Tiny, delicate, snowy-white moths that flutter around plant tops when disturbed may seem like an unlikely candidate for all-out war, but whiteflies are one of the most persistent and damaging pests to infiltrate growrooms and greenhouses. Anyone who has battled a persistent population knows they are not a pest to be taken lightly.
Apart from the contamination factor, whiteflies transmit numerous viruses, and can reduce a crop to a blackened, sticky mess in a relatively short period of time. These pests are also remarkable survivalists, with a wide host plant range, the ability to find their way inside even fairly well-protected greenhouses and growrooms, and a tendency to develop a genetic resistance to just about every single chemical pesticide that once controlled their numbers.
How do I know if I have a whitefly infestation in my garden?
There are a number of different whitefly species, including the greenhouse whitefly, the sweet potato whitefly, and the silverleaf whitefly. All three species can infest a wide range of plants. Large infestations of adult whiteflies, which look more like moths than flies, are usually the first introduction a grower will have to this pest.
Whiteflies are around 1.5 mm in length with four wings and are coated in a white, waxy substance. Adults are mostly found on the undersides of leaves in the upper regions of plants, and if disturbed, white clouds of flies will become airborne, making them impossible to miss and easy to inhale.
While heavy infestations of whitefly adults are easy to identify, the first stages of an infestation are often missed by growers. A single whitefly female will lay 200-400 eggs on the undersides of plant leaves, which will hatch in 8-9 days in warm growing conditions. The larvae wander around on the leaf until they find a vein into which they insert their mouthparts and start to feed on plant sap.
From this point until they emerge as adults, the nymphs will stay put, progressing through two more feeding stages. These juvenile stages of whitefly nymphs look like oval, pale, greenish-yellow scale on the undersides of leaves, and finding large numbers of them is a good indication a whitefly population explosion is about to occur.
Both adults and juvenile whiteflies feed by sucking plant sap, mostly obtained from the plant’s phloem. During this feeding process, the pests suck out more plant sap than they can digest, and the excess is excreted as a sweet, sticky substance known as honeydew. When pest populations are high, honeydew excretion can cover plants and other surfaces and support the growth of black, sooty mold.
Sooty mold not only interferes with photosynthesis, but it also contaminates the harvestable portions of the plants, including leaves and fruits, and is difficult to remove. Heavy feeding also results in discoloration, wilting and death of leaves, overall poor plant production and low yields as the plants are sucked dry of the assimilate produced via photosynthesis.
Where do whiteflies come from?
Whiteflies can enter an enclosed greenhouse or indoor garden in a number of ways. The most common is simply flying in through air intakes or unscreened vents. They can also hitch a ride on clothing, equipment or plants coming in from the outdoors.
While it may be possible to carefully scrutinize new plants for adult whiteflies, the eggs and juvenile nymphs residing on the foliage are virtually impossible to detect without a magnifying glass. Once inside the growing area, there is no over-wintering or resting stage with this pest. Whiteflies will continue to breed year-round while plant hosts are present and complete eradication is often extremely difficult without removing all plants from the growing area.
Why are whiteflies such a threat to indoor gardens?
Whiteflies breed in great numbers, so populations can become out of control relatively quickly. Secondly, they are common pests, inhabiting a wide range of host plants, so they tend to be available for invasion for much of the year in temperate and warm climates.
Thirdly, once the pest has invaded the comfort of a warm, secure indoor garden, they are usually free to wreak havoc without being pursued or annihilated by any of the natural predators that are present in outdoor environments. Furthermore, heavy rains, snow, and wind can’t wipe out the delicate adults when they are in a protected cropping situation.
Another serious problem when dealing with whiteflies is resistance to pesticides. In the early days of greenhouse growing, the whitefly was not a significant pest as it was easily controlled with insecticide sprays.
But over time, this pest has built up a genetic resistance to the vast majority of chemical pesticide compounds that were once effective, to the point where they no longer have any controlling effect on whiteflies.
Within any single whitefly population, it takes as little as 3-4 repeat sprays of the same chemical pesticide product before it becomes completely ineffective due to genetic resistance.
Because of this, growers should use a number of different control methods and rotate different classes of chemical compounds to prevent pest resistance from increasing.
Winning the War on Whiteflies
While it is often not possible to completely eradicate a heavy whitefly population, controlling the numbers is possible with persistence. Different control methods need to be used for both the flying adults and the juvenile scale, and different species may not all respond to the same treatment.
Exclusion and Entrapment
Adult whiteflies are attracted to yellow, so yellow sticky traps can be hung around possible entry points such as vents or doors and in the tops of plants. These sticky traps will not catch the majority of pests, and are only used to indicate that whiteflies have started coming into the growing area so control options can be started early.
Early detection is the key to winning the war against whiteflies, and steps need to be taken as soon as a threshold of two adults per plant is seen.
Exclusion is important when it comes to controlling the adult whitefly population. Insect screens on air intake vents, double-door entries and thorough hygiene practices all help prevent infestations.
Having a separate quarantine area for new plants, along with carefully inspecting them, is important. Some smaller growers have reported success with using hand-held vacuum cleaners to suck up the flying adults, slowing the egg-laying and population buildup, but vacuums can’t handle any eggs or juveniles already present on the foliage.
IPM, or integrated pest management, which involves the introduction into the enclosed growing area of natural whitefly predators and parasites, is a complex biological system that needs careful management and won’t completely eradicate the pest. The small parasitic wasp, Encarsia formosa, has long been used as a biological control for whiteflies and can be purchased on cards.
Encarsia larvae feed inside their hosts and eventually kill them, turning the whitefly pupal skin black. Encarsia requires specific temperatures and humidity levels to work, and some growers find the parasitic wasps don’t thrive in their particular growing environments. As with many other natural predators, Encarsia may also need to be continually introduced to the growing area, and if they are successful in significantly lowering the number of whitefly pests, they effectively eradicate their own food supply.
Other useful whitefly predators include Eretmocerus californicus, another parasitic wasp, and Delphastus pusillus, a small, black ladybird beetle. It is often more effective to introduce more than one biological control agent when using IPM to control whiteflies.
One important aspect of introducing predator insects for pest control is the fact that many pesticides and spray compounds will kill these beneficial insects, often more effectively than the pests they were intended for.
Soap and oil sprays used as smothering agents are a popular choice for growers. While these compounds do affect the whitefly adults they hit, prolonged use can cause considerable spray damage on many plants, particularly those grown under stress or indoors in a protected environment. Soap and oil spray damage is particularly damaging to young, developing fruits such as tomatoes and capsicums, but cases of severe foliage damage are also common.
Biopesticides and Insect Growth Regulators
Some of the most effective pest control technologies are some of nature’s oldest. Botanical compounds such as neem oil have been used for insect control for centuries, but recently, extracts and formulations that dissolve easily into water have become available for small growers.
Neem is, amongst other things, an insect growth regulator, so it is a longer-term approach to breaking the insect life cycle. Essentially a plant extract, neem is generally safe and non-toxic to use on all types of plants, but hydroponic growers still need to be careful when applying some neem products.
Many of these are oil sprays or emulsifiable concentrates and should always be tested on a small area first, checking for plant damage over a 48-hour period before spraying all plants. Neem contains the active ingredient azadiractin, which can also be found in some non-oil-based sprays.
While insect growth regulator products typically fall into the pesticide category, they have a complex mode of action that prevents insects from rapidly developing resistance. Insect growth regulator sprays need to make contact with the pest to be effective, so good spray coverage is essential. They are best used in conjunction with other control methods.
Biological control, or controlling insect pests with diseases, is another developing technology. Products containing spores of certain fungal pathogens that only target pests are available, including products containing the fungus Beauveria bassiana.
Many other biological control organisms are currently being investigated and it’s likely a number of effective products will be released in the future.
Whiteflies are sneaky, persistent and challenging to control. Long-term, effective control is heavily reliant on early identification with plant monitoring, sticky traps and inspection of incoming plant materials, combined with screening and the use of carefully selected control agents where necessary.
Due to this pest’s ability to develop a genetic resistance to many pesticides, the old hit ‘em hard approach with continual sprays of outdated insecticide products just won’t cut it these days when it comes to the war on whiteflies.
Read next: Beginner's Guide to Beneficial Insects