Stewardship: Reducing the risk of resistance

Stewardship refers to protecting the long-term effectiveness of the chemicals and technology used to control pests and weeds in the Australian cotton industry.

Resistance is an outcome that may result from exposing pest or weed populations to repeated selection pressure from long insecticide or herbicide use. Genes for resistance naturally occur at low frequencies in most pest and weed populations. Once a selection pressure is applied, such as an insecticide or  biotechnology trait, susceptible individuals are removed which over time can increase the proportion of remaining resistant individuals in a given population. As the proportion of resistant survivors increase, these are more likely to interact and produce resistant offspring leading to a snowballing effect.

With continued selection, the proportion of resistant individuals may increase to the point that the effectiveness of the toxin is observed to fail in the field.


What do you need to know?

Insect Resistance Management Strategy (IRMS)

The cotton industry has implemented an Insecticide Resistance Management Strategy (IRMS) to manage the risk of resistance of all major pests in cotton including SLW, aphids, mites and Helicoverpa spp. The IRMS is applicable to both conventional and Bt cotton, is updated annually and printed in the Cotton Pest Management Guide. It is also available to download directly here.

The results from insecticide and miticide resistance monitoring programs each season are used to identify any field-scale changes in resistance levels. With industry consultation, these results are used to inform changes to the IRMS.

The IRMS is split into two regions: Northern and Central/Southern, reflecting the different growing seasons from Central QLD through to southern NSW.

The IRMS aims to minimise selection pressure across consecutive generations of key pests, using a combination of tactics:

  • Rotation of chemical groups to ensure non-consecutive use of chemicals
  • Maximum use or application frequency restrictions of individual products and chemical groups
  • Product application windows, defined by pest life cycles and crop growth
  • End of season crop destruction and pupae busting to destroy resistant individuals.
  • Maintenance of farm hygiene to reduce resistant pests carrying over between seasons.

Biotechnology

Since the introduction of the first biotechnology trait (Ingard®) in Australian cotton in 1996, biotechnology has become a central component for cotton production. Today virtually all cotton planted in Australia contained at least one biotech trait (either Bt or herbicide traits).

Currently there are two broad classes of cotton biotechnology traits which are approved and available in Australian cotton varieties providing either insect protection or herbicide tolerance, or in varieties which are ‘stacked’ with a combination of both traits.

Bt cotton

Bollgard® 3 cotton, a third generation of Bt cotton, was released in 2016/17, expressing three proteins (Cry1Ac, Cry2Ab and Vip3a) providing excellent control of Helicoverpa. The stack of toxins each with a different mode of action provides an effective mechanism for reducing the likelihood of resistance when combined with other tactics as part of the Bollgard® resistance management plan (RMP)

The growing of transgenic Bt varieties has allowed Australian cotton growers to reduce insecticide use by more than 90 percent. However, the potential development of resistance is an ever present threat that requires effective technology stewardship to ensure technology longevity.

Screening has revealed that baseline frequencies of resistance in the Helicoverpa population to the proteins expressed in Bt cotton (Cry1Ac, Cry2Ab and Vip3a) were higher than originally anticipated. The continued efficacy of Bt cotton is therefore dependent on the effective implementation of the Resistance Management Plan (RMP).

The RMP for Bollgard 3 is based around five key elements that impose limitations and requirements for management. These are the mandatory growing of refuges, control of volunteer and ratoon plants, a defined planting window, restrictions on the use of foliar Bt, and pupae destruction. The combined interaction of all these elements effectively slows the evolution of resistance.

Planting windows

The aim of planting windows is to confine crop development and maturity to limit the number of generations of Helicoverpa spp exposed to Bt cotton each season, restricting the selection pressure over time.

Imposing a start and end to planting date is especially important in warmer or tropical regions where pupae do not necessarily enter diapause over the winter and where there is no climatic restriction for when sowing could theoretically occur.

Mandatory refuges

Refuge crops are planted to generate significant numbers of susceptible moths that have not been exposed to the Bt proteins in Bt cotton. Essentially these areas provide a nursery for Helicoverpa.  

The options for irrigated Bollgard 3 refuges are 100 percent sprayed conventional cotton, 5 percent unsprayed conventional cotton or 2.5 percent pigeon pea (relative to the area of Bollgard 3 cotton grown). Some variations on refuge requirements exist for central Queensland to accommodate the fact that cotton can be sown throughout the 5 month planting window.  In recent years almost 70 percent of refuges have consisted of pigeon pea. 

Refuge crops must receive adequate nutrition, irrigation, and weed and pest management (excluding Helicoverpa sprays) so that they remain attractive to Helicoverpa, attracting females to lay eggs in the refuge.

Volunteer and ratoon control

The presence of volunteers within a refuge diminishes the value of a refuge, as some of the moths emerging from that refuge have had exposure to the Bt proteins, potentially leading to an increase in the frequency of resistant individuals in the population.

The good farm hygiene practice of removing all volunteers in and around cropping areas is not only important in removing disease and pest carryover hosts, but also in reducing the resistance risk to Bt technologies.

Pupae destruction

Helicoverpa larvae enter a diapause phase (hibernation) in the soil as temperatures begin to cool and day length decreases in early autumn, allowing the pest to survive in winter months.

Cultivation between seasons, during the dormancy phase, is an effective way of preventing diapausing pupae emerging as moths that may be carrying resistance alleles from the previous season. This prevents them from contributing offspring to the population of the following year.

In Central QLD, due to the warmer temperatures and smaller changes in day length, Helicoverpa pupae produced late in the season are less likely to go into diapause, making pupae busting less effective. Late season trap crops are used as an alternative, and are timed to be at their most attractive after cotton has cut out. Once the cotton has been harvested, the trap crops are destroyed and cultivated to kill the remaining larvae and pupae.

In southern regions, the use of attract and kill technology (Magnet®) to target the last generation of moths that would oviposit eggs that would hatch to become the last larvae of the season that go on to enter diapause has also become an option within the RMP. This has been done acknowledging the difficulties encountered in some seasons with implementing pupae busting due to wet winter conditions. Consult the latest advice from Bayer for more information.

Evaluating the effectiveness of the RMP

Bayer Australia conducts a monitoring program of field populations of moths for resistance to Cry1Ac, Cry2Ab and Vip3a proteins. 

The results are used to make decisions about the need for modifications to the RMP from one season to the next to manage resistance.


So, what should you do on your farm?

  • Always consult the IRMS (found here and in the Cotton Pest Management Guide) when making spray decisions, even in Bollgard II cotton.
  • Always refer to the RMP (for either Bollgard II or Bollgard 3 cotton) for full details on how the RMP should be implemented on farm (found in the Cotton Pest Management Guide).
  • Manage refuges so that they are attractive to Helicoverpa throughout the season.
  • Destroy all ratoons and volunteers at the end of the season, both in field and elsewhere on the property.
  • Remember the bigger picture. Effective stewardship achieves long-term gains for the whole industry.

For more information on stewardship visit the myBMP Pesticide Management and Biotechnology modules.


Where should you go for more information?