Organic pest management: Cabbage Root Maggot
Cabbage root maggot, Delia radicum, was a huge problem for us last year. I've dealt with it in the past in other fields, but this was the first time we had it here. Because managing for a pest organically takes time and sometimes resources very specific to that pest, we usually don't attempt to prevent a pest until it becomes a problem. Sometimes that means that a lot of crop failure when a pest suddenly comes up. For me, it's all about expecting that pest the following season by doing all you can to learn about its life cycle and how to prevent damage organically.
Cabbage root maggot attacks plants in the brassicacea family (formerly known as crucifers). This family includes many major food crops such as cabbage, turnips, radish, cauliflower, broccoli, mustard, kale, etc. The adult stage of the cabbage root maggot is as a true fly. The female flies lay their eggs near host plants. White larvae hatch out and begin to feed on the roots, making their way to the crucial root core causing permanent and often fatal damage.
The white larvae pupate into adult flies and the cycle begins again. Depending on the length of the season, there can be up to four generations in one year. As climates warm and seasons get longer, this and other pests have more generations in a year.
Flies overwinter in the soil as pupae. In the spring they pupate to adults and lay their eggs near new plantings of brassicacea.
Signs and symptoms:
For me, the most obvious sign of feeding is that of wilting even where adequate water is present. Crops with damaged or destroyed roots cannot uptake as much water as they need, and therefore wilt. Where damage is suspected, I usually pull up the whole plant, roots and all to see the maggots. The maggots are white and easy to see. In root crops like turnips and radishes the maggots will tunnel inside, but leave very tell-tale damage like this:
Plants with damage will also have thin stems right at the soil surface. They might flop over or even break at this point.
The damage of cabbage root maggot is almost impossible to just live with because it stunts the plant so drastically and makes root crops completely unmarketable. However, it is quite preventable.
Organic pest management
Because flies overwinter in the soil as pupae near where they were feeding as larva, crop rotation where cabbage root maggot has been seen is critical. Just don't plant brassica plants where they grew last year. Just don't.
Row cover is your friend. When plants and surrounding soil are covered well with row cover, female flies don't have access to lay their eggs near roots. The maggots aren't adapted to travel far to get to their host roots, so they won't be smart enough to tunnel over to your covered crops.
With row cover, its imperative you cover the plants right after you plant and keep the cover on as much as possible. If you cover too late, you may just be trapping newly laid eggs in with their favorite food.
What won't work
Trap crops are crops you don't care about that you just plant to attract pests away from valuable crops. They aren't harvested. Trap crops only work when a pest clearly prefers one kind of plant and you plant that one as the trap crop. Cabbage maggots don't prefer one brassica over another, so they won't know to go to your trap crop instead of your valuable one. In this case, trap crops just allow the maggots and flies to grow their population.
Hoping they'll go away:
If you've had so much trouble with these guys in the past that you gave up on radishes or turnips or cabbage, I hope you'll now feel empowered to try again!