Reducing selection pressure key to resistance management strategies

Common tactics for managing agricultural chemical resistance

Weeds, Pests, Diseases
Pyrethroid resistance in redlegged earth mites is the result of single gene mutation. PHOTO cesar.

Pyrethroid resistance in redlegged earth mites is the result of single gene mutation. PHOTO cesar.

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There are many similarities between management strategies for weeds, insects and fungal pathogens.

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Key points

  • Rotate and alternate chemical Modes of Action (MoA) to reduce resistant organisms.
  • Non-chemical control options, as part of an integrated pest management (IPM) strategy, can reduce resistance.
  • Monitoring allows early detection of resistance and implementation of resistance management, which will increase the life span of available chemicals.

Although there are differences in how resistance evolves and should be managed in weeds, fungal diseases and invertebrate pests, there are some common tactics that can help growers tackle this growing problem.

Resistance to agricultural chemicals is an increasing challenge across the grains sector. All organisms in nature undergo genetic mutations occasionally, but most mutations damage, rather than help, the organism.

When agricultural chemicals are used, any organism that has developed a mutation to resist the chemical is favoured.

This process of evolutionary adaptation allows populations of pests, weeds and diseases to flourish, as they are no longer effectively controlled by particular chemicals.

The only way to slow the rate of resistance development is to reduce the selection pressure. This means that for weeds, diseases or insect pests, we need to reduce and, where feasible, stop using the agricultural chemicals that favour the resistance mutation.

The underlying genetic basis of resistance is the same across all organisms, regardless of whether it is the result of a single gene mutation or multiple genes working together.

The only way to slow the rate of resistance development is to reduce the selection pressure

Pyrethroid resistance in redlegged earth mites and DMI (demethylase inhibitor) resistance in barley powdery mildew both result from a single gene mutation.

Glyphosate resistance in some weeds and anilinopyrimidine resistance in botrytis fungus, which causes chocolate spot in faba beans, are the result of mutations that affect multiple genes.

Resistance based on multiple genes is more likely to evolve in organisms that breed through sexual reproduction, which brings together different combinations of genes (through recombination).

Asexual organisms are less likely to evolve multi-gene resistance, but it has been observed in asexual aphids when multiple mutations happen over time.

Most diseases have both sexual and asexual phases in their life cycle, allowing both multiple and single-gene resistance in the same species.

Fighting resistance

There are a limited number of different agricultural chemical mode of action (MoA) groups we can use to control insect pests, weeds and diseases.

Where we know resistance already exists in some groups, we need to use chemicals with more care to slow the spread of resistance and preserve chemical efficacy. Rotating and alternating between different MoA groups will reduce selection of particular resistance mechanisms.

For insect pests and diseases, limiting the application area will reduce the proportion of the population exposed to the agricultural chemical. This will preserve individuals that are sensitive to chemicals, which can then breed with the resistant population.

Using methods of control that do not rely on chemicals as part of an integrated pest management (IPM) strategy increases the strength of control programs and helps to protect growers from resistance development in pests, weeds and diseases.

Dr Fran Lopez-Ruiz, of Curtin University. PHOTO GRDC

Dr Fran Lopez-Ruiz, of Curtin University. PHOTO GRDC

Cultural practices, such as crop rotation, sanitation practices, such as burning to remove weeds, diseases and any dormant insect pests, and the removal of green bridges to control diseases and invertebrate pests are all key tools that help slow the evolution of chemical resistance.

Correct identification of pests, weeds and diseases, and the use of rapid monitoring strategies to follow population trends and resistance development, are also vital for effective control programs and sustainable resistance management.

These will ensure chemicals are only used when required. Economic thresholds are available for several grain pests and should be used to help guide spray decisions. Following these approaches should ensure chemicals are only used when necessary.

Finally, by conserving biocontrol agents that attack pests, weeds and vector insects that transmit diseases, fewer chemical applications are needed, which reduces the selection for resistance. These approaches are becoming increasingly important within Australian broadacre cropping systems.

Economic thresholds are available for several grain pests and should be used to help guide spray decisions

Early detection of agricultural chemical resistance provides growers with a better chance to implement strategies that reduce the risk of resistance building up and increase the life span of available chemicals.

There are also some important differences that need to be considered when designing resistance-management strategies for weeds, diseases and insect pests.

GRDC Research Codes UM00054, CSE00059, CUR00016, CUR00019, CUR00023, DAW00229

More information: Professor Ary Hoffmann, University of Melbourne, 0408 342 834, ary@unimelb.edu.au;Dr Fran Lopez-Ruiz, Curtin University, 08 9266 3061, fran.lopezruiz@curtin.edu.au

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