Cabbage maggots or cabbage root maggots are sporadic pests of various cole crops and a few root crops including cabbage, broccoli, cauliflower, Brussels sprouts, turnips, radishes, kale and rutabagas. They may occasionally attack beets and celery!
Identify the pests
Adult cabbage root flies emerge from the soil in early spring, fly close to the ground and lay eggs in cracks near plants and on the plants where the plant meets the soil. The flies are slender, long-legged and dark gray with black stripes on the back. They are about half the size of common houseflies making them very hard to spot!
The eggs hatch in 3-8 days. Cabbage maggots are small yellowish-white legless worms that grow to a maximum size of ¼ inch. They feed on the roots of the host plant for 3-4 weeks before pupating in the soil.
The pupal stage (brown elongated kernels) lasts for 12-21 days after which the 2nd generation flies emerge. There may be up to 4 generations a year.
Signs of an infestation
Often, infestations aren’t spotted until the damage is already done. Cabbage maggots feed on the root hairs of plants and form tunnels or holes in the roots or tubers of infected plant. This could make the plant look off-colored, sickly, and small and could eventually lead to plant death. While this could be a symptom of nutrient problems or unfavorable temperature conditions, check the roots to make sure!
Going about your crops looking intently for the cabbage root fly is the way to go especially in early spring when they are looking to lay their eggs.
Controlling and Prevention Techniques
Use row covers: Cover your seedlings with a very fine floating row cover and ensure that all edges are buried in the ground. This should prevent the root maggot flies from laying its eggs near your plants.
Use shields on the ground: Tar paper, weed fabric and old carpets can be used as effective shields against female maggot flies, preventing them from laying eggs around the base of your plants. Plant your transplants through a slit in the shield that is pressed to the soil to ensure no flies can creep in below!
Cultural control: Cultural practices have a major impact on cabbage maggot populations. Tilling the soil before seeding or before the onset of spring will reduce levels of emerging maggot flies owing to destruction of pupae that is overwintering in the soil. Destroy infested plants. Do not use them for composting!
Crop rotation will have a positive impact on controlling cabbage maggots, hence must be inculcated. Controlling cruciferous weed species like shepherd’s purse, stinkweed, wild mustard and flixweed that could succumb to cabbage maggots resulting in larger populations of cabbage maggots is another pest management option.
Environmental and biological control: Cool temperatures and damp soil conditions are ideal for laid eggs to hatch, thus influencing levels of cabbage maggots. Hence cabbage maggot infestations are very severe in cool, wet springs. Conversely, hot dry conditions do not favor cabbage maggots.
If you’re lucky, a naturally occurring soil fungus may attack and reduce cabbage maggot larvae populations. Parasitic wasps, grub-away nematodes, parasitic beetles and rove beetles are good biological controls.
Plant later than usual: By doing this you can avoid the 1st and most damaging generation of maggots. Start plants indoors and transplant them into your garden later on in spring.
Sticky traps: Yellow sticky traps laced with Vaseline or honey will attract adult maggot flies! Sweep nets are also an option for trapping adult maggot flies.
Insect repellents: Mounding wood ash, diatomaceous earth, hot pepper powder or ginger powder in and around the garden have all been used effectively against repelling insects.