There’s a new pest entering the scene and it is wreaking havoc on brassicas crops such as cabbage, cauliflower, brussels sprouts, broccoli and kale. It’s called the swede midge and it's larvae feed off of and destroy the growth point of plants As a new and invasive pest that currently has no organic approved pesticides that work to eradicate it, early detection and management are essential for controlling populations on your farms and gardens.

Swede midge adult. (Susan Ellis, USDA APHIS PPQ, Bugwood.org)
Origins

The swede midge was first discovered in the U.S. in 2004 on a broccoli farm in Niagara County, NY. Native to Europe and southwestern Asia, it has only been known to exist in North America since the year 2000 when it was discovered in Ontario, Canada. Since 2000, swede midge has spread to over 7 U.S. states and 6 provinces in Canada. Scientists have determined that swede midge prefer the climate conditions of Canada and the Northeast, Midwest and Mid Atlantic regions of the United States. It is predicted that swede midge will continue to spread from these regions to neighboring states and provinces.

Swede midge injury is often mistakenly diagnosed as damage caused by other issues including molybdenum deficiency, heat/cold stress, herbicide damage and genetic variation, among others. During feeding on the plants, larvae release a toxic secretion that results in malformation of the plant at its growth point.

Life Cycle

Swede midge pupae overwinter in the top 1-5 cm of soil. As the soil temperature raises in May, adult midge emerge. Unfortunately, not all of the overwintered pupae emerge at the same time, and instead emerge in numerous flights with three peak emergence events taking place between early May and the end of June. Adults will continue to emerge throughout the growing season, through to the beginning of October, which makes managing the pest very difficult. The midge appear to prefer regions where temperatures hover around 68-77 degrees F with continuous rainfall throughout the growing season.

The first order of business once the adults have emerged from the soil is mating. Males and females find each other using pheromones. This fact makes the use of pheromone traps a good method for evaluating the extent of your infestation and the potential peak emergence periods on your farm. Jackson White Traps are the low cost choice for Lepidopteran pests, fruit flies and many types of flying insect pests and have proven to work for luring and capturing male swede midge. Once swede midge have mated, the female begins scouting for a cruciferous crop to lay her eggs. Females lay 2-50 eggs on the newest growth of the plants. Though females only live for 1-5 days in total, they will lay several egg deposits on plants totaling around 100 eggs.

Within just a few days of being deposited, eggs will hatch and begin feeding on plants. The larvae are so small, around 0.3 mm in length, that they are difficult to detect with the naked eye. As the larvae develop, they will change from being translucent white to yellow and top out at a size of 3-4 mm in length. The larvae release a secretion that breaks down the plant tissue to release the plant's fluids which changes the physiology of the plant. This contributes to the abnormal growth, brown scarring and ultimate destruction of the new growth of the plants. Mature larvae can evade harm by flipping their bodies off the plant when the plant is disturbed. Generally, it is very difficult to detect swede midge damage until a plant has been completely infested.

The larval stage of the swede midge is the only stage that poses a threat to brassicas crops. This period can last 7-21 days. Pupae that reach the soil after this stage can cause new adults to emerge every 7-14 days after this initial cycle. During drought, pupae in the soil may remain dormant and emerge with the onset of wetter weather. Not all of the pupae that exist in the soil will emerge in the same year.

Swede midge are most likely to overwinter where a brassicas crop was grown in a previous season or in neighboring territory. They become a problem where brassicas crops are not destroyed post harvest. Swede midge are not very mobile and therefore prefer fields and gardens that offer up good shelter and protection from prevailing winds.

Control

Once swede midge have established on your land, it is almost impossible to eliminate them. That is why early detection and prevention are essential for protecting your crops. There are a few management techniques that can help keep populations low after you've discovered damage on your farm.

  • Check your plants early and often
    • If you begin to see abnormal growth, twisted leaves, brown scarring, or plants growing "blind" (ex: a cauliflower plant not making a head,) it may be time to take a closer look. Using a lens, you can investigate the growth point of the plant and look for signs of the tiny larvae. If larvae are identified, it might be time to take action to protect other plants.
  • Destroy all brassicas plants post harvest
    • Swede midge will infest crops that are aging out of their harvest cycle. Broccoli and cauliflower stalks that have had their main head harvested, that are sending up side shoots can harbor a growing population if not destroyed.
  • Maintain a 3-4 year crop rotation
    • Pupae will emerge from fields whether a brassicas crop or host weed is present in the field or not. Creating a strict rotation and managing host weeds (including field pennycress, wild mustard, wild radish, shepherd's-purse, common pepper grass and yellow rocket,) is essential in slowing population growth. While swede midge is prolific, it is not a good flier and can be kept in an area if properly managed.
  • Use floating row cover
    • In areas where swede midge is detected, using floating row cover or insect netting and protect growing plants. If the pupae exist in the soil where the brassicas crop is planted and covered, this method will not protect the plants.
  • Use clean transplants
    • Swede midge can infect greenhouses, especially where large numbers of brassicas transplants are grown. It is important to sanitize your greenhouse and or make sure that any transplants brought onto the farm come from a clean source. When hardening off your cole crops, protecting them with insect netting or floating row cover and prevent egg laying activity.
  • Heterorhabditis bacteriophora nematode
    • While lady bugs have been proven to feed on the larvae, they are often too large to squeeze into the growth point of the plants where the damage is being done. Heterorhabditis bacteriophora nematodes have proven to seek out and kill pupae overwintering or laying dormant in soils.
  • Plan out your planting and harvesting
    • Planting early successions of brassicas crops and avoiding late plantings can help where populations have become out of control.
  • Conduct cultivar trials on your farm
    • Studies in Canada and in the U.S. are being conducted now to determine what cultivars have the greatest resistance to infestation and damage caused by the swede midge. Trialing varieties on your own farm can be a key to understanding what to plant when, based on your regional population of midge.

While no single strategy works to completely eliminate the economic damage caused by the swede midge, coming up with an integrated plan can help your farm slow the progress of the midge and control its population. For more in depth information about swede midge management, check out this amazing resource list created by the Cornell University of Agriculture and Life Sciences.