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 last year in Farmington. An adult fly, only about two millimeters in length, has been circled in black on the sheet. File photo
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.”
UMaine Cooperative Extension Agriculture and Non-Timber Forest Products Professional Dave Fuller said a local farmer called him last August to say things didn’t look right with their broccoli. He visited the Farmington grower’s broccoli fields four times to examine crops that weren’t growing normally.
These broccoli leaves show the damage caused by swede midge larva. They regurgitate their stomach contents onto the leaves. Dave Fuller, UMaine Cooperative Extension
“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.”
Fuller said verification that it was swede midge was confirmed in September. 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 in Maryland for an official declaration.
Maine is the last New England state to have found these flies.
Since the initial identification, Fuller said he had a home gardener say they had broccoli missing heads (blind heads), an indication swede midge is present. He also saw Brussels sprouts missing sprouts on another farm later in the year, another indicator.
“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.
An adult swede midge fly. shutterstock
“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
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
‘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.
“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.
“Cultural management will reduce numbers but only one larva feeding can result in a cauliflower not being marketable.”
“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 growers can assume they don’t have swede midge if broccoli was not affected last year.
“Broccoli is the one brassica most visibly affected and the best indicator,” he said. “Farms should practice three-year rotations and be on the lookout for it on their farms.
“Other cultural control methods include deep plowing to turn pupae under and planting as far from previous brassica plantings as possible. Winds will blow swede midge around.”
Fuller said he monitored for the insect last year to see just when they stop.
“Through pheromone trapping, I found that swede midge persists until the end of September,” he said. “It starts in the spring when night time temps are in the 50s.
“Perhaps most importantly is how pervasive and damaging such a tiny fly is. It is active from early May through September with three to four generations causing damage along the whole way as opposed to some pests we have to manage with a much more narrow window of feeding and damage.”
“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. He plans to monitor the insect this year to gather more information.
“Pheromone traps will be placed in the next few days at two Farmington farms,” he said. “The traps will be monitored daily until the presence of swede midge has been detected, whereupon notice will be made to farmers.
“Local home gardeners would be wise to use lightweight row covers this year which will also minimize flea beetle and cabbage butterfly caterpillar damage to brassicas.
“There is reason to believe that swede midge will be with us for some time to come and that it will continue to spread throughout Maine. I am working on a publication about Swede midge in Maine, with a map of known locations plus information on its life cycle, signs of damage and management suggestions.”
Cornell University has an informative fact sheet on swede midge: http://web.entomology.cornell.edu/shelton/swede-midge/index.html.
Fuller said to his knowledge, only Franklin and Aroostook counties are known to have swede midge.
Those interested in learning more, or think they had damage last year, should call him at UMaine Extension, 778-4650. Or email him at dfuller@maine.edu. He is keeping track and posting notices when spread has been documented.
“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.”
Comments are not available on this story.
Send questions/comments to the editors.
