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Wheat Blossom midge is a very unusual and difficult pest to assess and control.Weather conditions now and during the next 3-4 weeks will determine if 2010 will result in damaging midge larvae levels. Understanding of the biology and life cycle of the pest helps explain why there could be problems again in 2010.

Larvae over winter in cocoons and can remain in the soil for more than 10 years if conditions are unsuitable. For the life cycle to progress suitable conditions (70 days at < 10⁰C) must be experienced to break diapause. Activated larvae move towards the soil surface.  They then require sufficient rainfall to wet the soil to a depth of 10 mm, and a rise in soil temperatures to above 13⁰C, to stimulate pupation. The duration of the pupal stage varies according to temperature, typically spanning a period of 2 to 4 weeks. Rising temperatures following rainfall stimulate hatch of adults from pupae. Air temperatures above 15⁰C are particularly favourable for flight.

Adult midges mate at the pupation site and the females then look for a suitable host crop. Flight occurs when light levels fall below 18,000 lux, usually 1-2 hours before dusk, especially if air temperatures are above 15⁰C and wind speed below 10 km/hr. Egg laying can continue until temperatures drop below 11⁰C. Adult midges live for about 7 days, with most eggs being laid on the third day of adult life. In good conditions, each female can lay around 80 eggs in batches of 2 or 3 per floret. Eggs hatch in 4-10 days, depending on temperature. Then the larvae move to a developing grain and feed for 2-3 weeks.

The average population of Orange Wheat Blossom midge in this year’s qualitative baseline sampling is 1.9 per kg soil, compared with an average over the last five years of 7.1.  The highest population (7.5 owbm/kg soil) was recorded in a sample from Boxworth in Cambridgeshire. No pupae were recorded at this round of sampling.

In the period 12^th - 18^th May the average soil temperature at 30 cm for England and Wales was about 10.8°C, well below the 13°C threshold.  In addition, there has been little rain in recent weeks, although temperatures have increased over the weekend.  Therefore overall it is likely that midge development is delayed.  As crop development is also delayed there is still potential for the emergence of midges to coincide with the susceptible stage of the crop.

Dow AgroSciences invest in Pestwatch to help you provide the best advice to farmers.  Please be aware that this data is not quantitative and the number of OWBM/kg soil should not be used as an absolute guide to the level of risk in any given area. It is a qualitative survey to indicate how pupation is progressing and thus give clues to when emergence might occur.

Action: Use the Dow AgroSciences reports which indicate larval development progression i.e. when emergence of adults can be expected, and the Risk Assessment Charts to identify fields at high risk, consider the use of traps at the most susceptible sites. These are typically wheat fields where the pest was noted last year, especially if no treatment was carried out. The economic risk is highest in crops intended for seed or milling. This is reflected in the lower threshold of one midge per 6 ears. The threshold for feed wheat is one midge per 3 ears.

Be prepared to apply Dursban* WG at 0.6 kg/ha in 200 to 1000 litres of water per hectare (leaving an unsprayed headland of at least 12 metres) if thresholds are met or exceeded.