Assessing the risk of Wheat Blossom Midge with Pestwatch

UK - May 27, 2010



The focus for Dow AgroSciences Pestwatch service changes from Wheat Bulb Fly to Leatherjackets and now to Orange Wheat Blossom Midge and starts with a warning that midge development may be delayed this year.  Weather conditions now and during the next three to four weeks will determine if 2010 will result in damaging midge larvae levels.  Indications from studying the biology and life cycle of the pest lead to the conclusion that these pests could be a problem again in 2010.  Growers and advisors can go to www.dowagro.com/uk to see when midges are likely to emerge. 

Dow AgroSciences invest in the Pestwatch service to help provide the best advice to farmers, says Sarah Hurry of Dow AgroSciences.  "It starts by sampling soils for larvae.  The average population of Orange Wheat Blossom midge in this year's qualitative baseline sampling is 1.9 larvae per kg soil, compared with an average over the last five years of 7.1.  The highest population (7.5 larvae/kg soil) was recorded in a sample from Boxworth in Cambridgeshire.  No pupae were recorded at this round of sampling.  It is important to be aware that the number of Orange Wheat Blossom Midge larvae/kg soil is not an absolute guide to the level of risk, but it is a qualitative survey to indicate how pupation is progressing and thus give clues to when adult emergence might occur."

Sarah points out that the risk relies on the vulnerable growth stages of the crop coinciding with egg-laying activity, which is influenced by the weather.  "Wheat Blossom Midge is an unusual and difficult pest to assess and control.  The larvae overwinter in cocoons and can remain in the soil for more than 10 years.  For the life cycle to progress suitable conditions (70 days at <10°C) must be experienced in order to break diapause, after which the 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 13°C to stimulate pupation.  The duration of the pupal stage varies according to temperature, spanning a period of two to four weeks.  Rising temperatures following rainfall stimulate hatch of adults from pupae.  Air temperatures above 15°C 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 15°C and wind speed below 10 km/hr.  Egg laying can continue until temperatures drop below 11°C.  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 emerged larvae move to the developing grain and feed for 2-3 weeks.  "With a number of opportunities to control the various life stages of the pest, it is important to select a product such as Dursban WG that will control adults, eggs and larvae, thus maximising control."

Sarah Hurry reports that in the period between the 12th and 18th of 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 recently.  Therefore it is likely that midge development may be delayed this season.  As crop development is also delayed, there is still potential for the emergence of midges to coincide with the susceptible stage of the crop."

Growers are invited to use the Dow AgroSciences reports on www.dowagro.com/uk which will indicate the pests life stage progression i.e. when emergence of adults can be expected.  Also available on the web site is the Risk Assessment Charts which help identify those fields which are at high risk.  The use of traps at the most susceptible sites should also be considered.  Susceptible sites are those 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.

Sarah Hurry advises that if treatment is justified, use Dursban WG at 0.6 kg/ha in 200 to 1000 litres of water, if thresholds are met or exceeded.  "Dursban WG will control all the life stages of the pest, giving the grower the widest window of opportunity and flexibility to control this pest.  In association with FWAG, Dow AgroSciences recommend a voluntary 12 metre buffer from the edge of the field as part of integrated pest management best practice."

Orange Wheat Blossom Midge is a sporadic pest of wheat in the UK which occurs in some crops in the UK every year.  Substantial loss of yield and quality can result.  Typically one larvae feeding per grain site can cause about 30% yield loss; with two or three larvae per grain site loss can be as much as 75%, or even higher if ear emergence is late.  In addition larval feeding can induce premature sprouting in the ear and a reduction in Hagberg Falling Number.  Secondary fungal attack can follow under damp conditions.