|A swede midge (Contarinia nasturtii)|
| Cabbage crowngall fly
Cabbage gall midge
The swede midge (Contarinia nasturtii) appears to feed only on plants in the Brassicaceae (cabbage or mustard family). In Europe it is considered an endemic and common pest of cruciferous vegetable crops (e.g. broccoli, Brussels sprouts, cabbage, cauliflower, kale, radish, swede, and turnip) often causing severe losses.
Swede midge injury is often difficult to distinguish from other factors that can damage the growing tip of a plant such as mechanical injury from cultivation, insect and animal feeding, molybdenum deficiency, herbicide injury, genetic variation of the plant, and heat or cold stress. For confirmation of injury due to swede midge, the larvae should be found on or within the plant tissue. During feeding, larvae produce a secretion that breaks down plant tissue, creating a moist environment. The secretion is toxic to the plant and results in swollen tissue, abnormal growth and brown scarring. Signs of swede midge feeding include puckered and crinkled leaves, distorted growing points, leaf and flower galls, brown scarring, blind heads, and plants with multiple small heads or shoots.
When scouting for swede midge, pay attention to sheltered areas, along field edges and buildings. Swede midge adults are not strong fliers and tend to prefer areas of low wind movement. Examine young crop plants for unusual growth habits, with emphasis on the growth point and any side shoots. Check the growing tips of young plants for galls and swollen leaf bases. Brown, corky scarring along leaf petioles and stems is a key diagnostic feature. Infested plant material often has a “moist” or “wet” appearance where larvae are actively feeding.
Insecticides are used with some success to kill adults or to prevent them from laying viable eggs. Systemic insecticides are required to kill the larvae, since they are usually well protected within the plant material. When swede midge populations are very high, insecticide sprays alone will not eliminate economic damage to the crop.
The best management is to limit the spread of the pest since, once it becomes present in an area, it is difficult to manage. Although adults are weak fliers, it is possible for them to be carried by winds. Other avenues include the movement of transplants, which may contain eggs or larvae, or soil, which may contain pupae. Since the pupae are located near the soil surface, working the soil may reduce the number of viable pupae. The most important management strategy is to destroy crop residue as soon as possible after harvest. This will minimize the population of the over-wintering generation. The second most important management strategy is to rotate away from crucifers as far as possible. If the over-wintering population does not have a suitable host when it emerges, a significant proportion of the population will die. Although most cruciferous weeds are less suitable host plants in comparison to cole crops, it is important that they be controlled during the cropping season as well as afterwards.