Using Insect Exclusion Netting to Manage Swede Midge in Organic Brassicas

Agriculture
Vegetables

Swede midge is an invasive insect pest that is threatening the viability of broccoli, cauliflower, cabbage, Brussels sprouts, kale, kohlrabi and turnip production within the Cornell Vegetable Program region and throughout the Northeastern US. Here's how organic brassica growers can use insect exclusion netting to manage the pest.

There are two methods to use insect exclusion netting (IEN), specifically ProtekNet, 25 g, to manage swede midge in brassicas:

As a cover over top of susceptible brassicas to exclude/protect the crop from swede midge; and
To include inside the netting swede midge that emerge from soil so that they cannot escape and attack susceptible brassicas outside the netting. In this case, a non-susceptible crop is grown under the netting.

Swede Midge is Tiny but Mighty

Swede midge is a tiny fly that lays its eggs deep in the growing points of brassica crops, and after the larvae (maggots) hatch, they feed on the young tissue which results in brown corky scarring, leaf puckering and twisting, multiple growing shoots or heads, and/or no head at all. Swede midge feeding damage in cabbage, broccoli, cauliflower/Romanesco and kohlrabi can render the marketable portions unmarketable, while feeding on kale and collards reduces marketable leaves/yield. Organic management of swede midge is dependent on disrupting the swede midge life cycle. Swede midge overwinter in the soil as pupae and emerge as adults (flies) around mid-May, which have 3 days to find a mate and a suitable host to lay eggs. After 3 days, larvae hatch and feed for 1 to 3 weeks, before dropping to the soil to pupate before emerging as adults in another 1 to 2 weeks. In total, the swede midge lifecycle is 21 to 44 days and there are 4 to 5 overlapping generations per season on New York.

Better Know a Pest: Swede Midge

Insect Exclusion Netting to Protect the Crop

IEN when used to protect susceptible brassicas from swede midge resulted in 100% control in Cornell studies (Hoepting et al. 2015-2017). However, IEN can only be 100% effective if swede midge does not emerge from the ground into the netting and provided swede midge does not enter the netting.;

Only use IEN to protect a brassica crop on ground that has not been cropped to brassicas in at least 2 years, the longer the better.;
- There is a small proportion of overwintering pupae that survive in soil for 2 years;
- When swede midge emerges from soil into the netting, it is even easier for the flies to find partners to mate and a suitable host to lay eggs, and the population can thrive and destroy the brassica crop, completely defeating the purpose of using the netting.
- Ensuring ground is free of swede midge works best on farms with permanent beds/fields.
- When under IEN, a brassica crop can be protected from swede midge even when grown adjacent to a bed/field that was infested with swede midge the previous year. This has been proven to be the only way to grow spring broccoli on a small farm that is infested with swede midge.
Set up IEN over brassica crops before spring emergence of swede midge, in early May.
For large brassica crops such as broccoli, cauliflower, Brussels sprouts, kale, collards, etc., IEN should be placed over 4-ft x 4-ft ¼-inch electrical conduit hoops that are anchored to the ground with electrical fencing stakes (cut in half) and held to the hoops with clamps. This will require IEN that is 14-ft wide.
- Shorter brassica crops such as kohlrabi, cabbage and turnips can be used with IEN over short hoops. IEN that is 10-ft wide will work.
- Hoops are typically placed 4-6 ft apart.
Using IEN in combination with mulch for weed management is very helpful so you do not need to enter/compromise the netting to hand weed, which may let swede midge in.
Secure edges of IEN to the ground using small rocks or sod/soil. If not secured, swede midge could enter from outside into the IEN when it is blown by wind or lifted by crops or weeds growing inside the netting.
- Using rocks requires getting the rocks from somewhere (rock picking).
- When IEN is sodded in, weeds often grow through it, and the netting tears when it is removed at the end of the season. It could be repurposed over short hoops, however.
Start with clean transplants that are free of other pests, including worms and aphids.
Conditions under IEN tend to be more moderate than the outside (cooler when it is hot outside and warmer when it is cold outside). This tends to result in less stressful growing conditions and healthier plants.
Sometimes, plant crowding in IEN tunnels results in heavy infestations of cabbage aphids and slugs (which prefer humid conditions). Creating a new pest problem while solving another is not ideal.
- Keeping the low tunnels free of weeds helps to improve aeration within the crop canopy.
- Sluggo slug bait may help with slug management.
- Controlling aphids organically can be challenging.
IEN works best for crops that do not need to be harvested multiple times, because the IEN may remain intact for majority of growing season.
- Once broccoli and cauliflower are ready to harvest, IEN can be removed, because swede midge will not cause economic damage to heads anymore.
IEN is expensive and labor-intensive to set up and take down. It generally only makes economic sense when high value crops are grown where swede midge pressure is high. For example, $5/lb broccoli crowns that would have suffered > 50% crop loss outside.
- IEN becomes more affordable every time the IEN is reused. Care must be taken when taking it down not to not tear it. Minor tears may be repaired with duct tape.

Advantages

Protects crops from other pests including cabbage maggot, flea beetles and worm pests.
Less stressed plants yield higher under IEN.
IEN can cover crops season-long unlike Reemay floating row covers that are too hot for brassicas in the summer and must be removed.

Disadvantages

Expensive
Labor-intensive
Difficult to weed and harvest under IEN

A section of 4-ft x 4-ft low tunnel covered with white netting. A row of small rocks is holding the netting to the ground.

Insect exclusion netting secured to ground using rocks so netting is not blown in wind or lifted by crops or weeds grown under the IEN.

Inside view of low tunnel covered with white netting. The 4-ft wide section has 2 rows of broccoli plants that are growing out of black landscape fabric.

Pairing insect exclusion netting with mulch such as landscape fabric to control weeds prevents netting from being compromised for hand weeding.

Looking through insect exclusion netting to lush green kale plants.

Red Russian kale grown under insect exclusion netting. Netting creates more moderate growing conditions relieving plants of stress and attack from flea beetles, resulting in larger and healthier plants.

Keep Swede Midge In

In this strategy, IEN is placed over top of ground that is heavily infested with swede midge so that they do not escape and infest nearby susceptible brassica plantings. A non-host is grown under the IEN instead. The swede midges emerge and perish in the absence of a host.

Ground that is heavily infested with swede midge would be where a brassica crop was heavily infested with swede midge the previous fall. This is the spring emergence site.
Set up IEN over crops prior to the spring emergence of swede midge, in early May.
Crops that work best are those that only need to be harvested once and that will remain in the ground until spring emergence has subsided (second half of July).
- Carrots, beets and onions work well.
Use of mulch for weed control will reduce need to open IEN for hand weeding.
If there are other sources of swede midge on the farm, this strategy will not result in 100% control, but it will damper the buildup of swede midge pressure on your farm.
The major advantage of this strategy is that ground does not need to be taken out of production, unlike with crop rotation and use of ground barriers.