Final egg counts confirm wheat bulb fly risk divide in 2020

Thursday, 8 October 2020

Autumn egg count results for the cereal pest wheat bulb fly show that 2020 is likely to be a relatively low-risk year, once again.

It follows a succession of low-risk years (figure 1); however, egg levels remain sufficiently high for farmers to consider using seed treatments to protect crops – in some situations.

Long-term wheat bulb fly survey results (1984-2020)

Figure 1.  Wheat bulb fly risk since 1984. Chart shows the percentage of fields above the level associated with economic damage in early-sown wheat (September to October, >250 eggs/m2). Dashed line indicates the overall mean.

Conducted by ADAS, the survey uses soil samples from 30 fields, split equally across sites located in the East and North of England that are prone to attack, to calculate the number of pest eggs per square metre.

The results indicate the potential risk of economic damage to wheat and show whether the use of treated seed could potentially reduce the risk of unacceptable crop losses (table 1).

Table 1. Autumn 2020 eggs count results (30 fields) presented by risk category.

Risk category

Risk category for monitored fields in autumn 2020

Sowing date
Sep-Oct Nov-Dec Jan

(<100 eggs/m2 )


No treatment required

No treatment required

Consider seed treatment

(unless no eggs present)

(100–249 eggs/m2)


No treatment required

Consider seed treatment

(250–500 eggs/m2)


Economic damage likely, but no effective treatment available

Very high
(>500 eggs/m2)


Economic damage likely, but no effective treatment available

Full results are on the project page

Seed treatment thresholds

  • Early-sown crops (before November) are unlikely to benefit from seed treatments, which lack persistence to offer full protection. Excess shoot production also makes these crops more resilient. However, wheat bulb fly pressure at which impacts on yield might be expected is 250 eggs/m2
  • For late-sown 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)

Note: Drill treated seed at the recommended depth of 2.44.0 cm in a firm, even seedbed. Signal 300 ES (cypermethrin) seed treatment must only be applied to cereal seed sown 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.

Although the actual damage threshold varies considerably, the simple rule of thumb is the greater the number of tillers at the time of attack, the lower the risk of economic damage.

Charlotte Rowley, who manages pest research at AHDB, said: “Our survey frequently detects large variations in wheat bulb fly egg numbers, both across and between regions.

“The early egg count data showed that 2020 would be no exception, and the final data confirms this.

“The role of sowing date in risk is important. For example, the latest sown crops are likely to benefit from a seed treatment, even when eggs counts are relatively low.”

Access survey results and threshold information via the dedicated AHDB web page: