that products are compatible.

Withholding periods and insecticide residues

  • Some products have long withholding periods (WHPs)
  • Harvesting a crop before the WHP has elapsed can increase the risk of exceeding minimum residue levels for particular markets
  • The presence or risk of residues may affect the marketability of the harvested product. Excessive residues can jeopardise overseas markets, especially if residues are from unregistered products
  • Be aware of regulations regarding the feeding of contaminated crop residues to stock. Export slaughter intervals and related periods are not generally shown on the product label, and are best obtained from the manufacturer or SAFEMEAT.

Toxicity to the environment and humans

  • Many older products, particularly organophoshates (e.g. chlorpyrifos) and carbamates (especially methomyl) are extremely toxic (schedule S7) poisons, and should be handled with caution
  • Users have a community and industry responsibility to minimise environmental, animal, surrounding crop and human contamination.


  • The cheapest is not always the best – or the cheapest (in the long run). For example, synthetic pyrethroids are cheap, but the combined impacts of insecticide resistance (especially in Helicoverpa armigera in northern regions) and flaring of secondary pests, can lead to the need for additional sprays and costs
  • In many cases, a single application of a more expensive but more effective and selective option will provide the best return on investment.
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