Post category: Cabbage root fly
Damage is done by the root fly grubs as they feed on the roots of cabbage family plants, including radish and turnip. Wallflowers and cleome can also be attacked. Sometimes in a wet season, the sprouts of brussels sprouts can be attacked as well as the roots.
When the small white grubs reach 1 centimetre, they pupate in the soil, emerging as adults after a few weeks, or else the following spring. The adults are like houseflies and can be seen hovering around young cabbage plants prior to laying eggs in the soil.
If the attack comes early in the life of the plant, it is usually severely stunted and killed by wilting in a hot spell. The main roots will have been destroyed. If the plant is fairly big when attacked, it often grows on after the initial check.
Early digging and the removal of old crops are important in reducing the numbers of adult flies that emerge. However, they can fly considerable distances, so others will appear. Discs or squares of polythene, about 15 to 20 centimetres across, can be placed around the stem of the plants at planting out to prevent egg-laying. A slit is make halfway across so as to fit snugly around the stem of the plant as this the key point of attack. Overlap the slit, so there is no gap and no access to the soil.