The Cabbage Root Fly - Delia (Hylemnyia) brassicae

Introduction

The cabbage root fly is one of the more serious brassica pests. Summer cauliflowers, broccoli, brussels sprouts, spring and autumn cabbages, savoys and kales all fall victim to attack. A large percentage plant mortality is unusual, however those plants which recover are of less marketable quality.

Morphology

Adult flies are grey in colour and resemble the common house fly.
Size: about 1cm long.

Life Cycle

• Spring
  With the advent of warmer weather in April or May the first generation of flies emerges from pupae of the latest generation of the previous summer, which have overwintered in the soil.
• Eggs
  Females lay eggs on or just below the soil surface close to the main stems of cruciferous plants. Eggs are very small but may be seen by the unaided eye.
• Maggot
  The creamy-white maggot emerges after about 6 days, and feeds on roots and may tunnel up the main stem.

• Symptoms of attack
  Plants which are attacked first exhibit a drooping appearance and later the leaves turn a blueish colour, as if nutrient deficient, and tend to wilt easily.
• Pupal Stage
  After about 3 weeks when the maggot is fully fed and has migrated a short way from the attacked plant, a reddish brown pupa, about 1 cm long forms.

Several generations can appear throughout the summer months - about every month or so. The result is a continuous presence of flies as generations overlap.

Natural Enemies

All immature stages of the cabbage root fly have natural enemies in the soil. The diet of small beetles may include eggs, maggots and pupae. Pupae may also be destroyed by staphinylid beetles and other insects. Conventional insecticide use may kill many beneficial insects as well as the targeted cabbage root fly.

Control

• Where a bad attack has occurred, it is inadvisable to plant another brassica crop until other vegetables have been taken in the rotation. In the seed bed affected, plants should be removed and burned and any grubs from the surrounding soil collected and destroyed.
• Chemicals (1948) : mercuric chloride, napthaline and calomel dust

Management Practises to minimise the effects of seasonal pests

• Cabbages bordering a Potato Crop
• Cabbages and turnips growing in alternate rows

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