Silverleaf whitefly (Bemisia tabaci Biotype B) 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.
Biotype Q was discovered recently in Queensland. There is also an Australian native species. These three biotypes are indistinquishable in the field.
Bemisia tabaci Biotype B, Q and AN
|Description of adult|
Adult silverleaf whitefly are small 0.8 to 1.2 mm 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 three to four weeks.
The oblong eggs are smooth and about 0.2 mm long and 0.1 mm 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 seven to 10 days.
There are four nymphal stages before the adults emerge:
Crawlers (first instar): When the eggs hatch greenish-yellow, flattened, oval first instar nymphs about 0.3 mm 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.
A small wasp parasitises some nymphs, these nymphs turn dark as the wasp develops inside them.
In warm weather the life cycle takes 18-28 days, but may take 30-48 days in winter, starting with eggs and going through four nymphal stages until the winged adults emerge.
Adults generally emerge in the morning, males first. Emergence is temperature dependant with earlier emergence at higher temperature. It takes about four 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.
SLW is a widespread pest in Queensland and Western Australia and could become a major pest in most irrigated agricultural areas of Australia.
SLW is a serious pest of many vegetable crops, including tomato, eggplant, cucurbits, sweetpotato, brassicas and beans.
Feeding by SLW causes severe damage to vegetable crops through:
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 (TLCV) 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 (7-9 a.m.). 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 three to five traps should be placed in a crop of 2-3 ha. 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.
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 including the use of pest-free seedlings, weed management, chemical control and cleaning up crop residue.
1. 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.
2. 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.
3. 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:
4. 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.
See the guide below for chemical control options for SLW:
Chemical registrations and permits
Check the Australian Pesticides & Veterinary Medicines Authority chemical database and permit database for chemicals registered or approved under permit to treat this pest on the target crop in your State/location. Always read the label. Always observe withholding periods.