Wheat bulb fly risk generally low, according to 2023 pest survey

AHDB’s survey of wheat bulb fly suggests that pest pressure is generally low for harvest 2024 crops. Jason Pole examines what this means for management.

The annual indication of pest pressure is based on the number of wheat bulb fly eggs in soil samples taken from 30 commercial fields (at elevated risk of economic damage) in the autumn.

Siobhan Hillman, who manages pest research at AHDB, said:

“In general, wheat bulb fly pressure remains low, as it has done for over a decade. However, even in low-risk years, it is not unusual to detect higher-risk sites, with two sites in the high-risk category in the 2023 survey.”

Despite the relatively low risk, nine of the sites still had egg populations at levels that could justify the use of seed treatments in late-sown winter wheat crops (above 100 eggs/m²).

• Ten were in the low-risk category (<100 eggs/m²)
• Three were in the moderate-risk category (101–249 eggs/m²)
• Two were in the high-risk category (250–500 eggs/m²)
• None were in the very-high-risk category (<500 eggs/m²)
• The average egg count was 81 eggs/m²
• The highest egg count was 370 eggs/m² (following sugar beet at a site in Cambridgeshire)

• Eleven were in the low-risk category (<100 eggs/m²)
• Four were in the moderate-risk category (101–249 eggs/m²)
• None were in the high-risk or very-high-risk categories
• The average egg count was 72 eggs/m²
• The highest egg count was 123 eggs/m² (following potatoes at a site in North Yorkshire)

• Early-sown wheat crops (before November) are unlikely to benefit from seed treatments*
• For late-sown wheat crops (November to December), consider seed treatments where populations exceed 100 eggs/m²
• For very late-sown crops (January), consider seed treatments** irrespective of the population size (unless no eggs are present)

The latest survey results are similar to 2022. Once again, about one-third (9/30) of sites were above the seed treatment threshold for late-sown wheat crops.

Notes:

*Seed treatments lack the persistence to offer full protection to crops sown before November. Excess shoot production also makes these crops more resilient, although egg populations greater than 250 eggs/m² may result in economic damage.

**Do not sow treated seed after 31 January (as this is defined as the end of the winter period by CRD).

For further information on wheat bulb fly risk and the latest survey results, visit ahdb.org.uk/wbf

The survey process is labour-intensive and requires bulky extraction equipment and taxonomic expertise for egg identification. As a result, it is costly.

At each sampling site (about 4 ha), 10 soil cores are taken from each half of the field. Typically, each core weighs 1kg (a standard bag of sugar), so about 20 kg of soil is extracted at each site.

A series of sieves is used to remove the larger components from each sample. The remaining sample is placed in a saturated magnesium sulphate solution, which causes the eggs to float (due to their density).

The eggs are collected on filter paper and the species identified under a microscope (by looking for characteristic features, such as egg size, shape and texture).

Due to the costs associated with the physical egg counts, the project research team has evaluated and refined a computer model to help predict pest risk.

This model now accounts for 70% of the variability in egg numbers and has successfully predicted risk categories but only in low-risk years based on regional averages.

As there is large site-to-site variation within regions, the project will continue to improve understanding of the numerous factors that influence risk.