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Developing pods of host plants turn yellow, ripen prematurely and may be twisted and deformed, with localized small swellings. Some descriptions of symptoms refer to the development of 'bladder pods', but this is misleading as midge infestations often cause little or no obvious swelling and there are other possible causes of enlarged pods, such as fungal infection or herbicide damage. Pods affected by the midge contain up to fifty white or yellow-white, gregarious larvae and normal development of seed is prevented. This reduces yields from oilseed crops, such as rape, and also affects production of seed crops. Infested pods dehisce prematurely, which can result in loss of healthy seed.
The dependence of D. brassicae on seed weevil activity for the provision of oviposition sites means that control of the weevils may be sufficient to prevent midge damage. This is particularly the case on spring-sown Brassica seed crops, but not on winter oilseed rape (Winfield, 1992).
D. brassicae adults disperse over relatively short distances and growing new crops of oilseed rape at least 0.5 km away from fields containing hosts in the previous 1-2 years is usually sufficient to limit infestations. In any case, infestations tend to be highest around field edges and large fields therefore sustain relatively little damage. Chemical control is not generally recommended because of the hazards to pollinating insects attracted to flowering crops. If used, selective spraying of crop borders and headlands may be adequate. Timing is critical and should be determined by monitoring adult midge activity.
Some potential for selective plant breeding exists in the differential responses of adult and larval D. brassicae to different species of Brassica and related genera.
The main impact of this species is on production of oilseed rape by reduction of seed yields at harvest. Barnes (1946) noted that severe infestations result in considerable losses either of seed for sowing or of seed for oil production, and that such attacks had occurred in Denmark and in Germany. Axelsen (1992) notes that it is a pest of oilseed rape in most of Europe, and that in Denmark, and probably elsewhere in northern Europe, it is important on winter oilseed rape but not on spring oilseed rape. Considerable damage in winter-sown oilseed rape crops was observed in eastern Sweden during 1992 (Lerenius, 1993).
Lahmer et al. (1992) reported damage to oilseed rape at one location in Morocco during 1988.