1. Identification

Identification

When to look for cabbage maggots
  • Multiple overlapping generations of cabbage root flies occur each year between about mid-March and the first frost.
  • The degree-day model for cabbage maggot can help identify when the first generation emerges in spring (available via US Pest).
  • Only pupae survive over the winter.
Adult (fly)
  • Adults are the most conspicuous phase of the life cycle, and look like small houseflies.
  • They are easiest to spot using yellow sticky traps at field edges.
  • They fly rapidly at approximately the height of the plant canopy and are difficult to catch by hand.
Egg
  • Adults lay eggs at the base of plant stems or in cracks in the soil.
  • Eggs are approximately 1/8 inch long, white, and cigar-shaped.
  • Cabbage maggots often lay eggs in clumps, as shown, rather than singly. Using a hand lens may make it easier to spot eggs.
  • Eggs hatch into larvae after 4-10 days, depending on temperature
Larva (maggot)
  • Larvae are the life stage that are destructive to plants.
  • Larvae hatch from eggs and feed on roots for around three weeks before becoming pupae.
  • Larvae are generally found in the top 2-3 inches of soil on plant fine roots.
  • Larvae can also leave tunnels in storage roots (such as radishes). Cutting a root into sections can confirm tunneling damage.
  • Many species of larvae look very similar, so knowing the crop and plant size is also important for identification. Cabbage maggots tend to feed on brassica crops with a few true leaves.
Pupa
  • When larvae have fed long enough, they pupate.
  • Pupae are brown and typically found a few inches under ground, possibly stuck to a root that the insect fed on.
  • Larvae pupate underground for at least two weeks, after which adult flies emerge.
  • Pupae are the only life stage that survives the winter.
  • This is the most difficult phase of the life cycle to find, and pupae look similar to those of other insects.
  • Pupae are also a difficult life stage to target for pest control.

Want more information?

Related species

  • There are other pest species related to cabbage maggot.
  • It is impossible to tell them apart visually in the field, but their preferred crops and plant life stages are different.
  • The main related species in the Willamette Valley is seedcorn maggot. In general, cabbage maggots affect plants with true leaves, whereas seedcorn maggots will feed on seeds prior to germination.
  • Jessica Green created a table (pdf) that explains the differences among the flies in this species complex.