Cabbage Maggot
Beginner
Scientific Name
Delia radicum
Identification
- The cabbage maggot adult is a grey fly, approximately half the size of the common housefly
- Small (1 mm, 1/25 in.) white eggs look like tiny grains of rice
- Larvae are small (7 mm or 1/4 in.), legless and white
Often Confused With
Onion maggot
Seedcorn maggot
Period of Activity
There are three periods of egg-laying throughout the season:
- The first generation is the most active and most damaging; it occurs from mid-May through June (flowering of yellow rocket)
- A second occurs in mid-July (flowering of day lilies)
- A third in late August (flowering of goldenrod)
- Dates are only approximate, and activity varies, depending on regional temperatures and differences in soil type.
Scouting Notes
To monitor for cabbage maggot, use the 5 x 5 method. Monitoring is done by looking for eggs. Eggs are laid in clumps of 2- 12 or occasionally as a single egg in the soil at the base or near seedlings. When checking for eggs, gently move the soil around with a small paintbrush. It is not necessary to count the eggs. Knowing that eggs are present is enough to consider the plant infested.
Thresholds
There is no threshold for treatment in Ontario. It is not unreasonable to assume that, at the seedling stage, most plants that have eggs on or around them will be lost to infestation by cabbage maggots. Cabbage root maggot management is primarily preventative.
Advanced
Scientific Name
Delia radicum
Brassica crop seedlings are very susceptible to damage by the cabbage maggot. Young plants from the seedling stage to about a month after transplanting are most vulnerable. They are a considerable threat to crucifers where the root is the marketed portion of the plant (e.g. rutabagas and turnips). They may pose a special threat to some leafy crucifers late in the season when they infest the heart leaves and maggots develop in the leafy portion of the plant.
Identification
The adult cabbage maggot is a grey fly about half the size of the common house fly. Female cabbage maggot flies lay eggs in cracks in the soil beside seedlings or on the stems of seedlings near the soil line. The eggs are white, oval and about 1 mm (1/25 in. ) in length. Though similar in appearance to seedcorn maggot, cabbage maggot eggs have longitudinal striations and a groove that extends along their ventral aspect. The larva, which is the damaging stage of the insect, is a small, legless, white maggot about 7 mm (2/7 in. ) in length when full grown. The head end of the maggot is pointed and the rear end is blunt with a number of small projections that are forked at their apex.
The extent of cabbage maggot damage depends on crop stage and the weather conditions in the field. As many as 90% of seedlings may be infested with eggs in untreated fields. While maggots tunnel along roots causing damage they also create entry points for soft rot bacteria to invade. Invasion by soft rot bacteria often results in death of the entire plant. Maggots that hatch later and enter roots already infected with soft rot bacteria can retain the bacteria in their gut. When this happens, they will carry the bacteria, as adults, to the next plant or field and thus spread this disease
Often Confused With
Onion maggot
Seedcorn maggot
Biology
Adult maggots emerge early in the spring from puparia that have overwintered in the soil. Adults mate, feed and about a week later, egg laying begins. Adults feed on the pollen of dandelions, spring mustards, marsh marigolds and white clover. Depending on temperature, eggs hatch in 2 to 10 days. Egg- laying activity in the spring appears to be associated with the bloom period of the wild mustard "Yellow Rocket”. Maggots feed for 3- 5 weeks then leave the roots to pupate in the soil. Adults will emerge from the pupal stage in about 2 weeks or may overwinter in this stage and emerge the following spring.
Period of Activity
The first and most active period of cabbage maggot egg laying occurs from mid May until early June, a second from late June to mid July, and a third in late August to early September. These are approximate dates. Fly development and activity varies depending on a number of factors, such as regional temperatures, differences in soil types, and local environments. When the temperature of the soil surface is greater than 26°C (79°F), survival of the eggs is reduced. High temperatures will also delay the emergence of adults. Another factor is soil moisture. Low soil moisture will adversely affect egg hatch and adult emergence. In warmer areas of the province, damage from the second generation is usually limited by hot, dry conditions that reduce maggot survival. In cooler regions, and for root crops and most ethnic vegetables, particularly those receiving irrigation, unacceptable levels of damage can be expected from all three generations.
A Growing Degree Day (GDD) model has been developed for cabbage maggot. Using a base of 6°C (43°F), first generation adults are expected between 314- 398 GDD. Second generation adults are expected between 847- 960 GDD. Third generation adults are expected between 1446- 1604 GDD.
Scouting Notes
To monitor for cabbage maggot, use the 5 x 5 method. Monitoring is done by looking for eggs. Eggs are laid in clumps of 2- 12 or occasionally as a single egg in the soil at the base or near seedlings. When checking for eggs, gently move the soil around with a small paintbrush. It is not necessary to count the eggs. Knowing that eggs are present is enough to consider the plant infested.
Check out dandelion and wild mustards around crucifer fields for adult cabbage maggot flies. Other maggots may be found but are not generally associated with rot diseases.
Thresholds
There is no threshold for treatment in Ontario. It is not unreasonable to assume that, at the seedling stage, most plants that have eggs on or around them will be lost to infestation by cabbage maggots. Cabbage root maggot management is primarily preventative.
Management Notes
- There are a number of natural predators and parasites of cabbage maggots in Ontario. Adults of the rove beetle (Aleochara bilineata) are predators of cabbage maggot eggs and larvae. Larvae of rove beetles parasitize from 9 to 77% of cabbage maggot pupae in Ontario. The fungus Strongwellsea castrans parasitizes and kills adult cabbage maggots. Infection rates in Ontario have been observed to range between 1 and 26%.
- Cabbage maggot development is dependant on temperature, and periods of egg laying have been associated with flowering of common plants.
- Egg laying by the first or overwintering generation of flies coincides with the flowering of yellow rocket (Barbarea vulgaris). Peak activity has also been associated with full bloom of McIntosh apples and chokecherry.
- Day lilies (Hemerocallis fulva) begin to flower at approximately the same time that second generation cabbage maggots are active. Flowering of these common roadside plants should be observed at mid-day when the large orange blossoms are fully open.
- Egg laying by the third generation begins around the time that early goldenrod (Solidago juncea) and Canada thistle (Cirsium arvense) flower.
*Flowering of these plants should be used only as a guide - it is not meant to replace proper scouting of fields for the presence of maggot eggs.
