It is likely the virus yellows forecast for the sugar beet crop will be ‘high risk’ for a second successive year.
At the beginning of last year’s sugar beet season, the British Beet Research Organisation (BBRO) warned growers of a ‘high risk’ of virus yellows infection in sugar beet crops, with up to 54% of the national crop likely to be showing symptoms in the worst-case scenario.
Thankfully, as the 2019 season campaign draws to a close, it is evident that, overall, this level of infection did not materialise. But there is no room for complacency in the coming season, growers are being advised.
From 1994 through to 2018 sugar beet pest control was relatively straightforward. Growers had the opportunity to use the neonicotinoid seed treatments Poncho Beta (clothianidin + beta-cyfluthrin) or Cruiser Force (thiamethoxam + tefluthrin), which provided control of soil pests and a range of above-ground insects, particularly aphids, for typically 12 weeks.
But in 2018 it was decided, at EU level, that for reasons relating primarily to bee health and soil residues, neonicotinoid seed treatments would be banned.
BBRO head of science Prof Mark Stevens says: “So 2019 was a change point. Emergency authorisations for the seed treatments were sought via British Sugar and NFU, with support from BBRO. Altogether 12 member states allowed their growers to use seed treatments in 2019 but the UK, along with the French, Germans and several others, enforced the ban.”
But did this contribute to the BBRO’s ‘high risk’ forecast for virus yellows last season?
Prof Stevens says: “We issued the virus yellows forecast at the beginning of March last year. We had to take out the neonicotinoid elements and, because of that and the particularly warm weather – we had places recording temperatures of more than 20degC at the end of February last year – the forecast depending on drill date and location was somewhere between 22% and 54% of the crop could go yellow.
“That was the worst-case scenario of having no treatments at all and bear in mind that we did have sprays, but it serves as a reminder of just how much virus yellows there could have been.”
As the 2019 season got underway the only approved insecticide option for control of Myzus persicae (peach potato aphid) for virus yellows control was one application of flonicamid (Teppeki), while a decision was awaited from HSE on an application for emergency use of Biscaya (thiacloprid) for foliar application. And at the time of writing (February 21), that is once again the situation.
Last season HSE approved the application, providing growers with an emergency authorisation for use from April 18, for up to two sprays, to be applied within 120 days.
“We held to that date so it coincided with the aphid migration period to make sure we could use the applications appropriately to limit the impact of aphids,” says Prof Stevens.
Last spring aphids were flying as soon as the crop started to come through the ground, but with the Biscaya emergency authorisation growers had the option of applying three sprays, with a programme of Biscaya – Teppeki – Biscaya advised.
“We were also concerned about managing resistance. You don’t really want Biscaya followed by Biscaya as aphids and particularly peach potato aphid can develop resistance,” adds Prof Stevens.
He also reminds growers that pyrethroid and carbamate insecticides are not approved for control of Myzus persicae but they do have approval for use against other pests, including thrips and flea beetle. However, they should be used with care as they may knock out useful natural aphid predators, he adds.
He cites observations from the Bury St Edmunds beet factory area where pyrethroids were used early to control these pests but also took out beneficial insects.
“We ended up with a scenario where the aphids were resistant, there were no beneficials and in one of the fields there was 70% virus yellows infection. It is just worth bearing that in mind because in 2019 beneficial insects clearly played a major role and we have to look after them going forward.”
In 2019 the Bury St Edmunds factory was hit hard with early peach potato aphid migration, with aphids then moving north to the Newark (Nottinghamshire) factory area.
“Once we got to July it was all over, but we had a record season; we physically identified 39,000 myzus persicae,” says Prof Stevens.
“If you put that into context, the year before it was less than 5,000; 75% were caught in the Bury factory area and local factors had a big impact,” says Prof Stevens.
Of the 10,000 aphids tested only 0.4% were carrying virus.
Part of the extended aphid monitoring programme conducted by BBRO in 2019 included counting the number of winged aphids caught in yellow water pans traps. At Battisford, Suffolk, for example, more than 1,000 winged myzus persicae were caught, starting in early May, with numbers peaking in early June.
“We were also recording the number of green wingless aphids and towards the end of May and again into June we were seeing quite large numbers per plant. The first application of Biscaya at this site was applied on May 23 and then Teppeki was applied on June 7. A combination of few aphids carrying virus and perfect use of products led to very little virus in that field.
“We’ve done that [winged aphid monitoring] at a number of sites and I think we have got tools and technologies available now to help use this data for winged aphids to give an early warning system for timing spray applications. We’re not there yet but it is something we are looking it,” says Prof Stevens.
Not all sugar beet crops received an insecticide. British Sugar 2019 field survey data revealed that while 38% of growers adopted a Biscaya followed by Teppeki approach, 16% of growers did not use any insecticide.
“From that same survey the national incidence of virus yellows was 1.8%, a far cry from the forecast we had given, but that was a worst-case scenario. I think we probably still have a legacy from using seed treatments; that will diminish over time and use of sprays clearly helped.
“But what is slightly unnerving is in 2019, 55% of fields had some level of virus; the previous year was 18% and the national incidence in that year was 0.4%, so we’ve gone from 0.4% to 1.8% incidence, and from 18% to 55% of fields with some level of virus, which is clearly building and which given the current scenario of very little cold weather, this probably means quite a high risk in 2020,” says Prof Stevens.
BBRO will issue its virus yellow forecast for 2020 in early March.