Scouting for Swede Midge

Christy Hoepting, Cornell Cooperative Extension Vegetable Program

Swede midge damage is likely to occur first:

• In the spring, in fields/sites where a cruciferous crop was grown the previous season, or downwind of last year’s crucifer crop, especially one that was not disked up shortly after harvest. These areas serve as the over wintering sites where swede midge will emerge in the spring.
• In the more susceptible cruciferous crops, such as collards and broccoli. For example, in a field of mixed crucifers, swede midge damage may likely appear first in collards and broccoli before it is noticed in cabbage or Brussels sprouts.
• In fields/sites that are sheltered compared to large wide open and windy fields.
• Along field edges near hedgerows, tree lines, buildings and other sheltered areas. Swede midge tends to be blown into sheltered areas which favor mating and the finding of suitable host plants to lay eggs.Swede midge can be scouted for when scouting for other pests in crucifer crops. Pay special attention to fields and crops that are at high risk for swede midge infestation (see above), and to sheltered field edges.Early signs of swede midge are very subtle and require a keen eye to detect. Realistically, swede midge damage is not observed until plants have been infested long enough for the toxins of the feeding larvae to produce more noticeable plant symptoms such as leaf puckering and scarring.

Step 1:
When walking down rows of cruciferous plants, look for leaf puckering, crinkling and drawstring effect, which are most noticeable on middle-aged leaves. This is damage that would have originally occurred at the growing point, but has grown out. (Figure 1 – collards). Look for plants with blind heads (Figure 2 – broccoli) or multiple shoots/growing points (Figure 3– broccoli).

Step 2:
Once you have identified a suspicious plant, i.e. one that has puckered leaves or multiple growing points, bend down and take a closer look. See if you can find any brown corky scarring amongst the crinkled leaves, along the leaf petioles or at the growing point (Figure 4). Brown corky scarring is a good indication of swede midge damage, especially when it occurs in combination with other types of swede midge damage. However, swede midge feeding is not the only cause of brown scarring in crucifers, and a cruciferous plant may be infested with swede midge and not have any brown scarring.

Step 3:
To confirm that suspicious damage is in fact caused by swede midge, it is ideal if you can find the larvae. To find larvae, look for growing points that may be twisted (Figure 5 – broccoli), have swollen petioles/leaf bases (Figure 6 – broccoli), and/or have puckered leaves and brown scarring (Figure 7 – broccoli). Note, you may need to look for suspicious growing tips in plants other than those that are showing the more obvious symptoms as described in step #1.

Step 4:
Peel back the leaves of the growing tip. Note, this is easier when you remove the growing tip from the plant. Look for tiny whitish to yellowish larvae (up to 3-4 mm in size when mature) nestled between the leaves, especially near the leaf axils (Figure 6 – broccoli). Swede midge larvae feed in groups and often you will see several larvae inside a growing tip (Figure 7). During feeding, larvae produce a secretion that breaks down plant tissue resulting in a moist environment (Figure 8– broccoli). If you find a moist growing tip, it is very likely that it will contain swede midge larvae. Using a 10x to 20x magnifying lens will help you to see these tiny larvae. Another option is to place the growing tip into a vial or other container of rubbing alcohol; this will cause the swede midge larvae to come out of the growing tip into the alcohol where you can easily see them. If you do not find larvae right away, keep looking! They can be tricky to find, until you get the hang of it.

Note, swede midge larvae may be confused with other things that you might find within the new growth of a cruciferous plant. Cabbage or other types of maggots (Figure 10) may look similar to swede midge larvae, but they have black mouthparts, while swede midge larvae (Figure 9) do not. Also, other types of maggots tend to be larger in size than swede midge larvae. Newly hatched diamondback moth larvae (Figure 11) have black head caps while swede midge larvae do not. Newly hatched imported cabbage worm larvae (Figure 12) have prologs and are fuzzy while swede midge larvae are not. Various maggot eggs (Figure 13) are similar in shape, but unlike swede midge larvae, they are bright white, rigid and do not move. Onion thrips adults are brown in color and the larvae are yellow, but unlike swede midge, they have antennae and legs (Figure 14). When swede midge larvae are nearing maturity, they are bright yellow and can curl up and fling themselves right off of the plant. If you ever see a larvae do this, it is very likely a swede midge.

Swede midge-like damage may not be caused by swede midge. If you experience leaf puckering, twisting or malformed growth on a serious scale with respect to the severity of damage and number of plants affected, but you cannot find any brown corky scarring or larvae, there is a very good chance that the damage is caused by something else, such as herbicide injury, for example (Figure 17).

If you are in doubt whether you have swede midge, it is a good idea to contact your local Cooperative Extension service, who can connect you with the expertise to make an accurate diagnosis.