UMaine Cooperative Extension Agriculture and Non-Timber Forest Products Professional Dave Fuller holds parts of a trap used to catch swede midge flies, a new invasive pest found recently in Farmington. An adult fly, only about two millimeters in length, has been circled in black on the sheet. Franklin Journal photo by Pam Harnden
FARMINGTON — A new invasive insect has been found in town which will be of significance to commercial and home gardeners raising broccoli and other members of the brassica family.
Swede midge is a member of the fly family. Once established it is difficult to get rid of. Native to Europe and southwestern Asia, the insect was first discovered in Canada in 2000 and in New York in 2004.
A report in the 2018 Journal of Insect Science said, “A single swede midge larva can render cauliflower unmarketable.”
A swede midge larva, seen near the finger has balled up prior to springing off a damaged brassica leaf. A second larva is seen to the left of that larva. Dave Fuller, UMaine Cooperative Extension
UMaine Cooperative Extension Agriculture and Non-Timber Forest Products Professional Dave Fuller said he visited a Farmington grower’s broccoli fields four times to examine crops that weren’t growing normally.
“As is typical in a new pest situation, I wasn’t able to figure out what was going on at first. I sent a sample to Extension plant pathologist Alicyn Smart. She said it wasn’t a disease,” Fuller said. “Then I looked at nutritional issues. The compost wasn’t a factor.
“I went out again, got a plant and brought it back to the office. I tore it apart, that’s when I found the larva. It’s difficult to determine the cause when we don’t have an issue here.”
Swede midge, a new invasive insect found in Farmington damages members of the brassica vegetable family. Seen is an affected Brussels sprout plant. Dave Fuller, UMaine Cooperative Extension
Fuller said it was three weeks ago that he discovered the insect. Verification that it was swede midge wasn’t confirmed until last week. He worked with Karen Coluzzi, the invasive insect specialist with the Maine Department of Agriculture. She sent specimens to the United States Dept. of Agriculture for an official declaration.
Maine is the last New England state to have found these flies.
“They have only been found in Franklin and Aroostook counties that we know of,” Fuller said. “Aroostook has thousands of acres of brassicas. They’re a big crop there.”
The adult fly is about two millimeters in size. The larva, or maggot, is what causes the damage.
“The maggots go after the active growing points, or meristems,” Fuller said. “In Brussels sprouts, the sprouts are missing. Broccoli plants are scarred, no heads form. It’s the same with cauliflower.
“Broccoli is affected the most, followed by cauliflower. Cabbage plants form several smaller heads. Kale and collards are also affected.
“One commercial grower in Farmington put in three plantings of broccoli. The first two were total losses. The last planting was a bit better.
“Swede midge is very significant economically.”
According to Fuller, the swede midge larva feed on new growth.
“The larva regurgitate their stomach contents on the leaf then lap it up,” he said. “It causes horribly distorted growth, the leaves shrivel.
“The larva are almost always found on new growth. It’s tender.
“When fully mature, getting ready to pupate, the larva constrict their muscles to ball up. Then they spring or ‘jump’ off the plant to a new location.
“I saw one balling up, looked again and it was gone. I was able to find it.”
Fuller said treatment for swede midge is challenging.
“For years we’ve been practicing Integrated Pest Management, trying to get away from spraying crops. This will go against everything we’ve done,” he said.
Fuller said for conventional growers, synthetic systemics provide the best treatment.
These broccoli leaves show the damage caused by swede midge larva. They regurgitate their stomach contents onto the leaves. Dave Fuller, UMaine Cooperative Extension
“Control will be extremely difficult for organic growers. Two to three year crop rotation, with plantings 300 yards from where they were last grown. The use of protective cover,” he said. “Most organic operations don’t have that much land. Row covers will have to be removed lest the larva overwinter underneath.
“It costs a dollar a foot for protective netting. Even using a medium weight row cover adds more expense.
“Another expense will come from tracking swede midge. Monitors to track most insects are relatively inexpensive. The one for swede midget is about $25 per trap.
“People may not be able to grow broccoli here for a few years.”
Fuller said he will continue monitoring for the insect this year to see just when they stop. He plans to monitor next year too.
“I think they go pretty late. There are three or four generations a year and they overlap. There is no period in between when they aren’t a problem,” he said.
Fuller said there may be a few predators that attack swede midge, but they usually aren’t effective in a massive infestation.
An educational display will be set up at the Extension booth in the Exhibition Hall at Farmington Fair. The insect will be discussed at the vegetable schools this winter in Portland and Bangor. Extension’s website and Facebook page will provide information. Information will be shared with the Maine Entomological Society and through Ag Matters.
Cornell University has an informative fact sheet on swede midge: http://web.entomology.cornell.edu/shelton/swede-midge/index.html.
“Now that we know what it is we have to educate people about this insect,” Fuller said. “There are more invasive insects coming that are going to be really bad.”
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