Wheat bulb fly risk and how to manage it

Wheat bulb fly can exploit bare ground after harvest. Ellie Dearlove examines the potential impact on pest risk this year and an integrated pest management approach to help mitigate it.

Most UK cereals have now been cut, with the dry conditions allowing many to complete harvest two weeks or more ahead of a ‘typical’ year.

This has implications for the following crop, including wheat bulb fly risk. While dry weather can suppress fly activity, recent rainfall in several regions has boosted soil moisture, making perfect conditions for egg laying.

Wheat bulb fly attacks all cereals, except oats. It mainly lays its eggs in bare soil during late summer (July and August), which includes exposed soil after combining (which may be exposed for longer in the egg-laying period following an early harvest) and on soil between rows of crops (such as in sugar beet).

It particularly likes freshly cultivated soils and fallow fields, with organic soils often recording the highest egg densities.

Despite these risk factors, long-term data shows that wheat bulb fly pressure has been low in recent decades, especially in drier summers.

However, local conditions (soil type, cultivation method, crop sequence) after an early harvest can still tip the balance towards higher risk, especially in the eastern and northern counties of the UK.

So, being proactive pays off, especially when chemical control options have dwindled.

A recent AHDB letterbox query asked us:

“What non-chemical control options are there to control wheat bulb fly?”

In Great Britain, there is only one chemical control option – the seed treatment (cypermethrin), which is only effective for crops sown after the end of October and needs to be drilled at depths of 2.5–4.0 cm (it is not recommended for broadcast seed).

Aside from a seed treatment, there are several non-chemical control approaches.

As ploughing buries eggs, it can reduce hatching success. In min-till or direct-drill systems, rolling after drilling improves seed-to-soil contact and encourages good establishment and strong plant populations that tolerate pest damage better.

Usually, eggs hatch from late December to February. A single wheat bulb fly larva can attack multiple shoots and plants.

As a result, plants with few shoots are the most vulnerable (and may even be destroyed). As early drilled crops have older, well-tillered plants (with more excess shoots) at the time of egg hatch, they stand up better to larval feeding.

Each day earlier a crop is sown potentially increases the maximum shoot number by about 4, 5 or 6 shoots/m² for 100, 200 or 400 plants/m² respectively (according to research in 2018).

Note: wheat bulb fly risk is also elevated in early sown spring cereal crops (before April).

The AHDB wheat growth guide suggests a target population of at least 150 plants/m² by GS31, if drilling in early October (and more if there is a high weed burden).

Sowing about 350 to 400 seeds/m² should be sufficient to hit this target. Although each additional plant increases the maximum shoot number per m², the extent to which this is seen is reduced for later sowing dates.

Therefore, increased seed rate is more important for later sown crops.

Eggs are often laid in the bare soil associated with potatoes, sugar beet or vegetables, so rotating away from these higher-risk scenarios and maintaining crop cover can make fields less attractive for egg laying.

Ground beetles, small rove beetles and other beneficial insects help reduce wheat bulb fly populations. Maintaining hedgerows, beetle banks and flower strips can encourage these natural allies.

A well-nourished crop (good access to nitrogen and phosphorus) can produce stronger plants. As access to early nitrogen promotes rapid and vigorous tillering, it will help crops tolerate and compensate for larval damage.

Early nitrogen applications in the spring can also promote recovery in affected crops.

There is still tremendous uncertainty about how the numerous factors combine to impact wheat bulb fly risk.

For example, risk increases with the number of August rain days and decreases with warmer summer temperatures (especially in July).

July 2025 was the UK’s fifth warmest July since records began in 1884. Summer rainfall is also tracking far below average across the UK, with England the driest.

With so much complexity, layering integrated pest management (IPM) strategies is arguably a good insurance policy.