Introduction
The Swede midge, an invasive agricultural pest (also known as the cabbage crowngall fly and cabbage gall midge) was first detected in New York in 2004 in Niagara County. Although the insect is a native of Europe and southwestern Asia, it is believed the midge was introduced into NY from the Canadian province of Ontario where it was first found on broccoli in 1996. By the end of 2007, the Swede midge had been confirmed in 12 NY counties (Allegany, Chenango, Franklin, Herkimer, Jefferson, Livingston, Onondaga, Otsego, Rensselaer, Steuben, Suffolk, and Yates).
Biology
This species is a small (1.5 - 2 mm), light brown fly that is indistinguishable from many other midges except by an expert entomologist. Adult midges emerge in the spring from pupae that have over-wintered in the soil. Adult flies mate soon after and females search for suitable host plants. Each female can lay about 100 eggs during their one to five day lifespan. The females lay their eggs on the growing point of young plants. Larvae hatch from the eggs after a few days and begin to feed in groups on the growing plant tissue. Larvae complete their development in 7 - 21 days after which they drop to the ground and pupate in the soil. Adults can emerge within two weeks, restarting the cycle. Depending on temperature and length of growing season, there can be up to five overlapping generations of Swede midge per year.
Damage
As they feed, Swede midge larvae produce a secretion that breaks down the surface of the growing point of the plant and liquefies the cell contents, resulting the formation of leaf and flower galls and a misshapen growing point. Damage caused by Swede midge larvae feeding results in distorted growing tips and may produce multiple (or no) growing tips; young leaves may become swollen or crumpled and leaf petioles or stems may exhibit brown scarring. Swede midges feed only on cruciferous vegetable crops, such as cabbage, cauliflower, broccoli, and Brussels sprouts, frequently causing severe losses. The insect also damages canola, collard, horseradish, kale, mustard, rutabaga, turnip, and radish.
Management
Insecticides can be used to kill adults or prevent them from laying viable eggs. However, controlling larvae is much more difficult because insecticide would have to enter the plant tissue upon which the larvae are feeding. Currently, the best way to manage Swede midge damage is to limit the spread of the insect into new areas. Adults are very weak fliers, so the primary vector of introduction is believed to be the movement of transplants which may contain eggs or larvae, or movement of soil which may contain pupae. Repeated working of infested soil can reduce the number of viable pupae. Also, because adult Swede midges cannot travel far, crop rotation using noncruciferous plants can help to reduce the likelihood of spreading an infestation.
Contarinia nasturtii, the swede midge, is a small fly, the larvae of which infest brassica plants, causing twisting and distortion of the leaf stems and foliage including death of the growing point in seedlings, or damage to developing flower heads. It is native to Europe and Turkey, and has been introduced into North America where it is regarded as an invasive species.
Adult swede midges are yellowish-brown and up to 2 mm (0.08 in) long, and live for up to three days. During this time the female lays about one hundred eggs in several batches on the leaves of suitable host plants. The eggs need moisture to hatch and the larvae emerge in between one and ten days at 30 to 10 °C (86 to 50 °F) respectively. They feed for between one and three weeks, again depending on temperature, and produce a gall. When fully grown they descend to the soil and either pupate in an oval cocoon at a depth of about 5 cm (2 in) or, in adverse weather conditions, become dormant in a globular cocoon buried deeper in the soil. The adults emerge one and a half to seven weeks after pupation and there are several generations each year.[1]
When the dormancy period is due to drought, development continues after wetting, but when it is due to low temperature, development restarts only after a prolonged period of cold weather. The larvae then make their way to the soil surface before burying themselves shallowly and forming an oval cocoon.[1]
A recent study found that C. nasturtii, despite being crucifer-specialist, may not depend on the crucifer-specific glucosinolates.[2] Therefore, other volatile compounds emitted by crucifer plants may mediate the C. nasturtii host attraction.[2]
Host plants for the swede midge include cauliflower, broccoli, cabbage, Brussels sprouts, kohlrabi and a number of wild plants in the brassica family including shepherd’s purse, field penny-cress, field peppergrass and yellow rocket.[3] When a seedling or young plant is attacked, the larvae conglomerate and secrete saliva which softens the plant cuticle and epidermis, sucking in the resulting fluid. On a leaf stalk, this damages the tissues and results in the formation of a corky gall. The undamaged side of the stalk continues to grow normally which results in twisting and distortion. The growing point can be completely killed and secondary bacterial infections can occur. If the infection occurs in the flowering stage of cauliflower, it causes a stunted, multi-branched, tuft-like appearance of the flower-head.[3]
Contarinia nasturtii is native to Europe and Turkey. It was first detected in Canada in 1996 and is now present in Ontario, Quebec, Nova Scotia and Prince Edward Island. In the United States it was detected in the western part of New York state in 2004 and is now also known from Connecticut, Massachusetts, Michigan, New Jersey, Ohio, and Vermont. It is regarded as an invasive species in North America.[4][5]
Contarinia nasturtii, the swede midge, is a small fly, the larvae of which infest brassica plants, causing twisting and distortion of the leaf stems and foliage including death of the growing point in seedlings, or damage to developing flower heads. It is native to Europe and Turkey, and has been introduced into North America where it is regarded as an invasive species.
