W. D. Hutchison and Suzanne Wold-Burkness
Department of Entomology, University of Minnesota
Swede midge, Contarinia nasturtii, (Diptera: Cecidomyiidae) is a pest of plants in the Brassicaceae family, including the Cruciferous vegetable crops broccoli, cabbage, bok choy, and cauliflower. In addition to vegetable crops, the midge will attack canola. Non--crop host plants include wild mustard, shepherd’s-purse, wild radish, field pennycress and yellow rocket. Native to Europe, it was first detected in North America in Toronto, Canada, in 2000. It spread to New York in 2004, and was recently found in Minnesota in 2016.
Figure 1. Swede midge adult.
(Susan Ellis, USDA APHIS PPQ, Bugwood.org)
Figure 2. Swede midge larvae on cauliflower stem.
(Mao Chen, Cornell University, Bugwood.org)
Swede midge adults are light brown, approximately 2 mm long, and nearly indistinguishable from other midges except under magnification. In Michigan and New York adults emerge in early-mid spring. Eggs are extremely small (<0.5 mm), cream colored, and are laid in clusters on young, actively growing tissue of suitable host plants. Larvae are cylindrical and transparent when young, darkening to a creamy yellow when they reach their final instar, and measure 3-4 mm in length. Larvae feed in clusters until their final instar, at which time they drop off the host plant to pupate in the soil. In Michigan, there are three-five generations from May to October.
Swede midge belongs to the gall-making family Cecidomyiidae, where their feeding leads to distorted plant tissue. Damage caused by swede midge larval feeding ranges from puckered leaf tissue, scarring, to plants with blind heads. Similar symptoms may be attributed to other sources, such as nutrient deficiencies and herbicide damage; therefore the presence of larvae is needed for accurate identification.
Although traps are available for monitoring the adult midge (males), it is a highly cryptic species, and adult flies may be laying eggs, with larvae causing damage, prior to adult detections in traps. That said, previous research in NY and Canada found that 5 flies/trap can be indicative of egg-lay and larval feeding damage. If flies are detected in traps it is best to take proactive steps to minimize the risk of swede midge infestations. Also, communicate with your transplant supplier to ensure you are purchasing pest-free transplants, so that you are not inadvertently introducing Swede midge to your farm. If transplants are infested, a transplant or soil drench containing an approved insecticide should be applied prior to planting.
Varietal Preference by Swede midge
Researchers at the University of Guelph (Ontario, Canada), conducted trials in 2001-2003 to determine Swede midge susceptibility to cultivars of broccoli, cabbage, cauliflower and Brussels sprouts. They found broccoli to be most susceptible. Of the broccoli evaluated, ‘Paragon’, ‘Eureka’ and ‘Packman’ had the highest damage ratings in the three-year study, while ‘Triathlon’, ‘Regal’ and ‘Everest’ showed lower susceptibility and slower symptom development in at least one of three years. In the 3-year study, there was no difference in susceptibility in red and green cabbages, nor between white, red or burgundy cauliflower cultivars; ‘Jade Cross’ Brussels sprouts showed low susceptibility to swede midge.
Swede midge populations have been shown to build up in the soil, therefore crop rotation is extremely important. Cornell researchers recommend rotating out of Brassicacae for at least three years. In addition, weeds should be managed in areas close to fields. Any suitable weed host should be destroyed.
Swede midge must be treated with preventative sprays. Trapping is an effective tool to determine when the population is emerging from the soil. When pressure is heavy in heading cole crops, seven-to-nine-day spray rotations have been effective in other states.
Information from Michigan State University, indicates that systemic neonicotinoids are a useful tool for controlling this pest. In a New York greenhouse study, foliar applications of Assail 30SG (acetamiprid) controlled larvae on cauliflower transplants using nine-day spray intervals. Field settings are more of a challenge because swede midge tends to hide from foliar contact insecticides under leaves near the growing tip. In New York field trials, clothianidin and thiamethoxam seed treatments offered 100 percent control of larvae for 3-5 weeks, and soil drenches of acetamiprid, imidacloprid and thiamethoxam were 100 percent effective for up to 7 weeks to control the feeding larvae.
These early season treatments can reduce the number of foliar applications required, but a field study between the University of Guelph and Cornell University indicated that supplemental foliar insecticides are especially important for mid- and later-season control of swede midge in field settings. Most pyrethroid, carbamate and organophosphate insecticides were deadly to swede midge, including λ-cyhalothrin, permethrin, acetamiprid, chlorpyrifos and dimethoate. Bt (Bacillus thuringiensis subsp. israeliensis) caused 50 percent mortality of larvae in the lab setting, but no organic certified (OMRI-approved) materials have been found effective in the field. These insecticides alone were not effective in knocking back populations. In order to effectively manage this pest, lengthy and spaced rotations as well as host weed control, will be essential to reduce population pressure.
Kikkert, J., C. Hoepting, A. Shelton, M. Chen, P. Wang, Q. Wu, J.-Z. Zhao. 2017. Swede midge information center for the United States. Cornell University. Retrieved from http://web.entomology.cornell.edu/shelton/swede-midge/index.html
Minnesota Department of Agriculture. 2017. Swede Midge. Retrieved from http://www.mda.state.mn.us/plants/insects/swedemidge.aspx
Phillips, B. 2015. Swede midge, an invasive pest of cole crops, has been discovered in Michigan, presenting a new management challenge where present. Michigan State University Extension. Retrieved from http://msue.anr.msu.edu/news/swede_midge_biology_and_management
Phillips, C., A. Ambourn, and L. Christianson. 2017. First detections of Swede midge (Diptera: Cecidomyiidae) in Minnesota. J. Entomol. Sci. 52(3): 297-300. Retrieved from http://www.bioone.org/doi/abs/10.18474/JES17-32.1