Orange wheat blossom midge: a literature review and survey of the 1993 outbreak

A substantial outbreak of the orange wheat blossom midge occurred in 1993, affecting the wheat-growing areas of England, Scotland and Wales. The larvae of this species overwinter in the soil for up to 13 years before pupating and emerging as adults from late May to the beginning of July. Larval reactivation and pupation are stimulated by rising soil temperatures and rainfall. Mass emergence of adults can occur under suitable weather conditions which may lead to substantial damage if, as in 1993, the flight coincides with ear emergence of wheat crops and a favourable period of warm, still weather. The degree of coincidence between flight and the span of time during which susceptible florets are present in a crop are the main factors determining the degree of infestation. Adult midges are strongly attracted to susceptible crops, to which they will migrate from a distance of at least one kilometre. The eggs are laid in the florets of wheat ears between ear emergence and flowering. The larvae feed on the swelling grain, and can cause shrivelling, pre-sprouting and induce secondary fungal attack. A substantial reduction in Hagberg falling number and other quality measures can occur if these factors are already under pressure in the crop. Yield losses can be considerable where attacks are severe.

No effective cultural control methods are available, but the pests can suffer significant reductions due to various natural enemies. Although chemical control measures are available, their efficacy is dependent on precise timing of applications within 4-5 days of the adult midges arriving in a susceptible crop. Successful control depends therefore on predicting which crops may be at risk in relation to forecasts of midge emergence and in being prepared to spray at short notice in case of need. Forecasts can be given based on monitoring of pupation in the soil and linking pupal populations to weather data. Soil sampling can be used to estimate levels of midge larvae in the soil in advance and thus estimate the probability of a significant emergence at a critical stage.

Action thresholds for spray application have been revised to take account of recent research findings and current costings to one ovipositing midge per ear for feed crops and one midge per six ears for seed and milling crops, between growth stages 55 and 59. The insecticide sprays approved for use in the UK are broad-spectrum in action and care should be taken to time sprays accurately to minimise effects on the midges' natural enemies and other
non-target insects.

Considerable differences in damage levels between varieties have been found in experiments, but are thought to be mainly due to patterns of coincidence at particular sites and show marked inconsistencies. Varietal traits of tightness of florets and hypersensitivity to midge damage have been identified which could merit further exploitation.