Hemiptera : Aleyrodidae
Image above: adult Whiteflies foraging on the underside of a mint leaf.
Whiteflies are not ‘flies’ but are sap-sucking insects related to aphids, scale insects and mealybugs. Common adult whiteflies are usually white, or pale coloured, so at least the ‘white’ part of their common name is fitting. Adult whiteflies are usually only seen when disturbed - they fly up from a plant like ‘flying dandruff’.
Immature whiteflies are scale-like nymphs hidden under leaves, so whiteflies often go unnoticed until the adults are seen. There are more than 100 species that occur in Australia but except for the native strain of Silverleaf Whitefly, most pest species of whitefly have been introduced from overseas.
Many gardeners only notice whiteflies in the adult form. Because of this, whitefly infestations can build up rapidly and undetected. It is best to detect whitefly incursions early and bring them under control, before infestations cause serious damage. To do this, you need to know what the different stages of a whitefly life cycle looks like (see Description below).
Adult whiteflies and their nymphs are sap-suckers and they excrete excess plant sap as honeydew, which falls onto the upper surfaces of the leaves below where the whiteflies are feeding. Sooty mould may form on the leaves where the honeydew falls. This is the easiest way to detect sap-sucking insect infestations. If you are out and about in your garden on a regular basis, you should notice the development of sooty mould on your plants.
There are several natural enemies that help to control whiteflies. The most effective natural enemy is the parasitic wasp Encarsia, which occurs naturally and is also available commercially. There are several insecticide products that you can apply against whitefly infestations. Try Yates PestOil or Yates Nature's Way Vegie & Herb Spray before trying others, as these two products are less likely to interfere with the activities of tiny parasitic wasps.
Whiteflies make up the family Aleyrodidae, which is part of the huge insect order Hemiptera that contains other common insects such as aphids, mealybugs, and scale insects. Pest species of whitefly include Greenhouse Whitefly (Trialeurodes vaporariorum), Silverleaf Whitefly (Bemisia tabaci), and Spiraling Whitefly (Aleurodicus dispersus). The most common and widespread whitefly that occurs in horticulture is the Greenhouse Whitefly.
Adult whiteflies are about 1.5 mm long, have four delicate wings and look like tiny moths when examined with a hand lens. The name ‘whitefly’ comes from the white, waxy substance coating their bodies. Adult whiteflies are not strong flyers, but they do fly when disturbed on a plant.
Nymphs are oval in shape and resemble scale insects and are always found on the underside of leaves. Greenhouse Whitefly nymphs are pale, almost translucent, greenish-yellow in colour with fine waxy filaments protruding all over their bodies. The less common Orange Spiny Whitefly (Aleurocanthus spiniferus), which feeds on citrus, has black nymphs with white waxy markings.
Whiteflies pass through a life cycle of modified gradual metamorphosis – egg, nymphs, adult. The life cycle is ‘modified’ because the last (4th) nymph stage is often termed a ‘pupa’ from which the adult emerges.
Females lay about 30 or 40 eggs at a time, often in a circular or arcing pattern on the underside of leaves. Each egg hatches into the 1st nymph stage, which is mobile and wanders around on the underside of the leaf until it settles into a suitable feeding place – all other nymph stages are sedentary. The 1st nymph stage is tiny - only about 0.3 mm long.
The nymph develops through two more sedentary nymph stages, feeding all the while, until it moults into the final 4th nymph stage, which is about 1 mm long. This final nymph stage stops feeding and a winged adult develops inside it. The adult eventually emerges through a split in the upper surface of the nymph.
The entire life cycle can be completed in about a month and there are several overlapping generations per year.
Image above: adult Greenhouse Whitefly (Trialeurodes vaporariorum)
Image above: underside of a kale leaf covered in whitefly eggs, juveniles and adults.
(Image courtesy of Elise Dando)
Image above: kale leaves covered in droplets of hardened honeydew
(Image courtesy of Elise Dando)