Whiteflies

Whiteflies are sucking insects that feed on plant sap. The adults are small, powdery insects with four white wings. Both the adults and immature stages are found on the underside of the leaves.

Training workers and growers to identify and report whitefly infestation is important for effective management. Employees who are responsible for pest control should be able to distinguish the greenhouse whitefly from the sweet potato whitefly and recognize life stages. Both whitefly species have six stages in the life-cycle. Adults deposit eggs connected by short hooks to the undersides of leaves, often in a crescent-shaped pattern. Eggs of the GHWF are oval-shaped.

They are white when young; turning dark gray before hatching. SPWF eggs are cigar-shaped and yellowish when young; turning light brown before hatching. The first nymphs, often called “crawler”, have legs and antennae. After a several hours, they settle on the leaf to begin feeding and lose their legs. The next two nymph stages are immobile feeders that are larger than the first stage.

The fourth nymph stage is immobile and does not feed. This “false” pupa is the best stage to distinguish between the two species. The pupa of the GHWF is round with sides raised perpendicular to the leaf surface, while the SPWF pupa is dome-shaped. The GHWF has a fringe of short hair-like filaments around the periphery that is absent from pupae of the SPWF.Adult whiteflies begin laying eggs 1 to 3 days after emerging from the pupae. SPWF adults are yellowish and smaller than the GHWF. The SPWF are narrower and hold the wings at a 45-degree angle over the body, while the wings of the GHWF are held flatter.

Whiteflies have a wide host range, including avocado, banana, cabbage, capsicum, cassava, cauliflower, citrus, coconut, cotton, eggplant, garlic, guava, legumes, mango, mustard, onion, pechay, pepper, radish, squash, soybean, tomato, and tobacco.

Life Cycle : All whiteflies have a similar lifecycle. They pass through four instars or stages from egg to adult. Eggs are laid on the undersides of the leaves. The first stage has legs and antennae, but these are lost after the first molt and the flattened, oval shaped larvae remain in one feeding spot until the adult stage.

Just before the adult stage the larvae stop feeding and form a pupa where the larvae undergoes a transformation into an adult with wings. The adults emerge from the pupae through a T-shaped slit and soon mate and reproduce. There are many generations each year. Whiteflies overwinter on weeds and ornamentals (thousands on spotted spurge now through the fall.

Damage: Both immature and adult whiteflies feed by piercing the undersurface of leaves and sucking the plant sap. The nymphs, which feed in one position for 2-4 weeks, may cause yellow spots which are conspicuous on the upper leaf surface. On older leaves the spots develop a brown centre. Some sweetpotato.

Cultivars respond to the damage by producing purple pigments, resulting in purple spots with a brown centre.Feeding by adults usually causes no visible lesion, but high whitefly populations may cause yellowing of infested leaves, and may stunt plant growth. Whiteflies produce honeydew, a sugary secretion, making the leaf surface sticky. In humid conditions, sooty mould may develop on the leaf surface, reducing its capacity to trap light required for the plant’s normal growth. The pest is more important as a transmitter of viruses such as sweetpotato mild mottle virus, sweetpotato yellow dwarf disease, sweetpotato leaf curl, and particularly sweetpotato chlorotic stunt virus, a key component of sweetpotato virus disease.

Control :Cultural control Ornamental plants in or around the home and plants in greenhouses or gardens often become infested with whiteflies through the introduction of infested plants. A slight infestation from one plant can quickly spread to other plants. Inspect all new plants thoroughly, and isolate them for a few days before placing them among established plants. During this isolation period, inspect the plants and treat those infested.Inspect plants regularly, and remove by hand older leaves that are heavily infested with whiteflies in the nonmobile nymphal and pupal stages.

Biological control :Whiteflies have many natural enemies in the garden setting, such as spiders, lady beetles and lacewings. However, frequent or widespread use of insecticides to control other garden pests prevents these predators from effectively controlling whitefly populations.The tiny parasitoid Encarsia formosa has been used successfully in managing whitefly populations in greenhouses and conservatories for many years. However, in an outdoor setting where favorable temperature and light conditions cannot be maintained, use of E. formosa is limited. E. formosa is available from commercial suppliers.

Mechanical control: The use of traps can be very helpful in controlling light infestations of whiteflies in greenhouses. You can make a trap from a 12-by-6-inch strip of cardboard or strong poster board. Paint both sides bright yellow and coat with a sticky substance, such as Tanglefoot, petroleum jelly, petroleum jelly/mineral oil mix, mineral oil or a heavy-grade motor oil (SAE 90). Hang the trap vertically or support it on stakes just above the plants. The adults are attracted to the yellow color and become trapped on the sticky substance. Whitefly traps are available commercially also.

The use of yellow traps outside may have limited value because of the large number of traps required. One trap for every two large vegetable plants is recommended. Clean the traps periodically to remove insects and other debris, and apply fresh adhesive to the yellow surface. To avoid trapping too many of the whitefly’s natural enemies, use traps when whiteflies first start to appear, but remove them when the whitefly population seems to be decreasing.

Another mechanical technique that can help control whiteflies is to vacuum them with a small, hand-held vacuum cleaner. Vacuum the adult whiteflies in the early morning when the temperature is cool and they are slow-moving. Put the vacuum bag containing the insects (not the vacuum cleaner itself) inside a large plastic bag and freeze for 24 hours to kill the insects. Vacuuming adults is most helpful and effective when an infestation is just starting and when the adults have not laid many eggs

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