Label rates best for effective control when managing resistance

Label rates for effective control of weeds, pests and diseases

Latest News
Irrespective of whether treating weeds, insects or diseases, management decisions about agricultural chemical rate should be based on effective control of the target species. PHOTO Chris Preston

Irrespective of whether treating weeds, insects or diseases, management decisions about agricultural chemical rate should be based on effective control of the target species. PHOTO Chris Preston

Aa

The application rate should not be seen as the core resistance-management consideration.

Aa

Growers and agronomists may question whether dose rates have an impact on how likely resistance is to evolve, but resistance comes in many forms and trying to manipulate rates should not be seen as the core resistance-management strategy.

The reality is that using the most appropriate rate for effective control is the best strategy for managing resistance. Label rates have been developed to provide robust and reliable control of the target pest.

The influence of agricultural chemical rate on resistance evolution is complicated by biological differences in the target organisms. As a result, there is no single answer regarding the influence of rate on resistance development.

However, irrespective of whether it is weeds, pests or diseases, management decisions should be based on effective control of the target species.

The application rate should not be seen as the core resistance-management consideration and should never prejudice effective control of the target organism.

Diseases

In many cases the full label rate is the most appropriate rate for control. However, for diseases, the lower rate from the label range of a fungicide can be used in conjunction with a crop variety that has a good disease-resistance rating, because disease pressure will be lower.

There is evidence that using a higher rate than necessary increases the risk of resistance, as removing all of the sensitive individuals provides more opportunity for resistant individuals to dominate the population and hence be the dominant strain colonising the plant.

The application rate should not be seen as the core resistance-management consideration and should never prejudice effective control of the target organism.

Weeds

For weeds, both low and high rates of agricultural chemicals select for resistance. Less-effective rates of herbicides will result in more weeds surviving and bigger weed populations, so robust rates should be used. High rates will typically select for resistance faster in self-pollinated weed species. For outcrossing species, low rates may allow weak resistance mechanisms to be selected and accumulate within the population. When the mutations for resistance are partially recessive, lower doses will control the most sensitive part of the population, effectively leaving a higher proportion of individuals that carry resistance genes.

Insects

For insects, the situation is similarly complex. Insects that reproduce sexually often evolve resistance that involves many genes (polygenic resistance). Of these genes, some are dominant or partially dominant and will confer complete or partial resistance when inherited from one parent. Others are recessive and must be inherited from both parents in order to produce resistance. Then there are insects that reproduce asexually, which are more likely to develop major gene resistance and pass this on to all of their offspring.

Dominance/recessiveness and single/multiple gene inheritance influence whether low insecticide rates will increase or decrease resistance issues. For instance, low rates might increase resistance issues if recessive genes are already present in a population and/or resistance is based on multiple genes. For these reasons, applying insecticides at the full label rate is recommended when managing insect pests that have already evolved resistance.  

Irrespective of whether treating weeds, insects or diseases, management decisions on agricultural chemical rate should be based on effective control of the target species.

More information: Nick Poole, FAR Australia, 0352651290, nick.poole@faraustralia.com.auDr Paul Umina, cesar, 0405464259, pumina@cesaraustralia.comDr Chris Preston, University of Adelaide, 0883137237, christopher.preston@adelaide.edu.au

​The authors acknowledge the assistance of Dr Fran López-Ruiz and Professor Ary Hoffmann.

Common terms in resistance management

  • Agricultural chemical resistance - When a previously effective agricultural chemical fails to control a pest, resulting in field failure.
  • Alternation or rotation - The sequential application of different agricultural chemicals, most typically (although not always) with different Modes of Action.
  • Cross resistance - When the resistance mechanism that makes an organism resistant to one agricultural chemical also makes it resistant to others, often those with a similar Mode of Action.
  • Diploid - An organism that has two copies of each gene (e.g. many insects and weeds).
  • Fitness penalty - Resistant individuals have reduced genetic fitness allowing susceptible individuals to out-compete resistant individuals if the chemical is no longer used
  • Frequency of resistance - The proportion of a population that is resistant in the field.
  • Gateway mutations - The first genetic mutation in a series of steps needed to allow expression of resistance in the field.
  • Haploid - An organism that has one copy of each gene (e.g. most fungi).
  • IPM/IDM/IWM - Integrated pest/disease/weed management. The collection of tools for the control of pests, diseases and weeds (e.g. use of agricultural chemicals, genetic resistance, cultural methods, physical destruction of weed seeds).
  • Label rates - The rates listed on the label. The full rate is the highest registered rate, while lower rates refer to label rates than can be used in some situations (such as low disease pressure, very small weeds or light soil types).
  • Mixture - The simultaneous combination of two or more agricultural chemicals from the same or different Modes of Action.
  • Mode of Action - The mechanism by which the agricultural chemical kills or suppresses the growth of pests or weeds.
  • Multiple resistance - When an organism possesses two or more resistance mechanisms to one or more Mode of Action groups.
  • Multi-site - An agricultural chemical that targets multiple pathways in an organism.
  • Reduced sensitivity - When the sensitivity of the pest to an agricultural chemical is reduced over time. This does not necessarily cause field failure.
  • Refugia - An area where particular conditions have allowed organisms (either beneficial or pests) to survive. The organisms living in these areas then colonise the surrounding crops.
  • Resistance mechanism - The biological process involved in the resistance to a given agricultural chemical. A common resistance mechanism is target site mutation.
  • Resistance monitoring - To actively survey insect, disease and weed populations for a particular agricultural chemical resistance.
  • Selection pressure - The evolutionary force that drives the evolution of resistance within an insect, weed or fungal pathogen population due to the repeated exposure to agricultural chemicals.
  • Single site - An agricultural chemical that targets only one pathway in an organism.
  • Tolerance - When an agricultural chemical has always failed to fully control a pest.

SOURCE: Dr Fran López-Ruiz, Dr Angela van de Wouw, Dr Andrew Milgate, Nick Poole, Dr Paul Umina and Dr Lisa Bird.

Aa