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Chaos in the Cole Crops – How to Control Cabbage Maggots

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Are your starters looking weak and wilted? One of the most common spring pests may be the cause.

By Jessica Dawe Posted Apr 14, 2016

Cabbage
Buzzing about the garden, the adult cabbage maggot is an easy miss for 99% of people. Appearing like a common housefly, this insidious pest can cause serious damage to cruciferous crops – arugula, bok choy, broccoli, brussel sprouts, cabbage, cauliflower, collard greens, and kale. If you’ve grown cabbages south of the 51st parallel in Canada or in the US as far south as Northern Carolina, you have no doubt seen this damage before. Cauliflower wilting for no apparent reason, broccoli seemingly dead overnight, these are signs that this pest is in your garden.

Cabbage maggot Hylemya brassicae is similar in shape to a housefly but has a somewhat smaller, 1cm grey body with 3 black stripes along its thorax and one black stripe running along the upper part of the abdomen. However, unless you are monitoring your garden with sticky traps, it is next to impossible to keep this fly still on a plant long enough for accurate identification. What is easier to recognize are the 1mm long, white elongated eggs laid around the base of host plants, or the classic white maggot found in the stem or root system of the plant. The cocoons containing the pupae of cabbage maggots are brown and cylindrical with slight ridges on one rounded end and tapered on the other. At this stage they are very similar to many other insects, thus it is not a reliable way of identification.

Lifecycle

The cabbage maggot can have anywhere from 1 to 4 generations in a season depending on where you live. The pest overwinters in its cocoon to emerge as an adult when the soil temperature warms in Spring. On the Pacific Coast and in parts of the East Coast such as Pennsylvania, the adult can be witnessed as early as the end of April, but the colder climes of Idaho and Montana have the adults emerging towards the end of May.

When the adults first come out they will immediately begin to feed on nectar and search for a mate. Multiple eggs are laid in singles not clusters just below the soil surface of host plants. These eggs will hatch in 3-7 days depending on temperature. The emerging juvenile maggots quickly begin searching for the roots of their favorite host plants and continue to feed for the following 3 to 4 weeks. When the maggot reaches its maturity it will form a cocoon to pupate for the next 3 weeks.

Damage

Healthy vs. infested broccoli
It is the juvenile stage that causes the destruction of early season crops. Cabbage, cauliflower, broccoli, and brussels sprouts are the most favored of hosts but additional hosts include beet, radish, turnip, and celery. Once the maggot emerges it immediately sources and feeds on the fine root hairs, eventually penetrating the larger root system making their way into the stem of the plant leaving a brown slimy tunnel in its path or visible brown streaks in the case of tap root crops. The area of feeding can be a source for secondary infection from diseases such as black leg or bacterial soft rot to which cole crops are also susceptible to.

The presence of cabbage maggots often goes unnoticed as they work away underground, mining the roots. It may take a week of feeding before you noticed that the leaves of your cauliflower are starting to yellow and wilt despite being well watered.

Severe infestations often follow cool wet springs and delayed germination.

Scouting

Scouting
Monitoring for cabbage maggot is a worthwhile task if you grow any quantity of crucifers and it is not a difficult activity.

  • After transplanting in the garden, place a yellow sticky card at plant height. Use 1 card per 25sq ft. Check weekly for the arrival of flies, identified by folded wings, the grey body and stripes on thorax and abdomen.
  • Gently scratch the surface around the base of the plant to inspect for the 1 mm eggs, generally found 2-3 per plant.
  • If wilting is observed, scratch around the base of the plant to expose the potential maggots. If discovered and damage is still external, the plant can be saved by the removal of the maggots. If damage is severe, remove plant being careful not to drop maggots on the ground, and destroy. Do not compost this infected plant material.

Management

Weak plants that are stunted from a cool wet spring or lacking proper nutrition are more susceptible to cabbage maggot attack than strong plants. It is common to observe only a few plants in a row suffering, most often those are on the perimeter of the crop.

Strategies to protect and manage this pest are as follows.

  • Crop rotation – Cabbage maggot overwinter as pupae in the soil of the crop. Limit their access to this season’s crop by planting in a new area.
  • Row cover – Immediately after transplant, cover crop with row cover to prevent access of adult flies from laying eggs around plants.
  • Delay planting – Cabbage maggot is more severe in early season plantings. Plant later and you will avoid the overwintering adults from laying eggs.
  • Tillage – At the end of the season after the crop is pulled, till the soil to a depth of 6” to expose any overwintering pupae.
  • Encourage beneficial predators – Ground beetles and Rove beetles love to dine on the maggots. Encourage them through companion planting and Nature will help do the job
  • Remove alternative host weeds such as; shepherd’s purse, stinkweed, wild mustard and flixweed
  • Paper collars – For smaller crops of only a few plants, great success can be achieved by wrapping the stem with a paper collar prior to time of transplant. Ensure this collar reaches the root system and is tight against the stem. This way if flies do lay their eggs next to the plant the maggots cannot penetrate the barrier to feed.

Paper collar

The difference between a home-grown crown of broccoli and one from a box store is night and day. Do not be discouraged by the threat of this small fly. With due diligence and sound gardening practices it is easy to prevent an infestation or limit the loss.

Related:
Natural Garden Insect Pest Control

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Jessica Dawe owns a garden center and has been practicing integrated pest management and permaculture since graduating in 1995 with a degree in horticulture.
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