With spray control options withdrawn and seed treatment best confined to later sowings, preventing wheat bulb fly larvae from hitting yields is becoming more of a challenge.
The pest can affect all cereals, except oats, and the currently accepted way to assess the threat and the need to apply the sole approved seed treatment, Signal 300 ES (cypermethrin), is by counting eggs which the fly lays in bare soil.
Based on those numbers AHDB offers treatment advice (see panel).
The latest research project*, led by ADAS entomologist Steve Ellis, continues to assess bulb fly risk by egg-counting but is also investigating modelling as an alternative to that laborious and expensive exercise.
Dr Ellis says: “The basic reason for the work is to provide information on egg numbers so farmers can decide whether to apply a seed treatment if they plan to sow from November onwards.
“Treatments are less effective for earlier sowings because they run out of steam before the main egg hatch period which is usually in January/February.” This season’s egg counts suggest a low risk year, as have been the past several years (see graph); but the impact of previous crops in the sampled fields (15 each in the East and North where the pest is most prevalent) is worth noting, says Dr Ellis.
“Over all sites, the highest risk was after French beans with a mean of 535 eggs/square metre, although only two sites were sampled, one of which had a low count.
The next highest risk was after sugar beet with a mean of 175 eggs/sq.m across eight sites sampled.
“Average egg numbers in the East were higher than in the North with 173 and 111 eggs/sq.m respectively.
The counts in the East were heavily influenced by two very high counts after sugar beet and French beans, as well as by a high count also after sugar beet.” Overall, the results showed 7%, 3%, 13% and 77% of fields in the very high, high, moderate, and low risk infestation categories respectively – identical to the breakdown in 2019, he adds.
• AHDB project 21120003 Autumn survey of wheat bulb fly incidence
• September 1, 2019 – October 31, 2021
• Funding: £32,000 (all AHDB)
• Early-sown crops (before November) are unlikely to benefit from seed treatments and excess shoot production makes these crops more resilient.
However, bulb fly pressure of 250 eggs/sq.m may impact yield
• For late-sown crops (November-December), consider seed treatment where populations exceed 100 eggs/sq.m
• For very late-sown crops (January), consider seed treatment irrespective of population size (unless no eggs are present) Note: Treated seed must not be sown after January 31