By Island County Extension Agent, Donald B. Meehan
WORMY" VEGETABLE ROOTS
This is the time of the year when many of you are harvesting root vegetables, as well as a number of other garden crops. A common problem found during the harvesting is the damage caused by insects feeding on roots below the ground. In most cases three different fly species are to blame for this damage. The first is the Cabbage Root Maggot which infests the underground portions of all members of the cabbage family and radishes, too.
The second pest is the larva, or maggot, that tunnels its way inside the roots of carrots and parsnips. It also can be found eating the roots of parsley and sometimes burrows into the hearts of celery, although these latter two cases are much less common. This insidious pest goes by the name of Carrot Rust Fly and is widespread in the Puget Sound region. It does considerable damage to carrots in home gardens.
Another critter which attacks onions and other members of this vegetable family is called the Onion Maggot. These maggots kill seedlings and damage bulbs of older plants.
All three of these monsters tunnel in the root parts of their host plants and this results in dwarfing, stunting, and wilting of the plants. As if this weren't enough, the wounds caused by these insects' feeding are also often infected by a bacterial disease called soft rot. It does exactly what it's called: it rots and decomposes the vegetables, usually causing some pretty severe odors.
The Cabbage Maggot is a thick, fat sort of white worm that causes most of its damage while the host plants are still young. Extensive feeding will often cause young plants to be killed. On warm days, the plants wilt and die. However, once the plants are established and begin to grow bigger, they are often able to keep growing in spite of being eaten by the maggot. They more or less outgrow the damage by putting on more roots than the insect can eat. If the maggot population is too high, even larger-rooted plants can be killed. With radishes, turnips and rutabagas, the edible root may not be edible because of the damage. Often cutting away damaged parts will help salvage the crop.
The Carrot Rust Fly's larva is a bit smaller, less fat and yellowish in color, a little like its carrot host. Usually the longer the carrots are left in the ground, the greater the damage becomes. If Carrot Rust Fly is a problem in your garden, harvest them as soon as they're ready and don't try to overwinter late carrots in the garden.
There are several methods that will give some control over these pests. One method is to cover the rows of susceptible crops with tight-fitting screen cover. This must be put in place at planting, so female flies are prevented from egg laying on the soil near the plants. Satisfactory results can be achieved by making a frame and covering it with nylon window screening. A well-made, tight-fitting frame will exclude the female flies very effectively, and it's particularly effective with radishes, onions, carrots and turnips, since you don't want any damage in these. These plants are smaller and are more easily covered. For cabbage, cauliflower, broccoli, etc., screen also works well, but your frame needs to be bigger.
A new material on the market called Reemay, an ultra-lightweight, spun, fibrous stuff, can be laid right on top of the rows and tucked under the soil at the sides to keep the flies out. Enough slack is left in the material so when the plants grow, they just push it up. It's so light in weight, it doesn't cause any undue stress to the plants.
In the past, only Diazinon was registered for use against these maggots on cole crops and radishes, but it never really did the job of controlling them because its effect lasted only about two weeks. The label says it can only be applied at planting time. Another insecticide, Dursban, has recently been registered for use to help deal with the worm problem. It works particularly well with radishes and most of the Cole crops. In the case of turnips and rutabagas, however, since their season of growth is so long, only early-season protection is gained. During the latter stages of growth they, too, may be infested, but the damage is not likely to be as great. Follow label directions for amounts to use, and how. Unfortunately, Dursban is not registered for use on carrots or onions, and if an insecticide is used, it must be Diazinon. Use according to label directions.
Finally, there is no scientific evidence that I am aware of, that using garlic sprays, ashes, sawdust, and paper collars to control maggots works. Testing done with these materials some years back at WSU's research center in Puyallup showed they were largely ineffective.
My thanks to my co-worker, George Pinyuh, King County WSU Extension Agent, for the use of his material for writing this column.
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