Current observations indicate that leaf miner populations are between first and second generations and overall, damage does not appear to be increasing. The recent wet conditions may have helped to reduce the activity of adults but new eggs will continue to be laid over the coming weeks and crops should be assessed for the need for any treatment. BBRO trials have shown that the most effective stage for treatment is at egg hatch/larval emergence. For many crops, this may be over the next few weeks but it is really important to assess crops to ensure your use of Biscaya is at the optimum time.
Growers are advised to remember, the threshold for treatment is when the number of eggs and larvae exceeds the square of the number of true leaves. For example, a plant with six true leaves would need a population of 36 or more eggs and larvae to warrant treatment and a crop with 10 true leaves would need a population of 100 or more eggs and larvae. Remember to check the underside of leaves for eggs and larvae.
More information can be found at the British Beet Research Organisations website:
http://bbro.co.uk/media/1140/bbro-advisory-bulletin-no-10.pdf