Armyworm - economic considerations for management

There are many economic and financial implications that need to be considered when choosing a management option. These may include:


Understand the potential yield losses associated with armyworm feeding damage.

  • Potential yield loss is variable and dependent on the size and growth rate of the local armyworm population, the growth stage of the plant at infestation, the effectiveness of natural enemies such as parasitic flies and wasps, predatory beetles and diseases and the potential crop yield.
Assess the costs and benefits of taking preventative action to directly control armyworm.
  • The cost of preventing armyworms should be weighed against the savings from not taking action in the event that no major infestation actually occurs during the year. Costs include the opportunity cost of unnecessarily treating seed or spraying and the biological costs such as also killing natural enemies of armyworms and potential risk of resistance build up. Biological control agents can be important in some years in suppressing armyworm numbers.
Assess the cost and benefits of controlling summer weeds to reduce feed source of armyworm.
  • Spraying out weeds in advance of sowing not only destroys a potential host for armyworm but  will also conserve soil moisture and lessen the probability of disease carry-over such as rust.


Compare the costs, benefits and risks of each management option against doing nothing.

  • What are the likely outcomes of each management option? When the result of treatment is unknown consider the most likely (expected), as well as the worst and best results likely to occur from each treatment option.
  • Observe the spray withholding periods and consider implications for selling grain at harvest.
  • When calculating the cost of non-treatment, assess the potential risk of yield losses and quality downgrades. This will depend on size of the armyworm population and how early the infestation starts or is detected in the crop. Infestation late in the season does not usually have a significant impact on yield. Quality reductions, such as lower hectolitre weights, can also reduce revenue.
  • Compare the costs of ensuring that you allow for the possibility of further treatment.
  • Selection of insecticide may be influenced by the opportunities to control other insects.
  • If applying insecticide simultaneously with other treatments, only assess the marginal cost of the application i.e. the application cost of going over the paddock is not included as it would have been incurred anyway. The marginal cost is the cost of the insecticide and any additional time needed to prepare and apply it.
  • Consider costs and benefits for both ground and aerial application methods.
  • Consider choosing a treatment option where the expected gain is sufficient to offset the downside risk of the treatment. We all have different attitudes to risk when making decisions. The probability (risk) of outcomes can be affected in terms of responsiveness (efficacy), application rates, products, application methods and climatic conditions. The economic calculator can assist with this decision.

Consider risk and associated costs or savings of no treatment or delaying treatment.

  • To avoid unnecessary spraying, monitor for armyworm in several different locations. Numbers are difficult to assess but may naturally reduce due to weather conditions or attack by natural predators.
  • Alternatively, if no action is taken and the armyworm population multiplies the treatment cost and yield losses could be higher.
Consider the potential risk of re-infection and costs of further treatment.
  • The effectiveness of insecticide is dependent on good penetration into the crop. This can be difficult to achieve in high-yielding thick canopy crops, especially when caterpillars are resting under leaf litter at the base of plants.
  • Seasonal weather conditions and length of remaining growing season will affect the likelihood of any follow up treatment being required.

Ignore all previous treatment costs in assessing current management options.

  • Costs associated with previous treatments are 'sunk costs' and should be ignored because they cannot be recovered,  i.e. provided the additional benefit of making the treatment is greater than the additional cost of treatment from here on, then the net return from investing in the treatment is still better than doing nothing.

Undertake a ‘what if’ scenario analysis to see what impact changing variables (e.g. grain price and seasonal conditions) have on the net income.

  • Some variables can influence decision outcomes but are not directly controllable, including fluctuations in wheat price, the value of the Australian dollar and seasonal influences. But they need to be considered, even if we cannot include them directly.


Consider using an integrated pest management system and include a resistance management strategy into your spray program.

  • Include a resistance management strategy into your spray program to reduce the chance of armyworms and other non-target pests becoming resistant. If monitoring indicates the need to spray, then insecticide choice (such as ‘soft’ on natural enemy chemicals) and rotation of chemical groups needs to be considered, along with appropriate spray application.

To assist in assessing the economic risk and financial costs associated with various treatment strategies go to MyEconomicTool

Page last updated: Tuesday, 2 September 2014 - 7:13am