Cereal crops are at the lowest overall risk from wheat bulb fly (WBF) since monitoring began, according to findings from the annual AHDB autumn survey of this pest.

Conducted by ADAS, the autumn survey involves taking soil samples in September from 30 fields prone to WBF attack (split equally across sites in England, located in the East and the North) and counting the number of WBF eggs.

Historically, the threshold of 250 eggs/m² has been used to provide a measure of overall risk. For the first time since monitoring began in 1984, none of the 30 soil samples was found to exceed this threshold (see Figure 1).

Despite the low risk, some sites sampled had egg levels above the seed treatment threshold relevant to later-drilled crops, with the risk higher at sites located in the north of England.

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Figure 1. The proportion of fields sampled each year exceeding the threshold of 250 eggs/m²

Good progress was made with winter wheat harvest, despite unsettled weather, and 90 per cent of the crop had been harvested by the first week of September, which is ahead of the five-year average.

Report author, Dr Steve Ellis, ADAS, said: “It’s possible the good progress with harvest limited the opportunity for the fungi that adult wheat bulb fly like to consume to develop in cereal ears.

“When food sources are restricted, we know the females produce fewer eggs and this could have contributed to the low-risk year for this pest.”

Treatment thresholds

Due to recent withdrawals, chemical options for control of WBF are limited to seed treatments. The thresholds for treatment are as follows:

> Early-sown winter wheat crops (before November) are unlikely to benefit from seed treatment, as they have more time to tiller and are better able to withstand WBF attack

> For late-sown winter wheat crops (November to December), seed treatments should be considered where WBF populations exceed 100 eggs/m² (moderate risk)

> For late-winter/spring-sown crops (January to March), seed treatments should be considered irrespective of the WBF population size

The threshold of 100 eggs/m² is, therefore, only relevant to late-sown crops. For the sites located in the North, four were above this level. Only one site, however, was above this level in the East (Table 1).

Table 1. The number of wheat bulb fly eggs/m² extracted from soil samples taken in 2017

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The WBF survey provides an indication of the overall risk level. As in-field risk depends on numerous factors, growers are advised to read the AHDB Wheat Bulb Fly publication to calculate the need to treat crops. The publication can be found, alongside the full survey results, at cereals.ahdb.org.uk/wbf 

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