Cabbage Whitefly

Identification

Whiteflies are pests which are instantly recognised by farmers and growers. The adults are pure white with 2 pairs of folded wings which are held ‘roof-like’ over the abdomen giving a ‘moth-like’ appearance. They are 2 mm in length and their bodies are covered with a powdery wax coating. They feed on the undersides of leaves

and when disturbed fly up in clouds only to quickly resettle on the host plants. The young nymphs are referred to as scales.

Symptoms

The adults and scales both feed on the plant phloem. The sticky ‘honeydew’ which is excreted is an ideal medium for the development of fungal moulds which give the lower leaves an unsightly dark discoloured appearance.

Life-cycle

The adults overwinter on brassica plants in sheltered locations and migrate into crops to lay eggs from mid-May onwards. After

a period of around 10 days the ‘nymphs’ emerge and after moving to a suitable feeding site they settle into an immobile stage and develop a hard scale-like external covering which darkens with age. After a couple of weeks the nymphs enter a pupal stage and after a further period of instar development the winged adults eventually emerge. The complete life-cycle from egg to adult can be completed in as little as 3-4 weeks but will be longer in cooler conditions.

Importance

This particular species, Aleyrodes proletella, only exists on brassicas including cabbage, kale, Brussels sprouts and broccoli. Where this pest is present in large numbers host plants can be severely weakened, especially where plants are under stress. Economic damage is sustained where crop quality has been damaged to the extent that produce becomes unmarketable.

Threshold

Whiteflies are an important pest of brassica crops and require treatment as soon as numbers start to establish especially in warm, dry conditions.