Wheat Blossom Midge

Key Points

• Wheat Blossom midge is only a sporadic pest but can be devastating when it does occur
• The main wheat growing areas of the UK are at risk
• Damage caused will affect crop yield, quality and acceptance for milling
• Some winter wheat varieties are genetically resistant to Orange Wheat Blossom midge, but not Yellow Wheat Blossom midge
• No product will kill larvae once they have entered the glume
• Fields at risk should be examined daily at dusk during ear emergence
• Where numbers of adults are found at or above threshold Dursban WG should be applied within 24 hours
• Water volume should be maintained to ensure good coverage and contact with the pest
• Only chlorpyrifos gives effective and persistent control of adults and larvae
• Only chlorpyrifos is approved in the UK for the control of Wheat Blossom midge
• Always carry out a Risk Assessment and always follow the Safe Spray Guidelines

Introduction

Wheat Blossom midge is a sporadic and occasional pest that occurs in some crops in the UK every year. Two species of Wheat Blossom midge are pests of cereal crops. These are Orange Wheat Blossom midge — Sitodiplosis mosellana — and Yellow Wheat Blossom midge — Contarinia tritici.

The larvae of both species feed within the floret and can be distinguished by their colour and key differences in their biology and life cycle. Outbreaks tend to be sporadic with both species able to attack wheat, barley, oats and rye. Significant damage to UK cereal crops has only been reported in wheat and rye.

Some winter wheat varieties are confirmed as having genetic resistance to Orange Wheat Blossom midge, but not to Yellow Wheat Blossom midge.

Biology

Orange Wheat Blossom Midge

Larvae overwinter in cocoons until diapause is broken (70 days at < 10°C) or, if conditions are unsuitable, can remain in the soil for more than 10 years. For the life cycle to progress activated larvae must move towards the soil surface. They then require sufficient rainfall to wet the soil to a depth of 10mm, and a rise in soil temperatures to above 13°C, 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 15°C are particularly favourable.

Adult midges mate at the pupation site and the females then look for a suitable host crop. They are orange in colour and grow to about 3mm in length. Flight usually starts 1 – 2 hours before dusk, especially if air temperatures are above 15°C and wind speed below 10km/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. Eggs are laid on emerged ears before flowering, GS53-59 of the crop. 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.

The orange coloured larvae move to a developing grain and feed for 2 – 3 weeks. Typically one larva feeding on a grain site will reduce yield by about 30%. If two or three larvae feed per grain site yield loss can be as much as 75% or even higher if ear emergence is late. In addition to direct feeding damage, larval feeding can induce premature sprouting in the ear and a reduction in Hagberg Falling Number. Secondary fungal attack can follow under damp conditions.

Yellow Wheat Blossom Midge

Adults of the Yellow Wheat Blossom midge are broadly similar to those of the Orange Wheat Blossom midge but tend to emerge earlier. All life cycle stages are pale yellow in colour. The life cycles are similar but with a few important differences.

Female Yellow Wheat Blossom midge adults lay eggs earlier, around GS51-55 of the crop, and will stop laying once the floret has hardened. Each female lays a few batches of about 15 eggs, of which 4 – 15 normally survive. The eggs must hatch before pollination occurs in order that flower development can be arrested, so that the flower retains its anthers upon which the larvae then feed. If pollination succeeds the grain develops normally. Adults emerge over a shorter period than Orange Wheat Blossom midge and cocoons only survive up to three years in the soil.

Product Choice

The first action must always be to carry out a Risk Assessment. This can be done using the Dursban* WG Wheat Blossom Midge Risk Assessment Chart (249 KB PDF). Some winter wheat varieties are confirmed as having genetic resistance to Orange Wheat Blossom midge. It is not necessary to undertake a Risk Assessment on such varieties as they will not need an insecticide treatment. No varieties currently grown have genetic resistance to Yellow Wheat Blossom midge. A Risk Assessment identifies those fields most at risk and where further monitoring should be carried out. A clearer understanding of likely risk can then be quantified by soil sampling. Samples from potentially high risk areas are analysed for the presence of Wheat Blossom midge as cocoons, larvae and pupae.

Be careful not to assume that a high risk means there will be a problem or that low risk means no risk. The conditions required for a potential problem to become a real one depend upon the weather conditions during May and June.

May:	Soil temperatures rising above 13°C Sufficient rainfall to wet the top 10mm of soil
June:	Intermittent rainfall keeping soils moist Daytime air temperatures at least 15°C Wind speed less than 11km/h at dusk Crop ears emerged but not yet flowering

1. All environmentally sensitive areas identified.
2. Crop growth stage between ear emergence and flowering, GS53-59.
3. Adult midges found in crop.
4. Thresholds met or exceeded:
   a. Seed and milling crops – 1 or more adults per 6 ears
   b. Feed crops – 1 or more adults per 3 ears
5. Leave an unsprayed headland of at least 12 metres.

If the answer to all criteria is YES, apply Dursban WG at 0.6kg/ha in 200 – 1000 litres of water per hectare.

If the answer to any criterion is NO, wait until conditions are met, if at all.

If treatment of Wheat Blossom midge is necessary and justified, then spraying needs to be undertaken in a way that will minimise effects on wildlife. Please follow the Safe Spray Guidelines promoted by Dow AgroSciences.

1. Walk the crop daily on warm still evenings to check for the presence of adults. Pay close attention to crops at ear emergence (GS53-59) and assess each field individually. Consider the use of pheromone traps or yellow sticky cards at the highest risk sites. Spray only where thresholds are met or exceeded — one or more adult midges per 6 ears in seed and milling crops; one or more adult midges per 3 ears in feed crops.
2. Assess the area to be sprayed carefully. Take note of surrounding wildlife features like ponds, woodlands and hedgerows and consider the whole farm in the context of the affected area. Only spray affected areas.
3. Ensure spray operating staff are fully trained and know which areas are to be sprayed.
4. Avoid spraying near field margins leaving a 12 metre headland unsprayed. At this time of year hedges will be rich in the insect species needed to provide food for fledglings. Damage done to young bird populations now may affect future bird numbers.
5. Use techniques to avoid spray drift. Check sprayers are calibrated correctly and produce the appropriate spray quality. Choose drift-reducing nozzles where possible.
6. Avoid spraying in windy conditions or if the day is warm and still. Spray early in the morning or preferably late in the evening when most flying insects are not within the crop. Always warn local beekeepers of your intention to spray.

Always check the Cereals Pestwatch pages for the most recent bulletins.

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