Management of Swede Midge on the Farm

The swede midge is an unwelcome guest, that once established on your farm will be virtually impossible to eradicate. However, with the use of best management practices you should be able to keep insect populations low enough to avoid economic damage. Proper management incorporates as many strategies as possible and should minimally include:

1) Use of clean transplants (physical)
2) 2- to 3-year rotation to non-crucifer crops (cultural)
3) Post-harvest crop destruction (physical)
4) Swede midge detection and monitoring (physical)
5) Insecticide applications as needed (chemical)
Details on these and other management practices are provided below:

Final Thoughts and Take Home Messages
No single strategy will provide 100% control of swede midge. However, using as many of the management practices described above as possible will help to keep swede midge populations manageable and reduce economic damage.

Swede midge tends to be particularly problematic when the pest is not detected early and builds up to high populations. Factors that contribute to high populations include lack of rotation to non-crucifer crops and leaving the stumps of cruciferous plants in the field after harvest. Insecticides can be an important tool in swede midge management, but may not be effective if populations are too high. Currently, there are no effective products that meet organic standards. Small farms that grow multiple crucifer crops are particularly at risk because they often have too small a land base for adequate crop rotation.

While we have learned much about swede midge in the last several years, much remains to be studied about its biology and management. Considerable research efforts are needed to identify less susceptible varieties and to document the ways in which swede midge spreads over short and long distances.

Swede midge is becoming an increasingly important pest of crucifers in North America, and it is imperative that management strategies be implemented while it is still a relatively new pest. Assume that you have swede midge, or that you will be getting it. Practice excellent crop rotation and immediate post-harvest crop destruct, and be sure you know how to identify swede midge damage.

Information for Home and Public Gardens
Tiny Pest May be Cause of Gardener’s Woe – Julie Kikkert, Cornell University Cooperative Extension, Cornell Vegetable Program

Do your broccoli and cabbage plants fail to form perfect heads? Is your ornamental kale twisted and sick? A tiny insect called the swede midge may be to blame. Since its first discovery in North America in 2000, this gnat-like pest has been spreading throughout Canada and the Northeastern, U.S., leaving damaged cole crops (broccoli, cauliflower, cabbage, radishes, turnips, kale, and the like) in its path.

The pest originated from Europe and parts of Asia and gets its name from the swede, a rutabaga-like plant. Adult swede midges are a mere 2 mm long, so gardeners will probably never actually see them. However, beginning in mid May, each female will lay as many as 100 microscopic eggs hidden in the lush growing tips of host plants. Within a matter of days, maggots (larvae) hatch from the eggs and begin to feed in groups on the succulent plant tissue. Initially microscopic and translucent, they mature to a size of 4 mm and become lemon yellow. Gardeners with a keen eye or a hand-lens may catch a glimpse of the larvae if they unfold the growth point of plants with suspect damage. Placing infested plant material in alcohol will cause the larvae to leave the plant tissue. The maggots don’t have any legs, eyes or hairs, so don’t confuse them with other tiny insects. Mature swede midge larvae have the unique capability to flip themselves off the plant, a tactic they use to move to the soil to pupate. The pest is a continual problem from spring planting through fall harvest because of its multiple generations.

Gardeners who notice Cole crops with puckered and crinkled leaves, distorted growing points, brown scarring, and plants with multiple shoots or small heads should suspect swede midge. A fact sheet with pictures of damage symptoms can be found at http://nysipm.cornell.edu/factsheets. For help with identification, take suspect plants to your local Cooperative Extension Office or Plant Diagnostic Clinic. Damage to the growing tips of Cole crops can also be caused by frost, rodent damage, slugs, cultivation equipment, and nutrient deficiencies. The only sure way to confirm swede midge is to find the larvae in the tissue, however, they have often left the plant by the time damage is noticed.

Everyone can avoid swede midge by using good management practices. Start with insect-free plants – growing your own plants from seed is the best way. If you can, move the location where you grow Cole crops each year – as far away as possible is best. Keep your garden free of weeds because there are many crucifer weeds such as Shepherd’s purse and wild mustard that are also hosts of swede midge. After the last harvest of your Cole crops, pull and destroy the remaining stalks and roots, since these often sprout new growth that is attractive to swede midge. High populations of swede midge in the autumn mean more that can overwinter and infest plants the following spring. The goal is to keep the population of swede midge as low as possible to lower the risk to your crop. If you continue to have a high level of crop damage, you may need to break the insect’s life cycle by growing Cole crops once every two to three years. Swede midge only affects plants in the cabbage family.