The unprecedented number of aphids being found in sugar beet crops are presenting a serious ongoing challenge to virus control this season, with risk of transmission said to be very high.
The sustained warm conditions have resulted in a continual movement of large numbers of winged aphids, which insecticides are struggling to control.
The British Beet Research Organisation (BBRO) says it has received ‘many questions’ about the situation.
At least 95 per cent of aphids counted in fields are peach potato aphids, the main virus yellows
vector, says BBRO’s head of knowledge exchange, Dr Simon Bowen.
He says: “Numbers are greatly exceeding the threshold of one wingless aphid per four plants, with tens of wingless aphids being recorded on some individual plants.”
In most situations insecticides are giving some level of control, but dry conditions may be reducing their systemic action, he says.
Following the recent emergency authorisation for use of Biscaya (thiacloprid), Dr Bowen says BBRO has seen good control from the new reduced rate of 0.3 litres/hectare, for up to four days post application, but the sheer pressure from aphid numbers is leading to rapid reinfection of crops.
BBRO has currently tested approximately 1,000 winged Myzus persicae (peach potato aphid) from across the four factory areas. The percentage carrying virus is currently 0.4 per cent of the population tested.
Despite this low number, with so many winged aphids being found this year, Dr Bowen says virus pressure will be high.
“Sprays should be applied up until the 16-leaf stage when aphids are found at threshold, although the threshold changes to one green wingless per plant above the 12-leaf stage,” he adds.
However, with variable plant sizes being reported in some fields, he advises growers to keep monitoring, and in such fields treat at the lower threshold value of one green wingless aphid per four plants until all plants are 12 leaves and above.
He also notes that ladybird numbers, which predate the pest, have been notably low this season.
“There are far fewer ladybirds present in crops compared to 2019, although evidence over
recent days suggests their numbers are building.
“It is not clear why this is the case, but the wet winter may have had an impact, or their lifecycle is out of synchronisation with the rapid build up of aphids this year. The good news is that other predators are also now being found.”
DR Mark Stalham, head of NIAB CUF says: “Just with the normal aphid migration, levels of viruses such as Potato Virus Y (PVY), Potato Virus A (PVA) and Potato Leaf Roll Virus (PLRV) - are predicted to be extremely high this year.”
The withdrawal of neonicotinoids and hotter weather has enabled aphid numbers to rise and transmit virus in recent seasons, says Gavin Prentice, technical and procurement manager at Agrico.
He expects to see little if any carry-over of infection as seed is grown to strict protocols regulated by Science Advice to Scottish Agriculture (SASA).
Each growing crop is inspected at least twice in Scotland. Tubers are also inspected before a seed lot is dispatched.
“We have still had a lot of samples tested with absolutely no virus this year,” says Dr Stalham.
“These are from growers that send in seed every year. We also have samples sent in by growers with the hope that they might make the grade. They have not treated the crop as seed with the associated spray programme and it shows. Some of these samples have anywhere between 70-90 per cent virus.”