Harvesting brassicas? Look out for a new invasive insect.

As the season starts to wrap and fall cole crops are harvested, keep an eye out for damage from a new invasive pest: swede midge.

Swede midge is an invasive fly that is a bit different than many of our insect pests.  It isn’t like Colorado potato beetle or cabbage looper, where you will notice the bug doing the damage.  In fact, you are unlikely to notice the damage until the culprit is long gone, and the damage can be so cryptic it is hard to figure out if swede midge is actually the cause.  

The midges themselves are small flies, and the larvae that do the damage are even harder to see.  They are only a few millimeters long and translucent, and feed directly on the growing point of brassica crops and weeds.

Swede midge larvae feed on the growing point, but in high numbers might spread out to other areas. Photo: Mao Chen, Cornell University, Bugwood.org.

So what are you likely to see if swede midge has been active on your farm?

• Plants with odd growth pattern and distorted leaves

• Plant with multiple stems

• Uneven heads

• Complete loss of head or crown

• Corky, brown scarring at the base of the stem/head/crown

A clue to look closer for swede midge scarring is plants that seem to have lost their way, producing distorted and twisted leaves in many directions instead of a head or crown. Photo: Julie Kikkert, Cornell Cooperative Extension, Bugwood.org

Many brassica crops and weeds are fed upon by swede midge.  As the larvae feed on the growing point, crops where the crowns are harvested are very susceptible to damage.  The most preferred crops are broccoli, cauliflower, collards, and kohlrabi.

A distorted cabbage plant, with the corky brown scaring present at the base of the plant and on the stems of the distorted leaves. Photo: Marissa Schuh, University of Minnesota Extension.

Where in Minnesota is swede midge?

Currently, swede midge is well-established in some metro-area community gardens.  How far will it spread beyond that? How far has it spread currently? We aren’t sure.  At first glance, the name “swede midge” may imply a scandinavian tolerance to the cold, but the “swede” part of the name comes from a type of European turnip.  We aren’t sure how cold-hardy swede midge is, though there are reports of swede midge in Manitoba.

In the next growing season, a team of UMN professors and extension educators will be investigating the extent of swede midge’s spread in MN.  As you harvest fall brassica crops, keep an eye out for swede midge damage.  If you suspect you are seeing swede midge damage, please report it in the form below.  It will help us connect with you as we work on this frustrating pest!