Pest Alert: Suspect swede midge found in Massachusetts (September 20, 2005)

The swede midge (Contarinia nasturtii) is an introduced pest of brassicas that has been found in Erie, Genesee, Orleans, and Niagara counties in western New York and in 15 counties in Ontario and 20 counties in Quebec, Canada. Massachusetts Department of Agricultural Resources and UMass have been working collaboratively to survey for this pest in Massachusetts as part of the USDA, APHIS Cooperative Agricultural Pest Survey (CAPS). Two suspect swede midge specimens were captured in Hampshire County this summer. One specimen was caught in a garden in Northampton with mixed brassica species. The other was from a vegetable farm in Hadley and was found in cabbage. The suspect swede midge specimens were identified visually and with PCR by Dr. Tony Shelton’s lab at Cornell University. Massachusetts Department of Agricultural Resources is continuing to trap in Hampshire County to find another specimen to have its identification verified by a USDA insect identifier. Several other states are also surveying for swede midge this year under the CAPS program. Once the full survey results are in, we should have a better idea of distribution of swede midge in the Northeast and what regulatory actions are most prudent to slow its spread.

How did it get here?
The swede midge is a major pest in Europe where it is endemic. No one knows how it reached North America or exactly how long it has been present in Canada. The presence of the swede midge in North America was not confirmed until 2000. The symptoms growers in Canada observed in their fields since 1996 were mistakenly attributed to nutrient deficiency. Since the swede midge is a poor flier, this insect is unable to move long distances on its own. The larvae pupate in the soil; therefore, soil movement is one way in which swede midge may move long distances. Transplants are also considered a vector for transport of swede midge eggs, larvae, or pupae. Canada has put a directive in place to regulate the importation and domestic movement of transplants of host species and soil that may contain life stages of swede midge to prevent the further movement of swede midge. Produce is not seen as a likely vector for movement of the midge.

What does a swede midge look like and what kind of damage does it cause?
An adult swede midge is a very small, brown fly only a few millimeters long. Since adults live only a few days, we use a pheromone trap that remains in the field for several weeks to catch their them during their flight period. Males are attracted to the pheromone lure in the trap and are caught on a sticky card inside the trap. We look for adults because they can be identified to species. Larval specimens can only be identified to genus. Larvae cause damage by feeding on plant tissue. When scouting for swede midge in the field, you will see the damage before you see this minute insect. Larvae are initially small (less than 1mm) and transparent. At maturity larvae are 3-4mm long and lemony yellow in color. If you suspect swede midge larvae in host plant tissue, you can place the tissue in a plastic bag in the sun or put the tissue in 70% alcohol to force the larvae out of the tissue.

Damage is more likely to be found around the edges of fields in more sheltered areas (near hedgerows or buildings) because the swede midge is a poor flier and prefers areas of low wind movement. Signs of damage include: brown, corky scarring; swollen and twisted leaf stalks, galls at the growth point of the plant, no head formation, and multi-headed or multi-stemmed plants. Swede midge damage looks like damage caused by cultivation, genetic variability of seed, heat stress, frost damage, or feeding by other insects (e.g. flea beetle or tarnished plant bug) that can damage the growing point of the plant. One needs to find larvae within the plant to confirm swede midge is the cause of the damage.

What can you do?
Swede midge is difficult to control, and cultural practices are a big part of the management strategy. So far, good crop rotation is one of the reasons damage seen in the United States is not as bad as in Canada. Suggested practices to minimize the spread of the swede midge include:
1) Start with clean transplants: Transplants are considered a likely vector for swede midge movement.
2) Crop rotation out of crucifers for 2-3 years: Pupae can remain in the soil for 2 years if they don’t have favorable conditions for emergence.
3) Field sanitation: Controlling cruciferous weeds eliminates an alternative host for swede midge.
4) Deep plowing and chopped of infested residue: Pupae are usually found in the top 5cm of the soil; so, deep plowing will reduce the viable number of pupae.
5) Early planting: Swede midge has between 3-5 generations per season. Early planting will allow you to harvest before populations reach their peak.
6) Plant in open fields: Early damage occurs along tree lines, buildings, and hedgerows.
7) Cultivar selection: Some varieties of broccoli such as Paragon, Eureka, and Packman are more susceptible to damage than Everest, Triathalon, and Regal. While the swede midge will attack any brassica crop, the highest levels of damage have been seen on broccoli, Chinese broccoli (gai lan), Brussels sprouts, cauliflower, Chinese cabbage, and other Asian greens.
8) Preventative insecticide applications: Canadian growers use acetamiprid on greenhouse transplants and lamda-cyhalothrin and acetamiprid in the field. Researchers are continuing to figure out the best pesticides and application times/techniques for use against swede midge.

If you suspect swede midge damage in your fields, please email Jennifer Forman Orth or call (617) 626-1735. Early detection of this pest will allow growers to implement management strategies to keep population levels low and take measures to prevent further spread of swede midge. As any new information about the swede midge is discovered, it will be posted on our website: (http://www.massnrc.org/pests).

Sources:
2005 Interim Best Management Practices to Control the Swede Midge (Contarinia nasturtii Kieffer) http://www.omafra.gov.on.ca/english/crops/facts/bmp_swedemidge.htm
The Swede midge - A pest of crucifer crops http://www.omafra.gov.on.ca/english/crops/facts/08-007.htm
Swede midge fact sheet from Cornell University http://www.nysipm.cornell.edu/factsheets/vegetables/cruc/sm.pdf
Canadian Food Inspection Agency: Interim Phytosanitary Requirements to Prevent the Entry and Spread of Swede Midge (Contarinia nasturtii)
http://www.inspection.gc.ca/english/plaveg/protect/dir/d-02-06e.shtml

The pest alert is from the Massachusetts Introduced Pests Outreach Project, a collaborative project between the Massachusetts Department of Agricultural Resources and the UMass Extension Agriculture and Landscape Program aimed at preventing the establishment of new pathogens and pests in Massachusetts. Visit the project website (http://www.massnrc.org/pests) for more information on Swede midge and other emerging pests or to subscribe and unsubscribe for pest alerts.