Initial egg count results from AHDB’s autumn wheat bulb fly survey suggest it is likely to be another relatively low-risk year for the pest.

Conducted by ADAS, this takes soil samples in September from 30 fields prone to WBF attack (split equally across sites located in the East and North of England) and calculating the number of wheat bulb fly eggs per square metre.

So far, results have been analysed from 14 sites – 10 located in the East of England and four in the North of England – and egg counts at five exceeded the 100/m2 seed treatment threshold, which applies to late-sown, November to December, winter wheat crops.

Those sown before November are unlikely to benefit from seed treatment, as they have more time to tiller and are better able to withstand WBF attack. Seed treatments also lack persistence to protect such crops. For later winter and spring-sown crops seed treatments should be considered, if eggs are present.

Although egg numbers are higher than 2017, the overall picture is of relatively low risk. Only one site had significantly more than 100 eggs/m2 – a potato field in Cambridgeshire which had 322 eggs/m2, said AHDB.

AHDB has also published a final report that looked at integrated pest management of this pest. WBF lays eggs in bare ground during the summer and larvae hatch during winter, enter plants and kill shoots. The potential yield loss depends on the shoot population in winter, the size of pest population and how much damage an individual larva can cause.

Modelling work showed that variation in the maximum shoot number had a large effect on the chance of yield loss, because crops that produced excess shoots were able to sacrifice them to the pest without affecting yield.

Five winter wheat field experiments, which used combinations of sowing date, seed rate, variety and insecticide treatments, conducted across two growing seasons (2015/16 and 2016/17), were used to test this model.

The model performed reasonably well. Crops sown early and at high seed rates produced more shoots, but the model was not able to deal with site-specific factors that limited tillering (eg soil capping).

Low pest pressure also made it difficult to test whether WBF prevalence can be reliably estimated through counts of adult flies caught in water traps. Charlotte Rowley, who manages pest research at AHDB, said: “Counting adults in water traps, before eggs are laid during the summer, would remove the need for laborious, slow and more costly egg counts from soil samples.”

The research also improved understanding of the relationship between WBF egg numbers in the soil in the summer and the number of shoots killed in the winter. AHDB will update its guidance next year after this research has been digested.