Silverleaf whitefly

Silverleaf whitefly (Bemisia tabaci) was first recorded in Australia in 1994. It is now a widespread pest in Queensland and Western Australia and could become a major pest in most irrigated agricultural areas of Australia. Silverleaf whitefly (SLW) is also known as poinsettia or sweetpotato whitefly and in USA literature it is now referred to as Bemisia argentifolii. SLW has a wide host range (over 500 species) of crops and weeds, and is difficult to control as it has developed resistance to conventional insecticides.

Silverleaf whitefly was previously called Bemisia tabaci Biotype Middle East Asia Minor 1 (MEAM1) and Bemisia tabaci Biotype B. There is also an Australian native species of Bemisia tabaci which is called Bemisia tabaci biotype AUS1. The Australian native species is indistinguishable from silverleaf whitefly in the field.

Scientific name

Bemisia tabaci

Description of adult

Adult silverleaf whitefly are small 0.8 to 1.2mm long (smaller than greenhouse whitefly), sap sucking, flying insects. Adults hold their wings vertically tilted, like a peaked roof. They have white wings and yellow bodies, and congregate on the undersides of leaves. Very high populations can develop within 3 to 4 weeks.

Immature stages

The oblong eggs are smooth and about 0.2mm long and 0.1mm in diameter. The eggs have a pointed end that is attached to the underside of the leaf surface. Each female randomly lays 50–400 eggs (average 160), usually on the underside of a leaf. The new eggs are whitish yellow then turn brown and hatch after 7 to 10 days.

There are 4 nymphal stages before the adults emerge:

• Crawlers (first instar): When the eggs hatch greenish-yellow, flattened, oval first instar nymphs about 0.3mm long emerge. They crawl a short distance until they tap into a sap source in the phloem tissue. They remain in that position until they emerge as adults.
• Second and third instar nymphs: During this stationary stage they look like soft scale insects, oval but slightly pointed towards the tail. They have no legs or distinguishing features, they suck sap from the plant.
• Fourth instar nymphs or pupae: Late in the third instar and through the fourth instar nymphs develop obvious red eyes and are referred to as red-eyed nymphs. They are yellow and about 0.6-0.8mm long. Late in the fourth instar they stop feeding and pupate, the yellowish-white body of the adult develops then emerges. The empty white cases the adults emerged from can be seen under the leaf.

A small wasp parasitises some nymphs, these nymphs turn dark as the wasp develops inside them.

Life history

In warm weather the life cycle takes 18–28 days, but may take 30–48 days in winter, starting with eggs and going through 4 nymphal stages until the winged adults emerge.

Adults generally emerge in the morning, males first. Emergence is temperature dependent with earlier emergence at higher temperature. It takes about 4 hours before adults can fly and 10–20 hours before the females can mate.

SLW populations build up rapidly during spring and summer. In Queensland the pest can complete 8–12 generations in a year.

Distribution

SLW is a widespread pest in Queensland, New South Wales and Western Australia and could become a major pest in most irrigated agricultural areas of Australia.

Host range

SLW is a serious pest of many vegetable crops, including tomato, eggplant, cucurbits, sweetpotato, brassicas and beans. It also affects cotton and grain crops and ornamentals.

Damage

Feeding by SLW causes severe damage to vegetable crops through:

• direct effect of feeding on plants
• injecting into plants a toxin which causes physiological damage
• producing honeydew which encourages sooty mould that contaminates the product
• its ability to transmit Begomoviruses viruses such as tomato leaf curl viruses (ToLCV and TYLCV).

Direct feeding damage

SLW adults and nymphs suck the sap from the plant causing reduced plant vigour, stunting, poor growth, defoliation and reduced yields. High populations may result in plant desiccation and death.

Injecting toxic saliva

While feeding, SLW inject toxic saliva into the plant causing physiological changes to plant tissue. On the outer skin of tomato fruit the external symptoms are green, yellow or orange streaks or blotches. Internally, the affected fruit have white or yellow tissue. In some tomato varieties the external symptoms may not be obvious, but internal damage is often very apparent once the fruit is cut open.

Silvering of leaves is a common symptom on pumpkin, zucchini and squash and fruit discolouration occurs in cucurbits and beans, pale stalks on broccoli and leaf yellowing and blanched stalks of lettuce.

Honeydew contamination

Both adults and nymphs excrete honeydew. This sugary substance can promote the growth of sooty moulds that affect the marketability of product. Sooty mould also reduces plant photosynthesis and the effectiveness of insecticides.

Transmission of viruses

SLW adults are efficient vectors in spreading Gemini viruses from infected plants into healthy crops. Gemini viruses include tomato leaf curl virus (ToLCV) and tomato yellow leaf curl virus (TYLCV) that infect beans, capsicums, tomatoes and a wide range of ornamentals and weeds.

Monitoring and sampling

Regular counts of adults and nymphs on leaves should form the basis of deciding how and when to apply control measures. Adults congregate and lay eggs predominantly on the undersides of younger leaves. The majority of the eggs and young nymphs are found on the young leaves while older nymphs are usually found on older leaves.

To determine the numbers of adult SLW gently turn over young leaves and count the adults on the underside. Adults should be sampled during early morning (7–9am). Rapid adult migration usually occurs when infested crops are in decline or about to be destroyed.

To assess nymph populations, sampling should focus primarily on the older leaves. A hand lens (x10) is necessary when inspecting leaves for the presence of eggs or small nymphs. Large nymphs can be counted with the unaided eye.

Yellow sticky traps are useful for monitoring whitefly adult movement or dispersal, especially the movement of SLW from mature or older neighbouring crops and host weeds. Around 3 to 5 traps should be placed in a crop of 2–3ha. Place them level with the tops of the plants, as whiteflies are most attracted to young foliage. Adult numbers on the traps will give an early warning of population increases within crops and an indication of the need for regular monitoring for nymphs.

Control

Controlling whitefly populations before they reach large numbers in crops is very important for successful management. If the adults occur in large numbers it becomes more difficult to control the nymphal stages. Adults move between successive crops, so management approaches must be employed in all crops within the area.

To reduce early season populations, best management practices require consideration of several approaches.

Pest free (clean) seedlings

Seedlings are potentially a major means of spreading whiteflies and leaf curl viruses into new plantings. Young plants are more susceptible to damage from SLW, so early infestations need to be avoided. Clean seedlings can be the first line of protection against the development of damaging populations. Growers should check their suppliers to determine how the seedlings are grown and what measures are being used to protect against whitefly infestation. Inspect transplants carefully upon arrival for whitefly eggs, nymphs and adults.

Weed management

The availability of a continuous source of hosts, whether they are crops, weeds or abandoned crops, is the major contributing factor to a severe whitefly problem. Even a small area of a favoured host can maintain a significant whitefly population.

Minimising whitefly hosts is important in reducing the base population at the start of the cropping season. A smaller base population then will delay the time it takes for SLW numbers to reach significant levels, reducing the number of sprays needed to control whitefly.

Common weed species that carry high numbers of SLW include sow thistle, bladder ketmia, bell vine, burr gherkin, native rosella and star burr. Milk or sow thistle is a regular weed host for whitefly and is common in Queensland vegetable production areas. Control these weed species in farming areas and seedling nurseries to minimise a build-up in SLW populations.

Clean-up crop residues

Movement of SLW adults from older crops and crop residues is the main source of infestation for younger crops. Post-harvest destruction of heavily infested crops often causes mass migration of SLW adults into adjacent crops. Therefore it is important to control adult whiteflies before they move into young crops.

Clean-up strategies for old crops/crop residues:

• For moderate whitefly infestations, use an insecticide or oil treatment effective against adults.
• Use high spray volumes, normally around 600–1000 L/ha for better coverage.
• Plough in the crop within 3 days to kill all remaining nymphs on the crop foliage.
• Remember that withholding periods still apply and that produce should not be taken from the fields for consumption. Don't feed crop residues to livestock.

Chemical control

Selecting the correct insecticides and applying them at the appropriate time is very important, both for achieving good SLW management and minimising the development of resistance to the insecticides. Base your spray program on the results of monitoring.

Insecticides vary in their efficacy on adult and immature SLW. Select insecticides according to the growth stage of whitefly, the infestation level, the age of the crop and the type of crop. Good spray coverage, particularly of the underside of leaves, is very important when using foliar insecticide applications as SLW adults, eggs and nymphs are found predominantly on the underside of leaves. Spray equipment should be correctly calibrated so that the correct amounts of insecticide are applied efficiently.

A systemic insecticide can be applied to some crops as a foliar spray, or as a soil treatment through sub-surface drip irrigation tubing, as a furrow spray or as a plant hole drench. Soil applications are more efficient than foliar sprays and are made shortly before, at or shortly after planting.

Short residual contact insecticides mainly control adults and are less effective against immature stages. Systemic insecticides can control both adults and nymphs. Organophosphate insecticides used alone provide no control for silverleaf whitefly.

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.