sustainability through science & innovation

the persistence of resistance - why the pesticide treadmill may be picking up the pace

29 Mar 2019

It is well known that pesticide resistance is a global issue. But, do you know where Australia stands in comparison to other grain growing countries? A recently published scientific review led by cesar and The University of Melbourne investigates.

Key Points

  • Pesticide resistance continues to rise
  • The Australian grain growing context and limited available chemistry supporting this trend
  • New Mode of Actions reach the market rarely and slowly
  • Existing and new tools, such as IRMS’s, predictive models, and gene editing will aid continued efforts to limit the evolution of resistance
  • Greater extension and collaboration are necessary to ensure adoption of these tools in future

More and more, pesticide resistance is a topic of conversation on the farm, at grower meetings, and at research updates. Evolution of resistance in multiple pest species, and tighter usage regulations placed on certain chemistries by regulatory authorities have resulted in a reduced pool of chemicals for use by the grains industry. In a nutshell, this is bad news for limiting the evolution of further resistance, as reliance on prophylactic applications, and insufficient practice of chemical rotation across the industry will likely see selection pressures increase.

Resistance is heritable. Continued use of the same Mode of Action over successive generations will favour the selection and reproduction of individuals carrying genes that confer resistance (denoted by blue flag). These individuals will increase over time. Credit: Jessica Lye, cesar

our decision-making environment

Let’s have a look at the context in which grain growers are making decisions about applying pesticides.

  • Growing operations are often large and mechanised
  • The climate is variable, making it difficult to judge the risk of a pest outbreak
  • Export requirements for grain quality are extremely stringent, with zero tolerance for live invertebrates
  • Genetically modified lines for food production are prohibited, reducing options available for pest management

A number of important invertebrate pests of grain crops have evolved pesticide resistance in Australia. These include cotton bollworm, diamondback moth, green peach aphid and redlegged earth mite.

But, what about those minor pests that may become peskier in the future? While it is difficult to pinpoint exactly which pests will evolve resistance, there are a few that you may want to keep an eye on. They include lucerne flea, Balaustium mite, and blue oat mite – all of which are common control targets as they often attack crops at the seedling stage.

Learn more about these pests on our PestNote database.

How much do you know about insecticide resistance management? Take the test and find out! Finish the test with a score of 100% and the fastest time to receive a hard copy iSPY Insect Identification manual!

what insecticides are most used by Australian grain growers?

Insecticides available to grain growers are extremely limited, and in terms of preference on-farm there are a few clear winners in the usage category. For cereals (wheat, barley, oats, rye and triticale), organophosphates and pyrethroids account for >85% of all estimated insecticide applications.

In legumes, the picture is similar, although pyrethroids are by far the most widely used Mode of Action and neonicotinoids are used far less than in cereals. Pyrethroids, organophosphates and neonicotinoids are also widely applied in canola crops within Australia.

You will by now be getting the picture that across the board in the grains industry, options are limited (often much more so than in other agricultural industries, such as horticulture) and it does not look like many new Mode of Actions will be forthcoming in the near future.

Let’s take redlegged earth mite as an example of one pest that is quickly emerging resistance to multiple actives.

This pest has only five unique Mode of Actions registered against it. Farmers are heavily reliant on three of them: organophosphates, pyrethroids and neonicotinoids, the latter of which may only be applied as a seed dressing in the case of ‘redlegs’. Only one insecticide formulations with a new Mode of Actions have been registered against the redlegged earth mite in 15 years!

Limitations in the number of actives available to grain growers and slow uptake of Integrated Pest Management practices will lead to continued selection pressure for resistance. When you have a toolbox full of hammers, everything is a nail! Credit: Jessica Lye

a word about seed dressings

One chemical application practice that has been on the increase is the use of seed dressings, which are often neonicotinoids. Canola seed is almost universally purchased as a dressed seed and in other crops, the threat of crop impacts by pests such as Russian wheat aphid has led to a rise in seed dressing usage.

The decision to use a seed dressing is made months before sowing, limiting opportunities to assess the risk before use. Thus, seed dressings are prophylactic in nature and will apply selection pressure to early season pest populations whether pest pressure is high or not. This means that selection pressure will always be maintained!

Reducing the selection pressure imposed by insecticides will in turn reduce the risk of resistance evolving, will give beneficial invertebrates the best chance of controlling your pests, and may lead to local pest populations regaining sensitivity to chemical controls (being resistant often comes at a ‘fitness cost’ and populations may revert back to ‘wildtype’ if the resistance gene is no longer conferring a benefit).

The speed at which resistance evolves is influenced by many factors, including the rate of reproduction, migration and host range of the pest, proximity of susceptible populations, persistence and specificity of the insecticides used, and the rate, timing and number of chemical applications.

current and new tools for the toolkit

Insecticide resistance management strategies (IRMS’s) are available to the grains industry for several key pests – redlegged earth mite, green peach aphid, diamondback moth, and cotton bollworm. However, uptake of the guidelines and principles in these IRMS’s is slow and has been constrained by several factors: 

  • disharmony between local objectives (e.g. ‘insurance’ sprays) and regional or industry priorities (e.g. reducing chemical applications to minimize resistance)
  • limited awareness of IRMS’s among growers
  • resistance issues in Australia are often not perceived as a priority for farmers

These IRMS’s set out the context of current resistance issues, provide guidance on rotation of chemicals, and emphasise how non-chemical management tactics may be integrated into farming systems. You can find the grains industry IRMS’s here.

Another tool that is beginning to be used for management of resistance is prediction modelling. Models can be used to identify production areas at high risk of local pest populations evolving resistance, or even predict what pest species are more likely to evolve resistance than others. These are a couple of examples for how prediction modelling may be used, but the opportunities for other practical uses go far further.

There are also future opportunities with CRISPR gene editing technology. This technology has recently been used to reverse engineer susceptibility to two different insecticide groups by targeting cytochrome P450 genes, which are commonly involved in pesticide resistance.

where does this leave us?

We are effectively playing a never-ending game of resistance chess, with informed choices in chemical actives and use of other management tactics becoming ever more important in order to stay on the playing field. Resistance issues will inevitably continue to increase. This is despite the overall quantity of insecticides decreasing in many regions of the world. However, there are existing and emerging tools at our disposal, making ongoing management of resistance achievable – as long as you keep your chess practice up!

This article draws on findings in Umina et al. (2019) Escalating insecticide resistance in Australian grain pests: contributing factors, industry trends and management opportunities’, Pest Management Science, DOI 10.1002/ps.5285

The full review can be accessed from Wiley Online.