Whiteflies are small white flying insects around 1mm long that prefer to hide on the undersides of young plant leaves, and when disturbed fly up in the air around the plant in a cloud of white flying insects, often in large numbers. They belong to the order Hemiptera (true bugs), family Aleyrodidae (Whiteflies), and are related to aphids, scales and mealybugs.
There are many different species of whiteflies, including greenhouse whiteflies (Trialeurodes vaporariorum) and silverleaf whiteflies (Bemisia tabaci), and all species are considered pests. Whitefly attack a wide range of produce crops, ornamental plants and weeds, which serve as host plants that they can feed on.
These insects suck the sap from the leaf and excrete honeydew (the sugary sap of the plant minus the proteins which they ingest) which leaves the lower surface of the leaf sticky to the touch, much like aphids do, causing considerable damage in the process.
What Damage Do Whitefly Cause?
The loss of a plant’s nutrient-rich sap can weaken it and stunts its growth. Heavy whitefly infestations can cause discolouration of leaves, defoliation (leaf drop), deformities in leaves and fruit, a reduction in yields, and may even kill plants.
Whitefly can breed very quickly to produce large populations which can significantly impact plant health. Their eggs hatch after 5–9 days and whitefly can complete their lifecycle from egg to adult in 18–28 days in warm weather! Newly emerged females are ready to mate in 12–20 hours, can live up to 60 days, and breed several times during their short lifespan. As such, they can produce large populations in 3-4 weeks, and 8–12 overlapping generations within a year, that can cause considerable plant damage.
The honeydew that whitefly excrete on the surface of a leaf is a sugary liquid that attracts ants which interfere with the activities of natural predators that control whitefly and other pests. It can also attract black sooty mould in humid conditions. This mould grows and feeds off the sugars, smothering the leaf surface with a thick black coating that restricts plant photosynthesis, but can be washed off fairly easily.
These sap-sucking pests have needle-like mouthparts which can pierce the plant tissue to reach the flowing plant sap inside. Feeding this way, they also act as disease vectors, organisms that carry and transmit infectious pathogens, spreading viruses from plant to plant.
While feeding on plant sap, silverleaf whitefly also inject toxic saliva is into the plant which can cause physiological changes to plant tissue. Common symptoms include:
- Pumpkin, zucchini and squash leaves turning a silver colour
- Discolouration of cucurbit fruit and bean pods
- Irregular ripening and blotching of tomato fruit
- White streaks or pale stalks in broccoli
Whitefly Prevention Methods
While it may be possible to or tolerate small outbreaks of whitefly, controlling large populations of this pest is difficult, even with toxic pesticides! Prevention of the problem is a far better long-term strategy.
The best way to prevent whitefly populations explosions is by minimising conditions favourable to them, and instead promoting environment that are detrimental to them. This can be done in the following ways:
- Avoid using excessive nitrogen fertilizer, as the weak, sappy growth produced increases whitefly populations.
- Don’t use synthetic fertilisers, as they’re very high in water-soluble nitrogen, which is fast acting, forcing rapid plant growth of soft, sappy tissue which sap-sucking insects love.
- Avoid using toxic synthetic insecticides as these will kill the natural enemies of whitefly that play an important role in their control. Products containing carbaryl, pyrethroids (synthetic, more toxic and more persistent versions of natural pyrethrins), or imidacloprid can be especially disruptive to beneficial insects. They’re more sensitive to these pesticides than pests, and take longer to recover their numbers after application, leading to pest population explosions in their absence.
- Control ants, as they protect whiteflies from their natural enemies, in order to collect the honeydew from them, as they do with aphids and scale insects. Use a homemade borax ant trap to eliminate their colonies or glue banding of trees as a pest barrier.
- Avoid or remove plants that are more prone to whitefly infestation and are observed to repeatedly host high populations of the pest.
- Use companion planting, plant nasturtiums to repel the pest.
Biological Controls for Whitefly
In many situations, the natural enemies of whiteflies can do a sufficient job to control pest numbers without having to resort to chemical treatments. Outbreaks often occur when the populations of these beneficial insects are disrupted by the use of toxic insecticides.
Whiteflies have a range of natural enemies, such as ladybirds, lacewings, hoverflies, parasitic wasps such as Encarsia species, predatory mites such as Montdorensis species, and predatory bugs such as big-eyed bugs, and minute pirate bugs.
Encourage these beneficial insects by growing plants for the Asteraceae (daisy) and Apiaceae (parsley) family, and other companion plants such as Sweet Alyssum, all of which produce shallow flowers that can serve as a nectar source to provide an alternative food source when pest numbers are low.
Physical Whitefly Control Methods
Physical controls include manual methods such as hand removal of small pest infestations, pest infested leaves, or hosing pests off and traps such as sticky traps.
- Hose plants with moderate pressure in the early morning, repeat 3 days in a row to wash pests off.
- Remove infested leaves covered in whitefly eggs and non-flying larvae, and either burn the plant waste or dispose of in a sealed plastic bag in the landfill garbage bin. Don’t put them into the compost, as the pests will mature into flying adults and head back to the garden.
- Use yellow sticky traps in small numbers to monitor, or in large numbers to reduce pest numbers. Whiteflies, aphids and fungus gnats are attracted to the colour yellow and stick to the adhesive. The concern with using these sticky traps is that they also capture some beneficial insects!
- Disturb the pests then vacuum them up with a handheld vacuum cleaner when they fly up into the air. Use a low setting to avoid stripping leaves off plants!
Organic Chemical Controls for Whitefly
There are environmentally safe pesticide sprays that can be used to control whitefly populations.
- Horticultural soap spray (Natrasoap). To use, mix 15-30 ml per litre of water. Spray all surfaces of the plant thoroughly to the point of run-off. The best time to apply is in the cool of the early morning or late evening. Repeat 5-7 days later or as required.
- Horticultural oil spray (Eco-Oil, White Oil, Pest Oil). To use, mix 5mL per litre of water. Spray when pests first appear. Apply two sprays 3 to 5 days apart. Repeat applications at signs of reinfestation. Spray both upper and underside of leaves to run-off. Do not apply more than three sprays to plants within a 4-to-8-week period. To make your own, see article – How to Make Horticultural Oil Spray for Organic Pest Control
- Pyrethrum insecticide spray. To use, mix 20ml per litre of water, spray when pests first appear and repeat at weekly intervals if required. Thorough coverage of all foliage and surfaces, including under leaves, is essential. Please spray when bees are not foraging (late evening is best) as pyrethrum can be toxic to bees.
- Neem insecticide spray. Effective against a range of chewing and sucking insects. Mixing rate as directed. Apply when pests appear. Re-apply every 7 days while pests are present. Good coverage is important so apply spray to upper and underside of leave s to point of run-off.
The fairly frequent application of these sprays is necessary to keep up with the very fast breeding cycle of the pest.
It is worth noting that both horticultural soap and horticultural oil work by smothering the insect and stopping it breathing, effectively suffocating it. Since they don’t work by chemical action like other pesticides, the pests can never develop resistance.
As a general precaution when using garden sprays, do not spray when shade temperatures are near 30°C or higher, or when the soil is dry and plants are suffering from moisture stress.
Whitefly Resistance to Toxic Synthetic Pesticides
Whitefly can quickly develop resistance to toxic pesticides, and many are already resistant to the most common ones, including some of the heavy-duty agricultural pesticides.
According to the New South Wales Department of Industry,
silverleaf whitefly arrived in Australia with resistance to synthetic pyrethroids, organophosphates, carbamates and insect growth regulators. Since their arrival, they’ve developed measurable resistance to endosulfan (similar to DDT and now banned), amitraz, bifenthrin and imidacloprid.
By using a combination of environmentally safe control methods together, the populations of whitefly can be reduced to such a low level where their impact will be negligible.
References
- Agriculture Victoria, Biosecurity, Silverleaf white fly, <https://agriculture.vic.gov.au/biosecurity/pest-insects-and-mites/priority-pest-insects-and-mites/silverleaf-white-fly>
- New South Wales through Department of Industry and Investment, December 2009 PRIMEFACT 974 Silverleaf whitefly in vegetables, Sandra McDougall, <https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0019/312805/Silverleaf-whitefly-in-vegetables.pdf>
- Queensland Government, Department of Employment, Economic Development and Innovation (DEEDI), Whitefly-transmitted viruses in vegetable crops, Integrated virus disease management, <https://www.daf.qld.gov.au/__data/assets/pdf_file/0009/73926/Whitefly-viruses-veg-crops.pdf>
- University of California Statewide IPM Program, Pest Notes: Whiteflies, UC ANR Publication 7401, Author: M.L. Flint, Extension Entomologist Emerita, Department of Entomology, UC Davis, <https://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html>