A cereal pest causing similar symptoms to wheat bulb fly is attacking crops in eastern and northern counties in the highest numbers seen for a decade. But growers are being warned not to rush out and spray.

Yellow cereal fly, opomyza, can easily be mistaken for wheat bulb fly as the larvae are present at the same time, between January and May. Symptoms are also similar, with “deadhearts” developing due to the death of the central shoot.

Opomyza populations are the highest seen for 10-15 years, says TAG’s Dave Robinson. “I don’t know why there are so many- you see some every year, but not at these levels.”

But growers shouldn’t spray until they are sure which fly they are dealing with, he says.

“There is little point spraying opomyza-infested crops as the tiller will already be dead and unlike wheat bulb fly they will not migrate from the tiller they are lodged in – you would just be revenge spraying.”

It is relatively easy to spot the difference by cutting the stem open and examining the pupae, he says.

Wheat bulb fly are 5mm in length with “a chunky maggot-like appearance” and are flat at one end, he says. Opomyza are the same length, but have a more slender appearance and are pointed at both ends.

Lincolnshire, Nottinghamshire, Yorkshire and Northumberland are experiencing the highest numbers. “In the worst patches you could get a 25-30% yield reduction, but across the whole field, this is likely to be no more than 5% or 6%.”

Opomyza often strike where the crop is at its thinnest, which can make the damage look worse, he notes.

AICC agronomist Patrick Stephenson has also seen large opomyza populations in his area of North Yorkshire. “It’s common throughout the county and I’ve seen high levels from Doncaster to Darlington.”

He is advising growers not to treat opomyza and to only spray wheat bulb fly if a sufficient number of tillers are infested and the pupae are not too large.