Low-to-moderate wheat bulb fly risk, says autumn survey

AHDB’s autumn survey of wheat bulb fly indicates the general risk trends for this potentially serious cereal pest. Sue Cowgill, who manages pest research at AHDB, explores the latest results and how a lower-risk year can mask higher-risk pest pockets.

At present, chemical control for this pest is limited to seed treatments. Only effective in certain situations, our wheat bulb fly survey raises awareness of when they should be considered. And, in line with all integrated pest management (IPM) approaches, targeted treatment requires an understanding of the components of risk.

For wheat bulb fly, there are two broad risk components:

1. Pest pressure – the number of stem-boring larvae (January to March).
2. Crop tolerance – the number of stems (each larva attacks multiple shoots, so crops with more shoots are more tolerant).

The seed treatment thresholds reflect these components.

• Early-sown wheat crops (before November) are unlikely to benefit from seed treatments, which lack the persistence to offer full protection. Excess shoot production also makes these crops more resilient – although populations greater than 250 eggs/m² may result in economic damage
• 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)

Notes: Drill treated (cypermethrin) seed at the recommended depth of 2.4–4.0 cm in a firm, even seedbed in the autumn/winter. Treated seed must not be sown after 31 January, as this is defined as the end of the winter period by CRD.

Conducted by ADAS, the survey involves taking soil samples in September from 30 fields prone to attack* (split equally across eastern and northern England) and calculating the number of wheat bulb fly eggs per square metre – acting as a proxy for eventual larval pressures.

*The pest-prone fields are selected based on the prior crop in the rotation. Adults prefer to lay eggs (July to September) on bare soils, such as between row crops or in freshly cultivated ground.

Of the 15 sites assessed:

• Eleven were in the low-risk category (<100 eggs/m²)
• Three were in the moderate-risk category (101–249 eggs/m²)
• One was in the high-risk category (250–500 eggs/m²)
• None was in the very high-risk category (<500 eggs/m²)

The highest egg count in this region was 253 eggs/m², following vining peas at a site in North Yorkshire. The average egg count was 95/m².

Of the 15 sites assessed:

• Nine were in the low-risk category (<100 eggs/m²)
• Two were in the moderate-risk category (101–249 eggs/m²)
• Three were in the high-risk category (250–500 eggs/m²)
• One was in the very high-risk category (<500 eggs/m²)

The highest egg count in this region was 582 eggs/m², following beetroot at a site (organic soils) in Norfolk. The average egg count was 143/m².

Overall risk has been relatively low for the last 10 years. Although the 2021 results make it one of the higher-risk years in this period, it is still below the long-term (1984–21) average. In 2021, 17% of sites were above 250 eggs/m² – the long-term average is 19%.

For late-sown wheat crops (November to December), seed treatments should be considered where populations exceed 100 eggs/m² – 40% and 27% of sites were above this threshold in eastern England and northern England, respectively.

The influence of soil type is also assessed at the sites in eastern England. Results suggest that egg counts tend to be higher in organic soils than mineral soils.

Even in the lowest-risk years, it is not unusual to detect higher-risk sites. As a result, nothing can substitute for an in-field assessment of risk.

Counting wheat bulb fly eggs is labour-intensive, requires the use of bulky extraction equipment and taxonomic expertise for egg identification. It can only be undertaken by a specialised laboratory. For these reasons, we have invested in research to improve risk modelling.

The models look at the two risk components outlined earlier – pest pressure and crop tolerance. Although the work has improved the prediction power of the models, further work is required to make them sufficiently accurate for commercial use. Pest, crop and environmental interactions are notoriously difficult to predict.

For full survey results and information on the risk-modelling research, visit our autumn wheat bulb fly survey page.