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Insecticide resistance – why monitoring is important

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Insecticides are an important component of integrated pest management programmes, and are critical in maintaining crop yields and quality. However, insect populations can develop resistance to insecticides, hampering long term control.  There are an increasing number of  regulatory restrictions that affect the availability and application of insecticides, and this can limit the options available to growers when resistant pest outbreaks occur.

 

Insecticide resistance – why monitoring is important

Resistance has been found in many important insect pests in the UK, including the peach-potato aphid, Myzus persicae, which damages several arable and horticultural crops, particularly those grown under protection.The abundance and distribution of resistant strains varies over time, and these are difficult to predict without an underlying knowledge of how resistant populations emerge and spread.  Long -term monitoring of resistance in the peach-potato aphid and other aphid species alerts growers to changes in resistant populations and the development of novel resistance, helping them make informed pest control decisions, and improve the sustainable use of insecticides.

Challenge

Why is insecticide resistance a problem?

Insecticide resistance prevents effective control.  Resistance occurs where a genetic mutation in an individual insect prevents an insecticide from having a lethal effect.  This mutation is then passed on from the resistant individual  to its offspring.  As aphids can produce dozens of genetically identical offspring asexually over very short periods of time, a single resistant individual has the potential to create a large resistant population.  Resistance can come at a cost to the individual, for example; peach-potato aphids carrying organophosphate resistance may be less likley to survive the winter, reproduce more slowly, and are less able to respond to natural enemy attacks1. Because of this, aphids carrying resistance mutations are less likely to survive under times of stress compared to  susceptible (non-resistant) aphids. When the relevant insecticide is applied, however, the benefits of being resistant far outweigh these fitness costs, allowing those carrying resistance to increase in numbers and spread through the population. This interaction between susceptible populations, which are fitter in the absence of insecticides, and resistant populations, which are fitter in the presence of insecticide, can lead to fluctating frequencies of resistance occuring across the UK2. Resistance can be managed to some extent by using a range of different insecticide groups, and by integrating the use of chemicals with other forms of control, such as releasing biological control agents and enhancing the role of natural enemies3,4.  As the number of insecticides available is becoming more restricted, it will become even more important to use the remaining chemicals wisely, and this requires a greater understanding of how resistance develops  and spreads.  

Approach

Why is resistance monitored in the peach-potato aphid?

The peach-potato aphid is known to have developed resistance to several important insecticide groups, and is found in a number of arable and horticultural crops, particularly those grown under protection . ADAS is involved in a long term monitoring project led by Rothamsted, in which samples of the peach-potato aphid are collected from crops across the UK every year.  The known mechanisms of insecticide resistance in the peach-potato aphid in the UK are against organochlorines (including DDT, which was banned in the UK in 1984), organophosphates, carbamates (e.g. Pirimicarb), and pyrethriods1.  A further mechanism giving strong resistance to neonicotinoids was found in southern France in 2009, and has subsequently been recorded in Spain and Italy1 but not yet in the UK. The ability of the peach-potato aphid to rapidly increase in abundance in several different crops and to spread plant viruses makes it one of the most important pests in the UK.  It can infest not only potatoes, but also oilseed rape, sugar beet, lettuce, cabbage and other brassica crops,  ornamentals and several other horticultural crops. It is this wide host range, exposing aphids to a multitude of insecticide applications, that drives relatively high selection of resistant individuals in the UK.  Resistance develops faster on crops grown under protection due to the higher temperatures allowing faster aphid population growth, and thus more generations per year than on outdoor crops. The damage the peach-potato aphid can inflict on potential host crops, its dispersal ability, its high reproductive rate, and its high capability to evolve resistance makes it the most important economic pest aphid worldwide1.

How does monitoring populations help?

Monitoring the frequency of resistance in aphids collected across the UK allows a benchmark to be maintained against which changes in resistance can be identified and compared.   Detecting changes in the occurrence of resistant strains enables  the application of effective pest and resistance management strategies.  Crucially, ongoing monitoring can also identify new mechanisms of resistance.  Long term monitoring projects, such as that led by Rothamsted Research,  provide a platform for  further research to improve our understanding of resistance development and spread.  Alongside monitoring samples for the presence of resistance, additional work has helped our understanding of how resistant aphids respond to other forms of control. For example, it has been found that resistant peach-potato aphids  are less able to repel  parasitoid attacks as they  respond less to alarm pheromone, which is used to warn other aphids in the colony of an attack5. Results from these studies are feeding into the development of regional aphid managment strategies.

Outcome

What are the latest results of the monitoring programme?

In 2015 there continued to be an extremely low frequency of the peach-potato aphids with esterase-based resistance to organophosphates.  This is likely due to the low use of these insecticides in the UK, and high fitness costs associated with resistance to these insecticides.  Modified acetylcholinesterase (MACE) resistance to pirimicarb insecticides continues to be common and widespread in M. persicae in the UK.  Knockdown resistance (kdr) to some pyrethroids is rarely found now, however ‘super-kdr’ (a mechanism that confers stronger resistance to pyrethroids) continues to be recorded at a very high frequency.  No samples were found to have resistance to neonicotinoids, suggesting this has not yet spread to the UK from mainland Europe.

Insecticide resistance poses a huge challenge to crop production, especially as increased reliance on fewer active ingredients increases the risk of current resistance mechanisms spreading and new mechanisms of resistance appearing.  While there is growing pressure to reduce chemical inputs on crops, there is no question that insecticides are still an important tool in achieving the yields necessary to feed a growing human population.  By monitoring the presence of resistance across the UK we can better understand the situation, and how we can adapt management to ensure crop yields and quality are protected.

Find out more

The main outputs from the ongoing monitoring project are published on the AHDB website, which also provides weekly updates on regional aphid activity, including that of the peach potato aphid, during key times of the year. View the latest or archive editions of AHDB Aphid News. Guidance is available to advisors, growers and the scientific community through the Insecticide Resistance Action Group (IRAG-UK).

You will find more information in Pesticides under pressure – what will it mean for your business?, or contact Sarah Wynn.

Project Team

For more information on how ADAS can help in the management of insect pests, contact Mark Ramsden.

Pest Management and Entomology

References

  1. Bass, C., Puinean, A.M., Zimmer, C.T., Denholm, I., Field, L.M., Foster, S.P., Gutbrod, O., Nauen, R., Slater, R., and Williamson, M.S. (2014) The evolution of insecticide resistance in the peach potato aphid, Myzus persicae. Insect Biochemistry and Molecular Biology, 51, 41-51
  2. Foster, S.P., Harrington, R., Dewar, A.M., Denholm, I., and Devonshire, A.L. (2002) Temporal and spatial dynamics of insecticide resistance in Myzus persicae (Hemiptera: Aphididae). Pest Management Science, 58, 895-907
  3. Foster, S.P., Denholm, I., and Devonshire, A.L. (2000) The ups and downs of insecticide resistance in peach-potato aphids (Myzus persicae) in the UK. Crop Protection, 19, 873-879
  4. Gentz, M.C., Murdoch, G., King, G.F. (2010) Tandem use of selective insecticides and natural enemies for effective, reduced-risk pest management. Biological Control, 52, 208-215
  5. Foster, S.P., Denholm, I., Thompson, R., Poppy, G.M., and Powell, W. (2005) Reduced response of insecticide-resistant aphids and attraction of parasitoids to aphid alarm pheromone; a potential fitness trade-off. Bulletin of Entomological Research, 95, 37-46

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