WHITEFLY Order; Hemiptera Family; Aleyrodidae
Symptoms The presence of the various species of whitefly is indicated by large amounts of sticky honeydew and sooty moulds on the upper surfaces of leaves, and by a yellow mottling where the whitefly have fed. Whitefly can transmit several strains of plant viruses and infested plants suffer from a lack of vigour. Of the greenhouse crops, tomatoes and cucumbers are usually most severely infested, although this pest will attack many other common glasshouse and houseplants. Also at risk are cabbages, brussels sprouts and other brassicas, rhododendrons and azaleas.
Description and Life Cycle Whiteflies, both adults and eggs, are found on the underside of leaves. Adults are small grey to white winged flies, measuring 1.25 –2mm, and usually live for a month. Each female adult will lay up to 200 eggs on the underside of leaves, often in neat circles. These eggs are initially white but darken to an almost black colour before hatching into a nymph, which will crawl around the leaf surface before settling to feed. The legs of the nymph then degenerate and the nymph becomes an immobile scale, feeding for 2 weeks before pupating. Adults emerge 10 days later and begin feeding immediately. The development from egg to adult can be as rapid as 3 weeks at 21 degrees centigrade but will take much longer at lower temperatures.
Biological Pest Control Treatment The parasitic wasp, encarsia formosa, has been used with great success to control whitefly populations since 1926. Encarsia females lay eggs directly into the immobile whitefly scales which remain white and develop normally until the encarsia pupates, turning the scale black. The adult encarsia will emerge around 10 days later. The presence of encarsia formosa is indicated by the black parasitised whitefly scales alongside the un-parasitised white scales. Leaves containing black scales are introduced to the greenhouse environment, and under ideal conditions the adults, when they emerge, can lay between 12-15 eggs per day. Keeping the temperature between 18 – 21 degrees centigrade with good light intensity will result in maximum activity. As with all biological predators, it is important not to introduce an excessive number of encarsia formosa as they depend on the whitefly scales to continue their life cycle.
