Unprecedented levels of aphids threaten beet crops

Threshold

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.

Virus

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.

Beneficials

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.”