Monitor for abnormal cole crop injury to slow the spread of swede midge

By Doreen Muzzi

The question is not if, but when, swede midge will advance farther south into the heart of U.S. broccoli, cabbage and cauliflower production, say entomologists.

There's certainly the potential for concern for U.S. growers in 2005 because of the swede midge damage that has occurred in Canada, especially in the Ontario area, says Tony Shelton, a Cornell University entomologist in Geneva, N.Y. The potential host range in the United States for this particular pest is enormous. Studies completed so far indicate that based on temperature and rainfall requirements the host range could include virtually the entire U.S. vegetable production region.

In 2004, swede midge was discovered in Niagara County, N.Y., using experimental pheromone traps placed along the edges of vegetable fields. The pest, which can severely limit harvest potential if allowed to attack young cole crop plants, prefers moist soils and sheltered locations.

Shelton advises cole crop producers to be on the lookout for any abnormal injury, such as twisted or missing heads, split terminals, crinkled heart leaves or other distortions. Infested seedling plants will produce no marketable yield, whereas plants infested in the early development stage will have a severely reduced yield.

Meanwhile, he says, researchers will continue to monitor for swede midge in the United States.

Through the use of pheromone traps, we did find swede midge adults in New York fields last year, he says. We don't know if they were there previously and we just didn't find them or whether they moved into the region in 2004.

University of Georgia Extension entomologist Stormy Sparks says he doesn't expect the pest to move into the Southeastern vegetable growing region this year. However, he says, the pest could potentially migrate south.

So much plant material gets moved around that the potential does exist for swede midge to infest Southeastern vegetable crops, says Sparks, who's based in Tifton, Ga.

Mistaken identity

Because the adult swede midge is less than 2 millimeters in size, it is difficult even for entomologists to differentiate from other closely related species, Canadian government officials say.

Scouting for injury caused by very low infestations of swede midge is very, very tough, Shelton says. You might just find less than 1 percent of the pests causing the plant injury.

To complicate matters, Sparks says the damage caused by swede midge can easily be confused with other types of pest damage.

While we are not specifically scouting for swede midge populations, we do have the systems in place to monitor new pest problems, Sparks says. We are hopeful that any unexplained damage to cabbage, broccoli or cauliflower crops will be turned in to our pathology laboratory. Often, any unexplained damages or production factors that seem out of place show up rather rapidly.

In Canada, damage caused by the pest was misdiagnosed for years. It can resemble molybdenum deficiency, heat stress, hormonal herbicide injury, genetic variability or frost damage.

By the time people started paying attention to swede midge, the infested plants had been moved around to such an extent that the infestation became much more widespread, Sparks says. We are trying to be more proactive before the pest becomes problematic in the United States.

It is vitally important for us to develop management practices for our growers prior to this becoming an endemic pest in the United States. We need to have strategies in place so that if and when swede midge infestations do occur, we'll have the knowledge needed to adequately manage the pest.

A battle is drawn

The plan of attack includes monitoring the pest's movements through pheromone traps and researching potential control methods.

In 2004, Cornell University researchers conducted quarantined, lab-based insecticide studies into promising control options. This year, they hope to collaborate with Canadian counterparts to have the top contenders included in Canadian field trials.

Currently the only insecticide labeled for swede midge is Assail 70WP, a foliar spray that only has a supplemental label in New York.

There are foliar insecticides, drenches and even seed treatments that could provide some systemic activity on swede midge that could dramatically reduce the amount of insecticides being used and still provide excellent control, Shelton says. There is also a lot of value in trying to test for any varieties that might be more tolerant to sweet midge injury. We've just begun some tolerance tests, and they are proving to be very encouraging. If we can combine host plant tolerance with a judicious use of insecticides, that could go a long way to managing this pest.

Entomologists are also studying whether there are connections among wild weed hosts, such as wild mustards and shepherd's purse, soil types and swede midge populations.

In addition, the U.S. Department of Agriculture will continue to use pheromone traps to monitor selected areas, including New York, for swede midge populations.

If growers suspect a swede midge infestation in their fields, they should contact their county Extension agent or local USDA Animal and Plant Health Inspection Service office. G

For questions or comments, contact Vicky Boyd, editor of The Grower, at (209) 571-0414 or

What is the swede midge?

A native of Europe and Asia, the 2-millimeter-long brown fly (upper left) is indistinguishable from other midges except by insect experts. The pest was first found in Ontario, Canada, in 2000. The resulting injury to Canadian cole crops has been so substantial that some growers plowed down the crop and replanted, says the Ontario Ministry of Agriculture and Food.

It appears to have a wide host range within the Brassica family, including cauliflower, broccoli, cabbage, leafy Asian crucifers and raddishes.

According to the Ontario Ministry of Agriculture and Food, the swede midge overwinters as a pupa in the soil and emerges as an adult in northern climates between mid-May and early June.

Mated females lay eggs, typically in clusters of two to 50, on the youngest and most actively growing portions of the host plant.

Once the eggs hatch, the small maggot (lower right) feeds on plant tissue, resulting in stunting, distortion and/or plant death.

Once mature, the larvae fall to the soil to pupate and continue the cycle. The pupae can survive in the soil for up to two years before emerging as adults.

Internet Hotlinks

Cornell University Swede Midge Fact Sheet

Ontario Ministry of Agriculture and Food:

The Swede Midge A New Pest In Crucifer Crops In Ontario