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.
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.