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09 Jun 2022

Keeping aphid enemies in sight

As featured in Arable Farming Magazine

Keeping aphid enemies in sight

by Arable Farming Magazine April issue

Aphid populations can evolve to resist the insecticides available to control them and now the future of monitoring them is uncertain.

Resistance to insecticides and the loss of insecticidal actives through tighter regulation pose well-documented challenges to the effective control of aphid crop pests.

And now climate change which could mean aphids present a threat to crops over a longer duration must be added to the list, according to a leading entomologist.

Insecticide resistance in aphid populations is constantly evolving, with the latest concern for UK growers the identification of neonicotinoid-resistant peach potato aphids (myzus persicae) in sugar beet crops in Belgium, says Rothamsted research entomologist Dr Steve Foster.

We know in mainland Europe that a strong resistance has evolved and this is related to a target site mutation, which means neonics would no longer work against those resistant forms.

The good news is we have not seen any evidence of those forms in the UK so far, Dr Foster says.

From a UK perspective there is, however, more reassuring news in terms of other insecticide actives, with no evidence of reduced sensitivity or resistance in peach potato aphid to Teppeki (flonicamid), Movento (spirotetramat) used in brassica and root crops and Sequoia (sulfoxaflor) used in some protected crops.

On the other side of the coin, we have seen consistent and widespread resistance to pyrethroid insecticides in this species, Dr Foster adds.

Once resistance has been identified, aphids can be screened to determine what is causing the resistance.

Diagnostics Once you have evidence of a resistance mechanism you can then develop diagnostics to allow you to screen the aphid population to find out what its resistance genotype is.

In the past, weve been looking for MACE, which is a target site mechanism associated with resistance to pirimicarb.

We are also able to look for kdr knockdown resistance and super-kdr, which are mecha nisms which confer moderate and very strong resistance to pyrethroids, says Dr Foster.

Resistance to insecticides occurs when there is a large population, in this case of aphids, in which genetic mutations are occurring and these mutations are being selected for in the presence of insecticides.

For this reason, it is important to monitor aphid populations so that up-to-date data is available on the frequency and type of resistance mechanism present in the population.

Kdr resistance provides a good example.

It has been a bit of a rollercoaster ride with kdr.

What you see in certain years does not necessarily affect what you are going to see in the future.

For example, in 2012/13 we didnt find any myzus persicae carrying this mechanism, says Dr Foster.

So-called super-kdr was first discovered in the UK in 2012 and has been common in the UK aphid population since then, he adds.

Interestingly, if the aphid carries super-kdr, it also tends to carry MACE, so possibly MACE is maintained in the population, not because it is conferring resistance to a compound that is no longer used, but because it is hitch-hiking alongside super-kdr the aphids are carrying both mechanisms together, says Dr Foster.

The UK peach potato aphid population is considered to be made up of ‘super clones, where the population is composed of a limited number of dominant clones.

The aphid population of M.persicae in the UK tends to be asexual, made up of super clones and primarily when you go looking for the super kdr and MACE clone genotype using a DNA fingerprint-type approach you find the population is very much made of this one type of aphid.

Essentially the diversity in the population in the UK is very limited; a few lines have done very well, in this case probably because of their resistance to pyrethroids.

Following the loss of neonic seed treatments, growers are now reliant on pyrethroid sprays for control of the three main aphid vectors of BYDV in cereal crops the grain aphid (sitobion avenae), the bird cherry-oat aphid (rhopalosiphum padi) and the rose-grain aphid (metopolophium dirhodum) so keeping track of the resistance status of these species is essential.

In work funded by AHDB and undertaken by Rothamsted Research in conjunction with ADAS, cereal aphids are being screened to determine the level of resistance or sensitivity within the UK population.

Moderate resistance What we are finding is in the case of S.avenae we have another super-clone, like in M.persicae, and this is called SA3.

This clone carries kdr, which gives moderate resistance to pyrethroids not strong resistance.

In this case it is in the heterozygous form, so the diploid aphid only has one copy and interestingly we have never found a homozygote out of the thousands of aphids that have been screened possibly because homozygotes may carry a fitness cost.

The and 2020, with various response ratios, importantly none of these response ratios were above and beyond what was seen for the standard kdr clone, which means there was no evidence of any increased selection or any increased resistance in the grain aphid.

This work also revealed that in the York area in 2020, there was a high frequency of grain aphids carrying kdr.

The data for 2021 is not yet available.

There is no recent evidence of reduced tolerance or resistance in the UK bird cherry-oat aphid population, so pyrethroid insecticides should work well against this species.

However, one species in which there have been new reports of poor control with pyrethroids is the willow-carrot aphid, which is a major pest of carrots, celery and parsnips.

Based on samples collected going back to 2018, the suggestion is there is a problem, particularly at the field rate, says Dr Foster.

We are getting survivorship of these aphids.

There is strong evidence of resistance, even to the level that would compromise pyrethroid sprays aimed at this particular aphid pest.

Resistance and the loss of insecticides are continuing causes for concern as pest pressures look set to accelerate.

We are losing compounds, not only through resistance as we see with the willow-carrot aphid but also due to legislation, as we saw with the loss of neonic seed treatments in all outdoor crops, although there is a derogation to allow the use of neonics in beet in this country and that is triggered by predictions made by the Rothamsted Insect Survey.

Climate change is set to bring additional challenges with cereal aphids present in and around crops for longer and transmitting viruses over an extended period, particularly in the autumn because of warmer weather, adds Dr Foster.

But while monitoring and tracking of aphids is vitally important, the resistance monitoring work carried out by the Rothamsted Insect Survey is currently in jeopardy due to the withdrawal of some of its funding.

Monitoring could end as soon as May this year if efforts to secure additional funding are unsuccessful, says Dr Fresistance response we have is 35-fold, which is on the edge of conferring resistance to a full field rate, but it is not dramatic like super-kdr, which would give a resistance ratio of thousands, adds Dr Foster. Of five samples collected in 2019

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