Biological control of the cabbage root fly, Delia radicum
What can you do against the cabbage root fly?
The cabbage root fly, Delia radium (Order: Diptera; Family: Anthomiidae), attacks all crucifers and is a serious damage on cabbage, radish, rape etc. Roots of cabbage can be eaten away totally by the maggots. In radish and rape, the maggots form galleries in the fleshy parts, rendering the plants unsuitable for consumption.
The fly, which is also called cabbage fly, is a 6-7 cm long, grey fly, that looks like a normal housefly. The larvae, or cabbage maggots, are yellow-white, legless maggots. They can reach a length of 10 mm. They have no head, but only two dark-coloured mandibles. They pupate in a barrel-shaped, brown puparium.
Two cabbage maggots and a puparium
Photo by Rasbak
Biology of Delia radicum
The first flies appear in April/May. Females lay their eggs in small clusters in the ground around the stem of cruciferous plants.
At 15 ÂºC, the larvae hatch in about a week and immediately start feeding from the roots. While feeding, they penetrate the roots to form galleries in the most tender parts.
When mature, after three to four weeks, they leave the plants to pupate in the soil. In the UK there are usually two to three generations per year, depending on temperature. If ground temperatures in summer rise above 22 ÂºC, the pupae remain in a state of rest and only continue their development when the temperature returns to below 20 ÂºC.
Pupae that are formed in September or early October overwinter in the soil.
Damage by the cabbage maggots
The cabbage root fly attacks all crucifers and is a major pest of cabbage, radish and rape.
Roots of cabbage and rape can be more or less destroyed, and crop losses in the UK can be up to 60%. In root vegetables, such a radish, the larvae form galleries in the fleshy parts, rendering the plants unsuitable for consumption. Purplish red leaves turning yellow are often a sign that the plant is attacked by this pest.
Natural enemies of the cabbage root fly
The main parasitoids of the cabbage root fly are the wasp Trybliographa rapae (Order: Hymenoptera; Family: Cynipidae) and the rove beetle Aleochora bilineata (Order: Coleoptera; Family: Staphylinidae).
T. rapae females lay eggs inside first or second instar larvae of the fly, whereas female A. bilineata lay their eggs in the soil around infected plants.
The larvae of A. bilineata, locate the pupae of the cabbage root fly and develop inside the puparium by feeding on the pupa.
Predators are the ground beetles Pterostichus vulgaris (Order: Coleoptera; Family: Carabidae) and various Harpalus species. The small Bembidion species of the same family, eat eggs of the cabbage root fly.
Bembidion quadrimaculata - One of the predators of the cabbage root fly
Photo by courtesy of @entomart
Treatment of the cabbage root fly
- Root vegetables should be covered with a horticultural fleece or row covers during the first egg-laying period in April/May.
- As females prefer a dry environment for laying eggs, regular watering at root level will decrease the number of eggs and, therefore, damage later in the year.
- Place a piece of cardboard, carpet or felt on the ground around the stems of cabbages. The females will lay their eggs on it, instead of on the ground, which results in drying out of the eggs, before they hatch.
- Once you have harvested the brassicas, do not grow them in the same place for at least four years
- Allow chickens to roam on the land during the winter. They will expose the over-wintering pupae and consume them.
- Hang up nest boxes and feeders for insect-eating birds
- Lure ground beetles to the plot by providing suitable shelters.
- And last but not least: destroy the roots of cabbages after you have harvested the tops.
Applying the protection around the stems of cabbages
Note that the lady gardener talks about "beetles going down the stems to eat the roots". This is, of course, not what she places the ring-shaped pieces around the stems for. They are to prevent the flies from laying their eggs on the soil near the stems!