Those tiny white insects that take off when you approach your plants are whiteflies. Photo: warwick.ac.uk
As the days get longer in late winter and spring, our houseplants come back to life … and so do the whiteflies. They are easily recognized: when you approach an infested plant, whiteflies rise in mass, apparently in panic, and flutter about in all directions. They’re often said to look like flying dandruff! They don’t stay airborne long, but very quickly land on the same plant or a neighboring one.
Although whiteflies often appear to come out of nowhere in February or March, their abrupt appearance is not due to spontaneous generation as scientists in the mid-19th century once believed. The insects enter homes in the fall on plants and cuttings we brought in from outdoors or even on our clothes when we come in from working in the garden. They then settle on indoor plants … and seem to disappear! That’s because they aren’t too active in the fall to start with, then quickly go into full diapause (dormancy) as days grow shorter and often cooler. Then they sleep through the winter on our plants, sight unseen.
Now that the days are getting longer and temperatures are rising, though, they start to become active and reproduce, settling on our seedlings and houseplants. Their life cycle can take as little as three weeks at 21–24°C (70–75°F) and that means their numbers can increase very quickly.
What is a Whitefly?
The whitefly is a small insect of the Hemiptera family (aphid family) and is therefore not a true fly (Diptera family).
The main species found in our seedlings and indoor plants is the greenhouse whitefly (Trialeurodes vaporariorum). Native to Central America, it was accidentally introduced into the temperate and tropical regions of all the continents and is now widespread outdoors. In very cold climates (it’s not very frost hardy), it overwinters indoors in homes and greenhouses, then moves outside again for the summer.
Another species, the more recently introduced tobacco whitefly (Bemisia tabaci), is almost as common.
There are more than 1,500 species of whitefly found especially in the tropics, but the above two are the most common in our indoor plants and seedlings.
To simplify the explanations, the following information specifically describes the greenhouse whitefly, but from the point of view of the average gardener, the differences between this species and the tobacco whitefly are of little consequence, especially in terms of their control.
Up close, the “flying dandruff” turns out to be a tiny, pale yellow insect with white wings that is only 2 mm long.
The adult female lays eggs on the underside of plant leaves. She can lay over 100 eggs (sometimes as many as 500) over her 3- to 6-week life span. The tiny and therefore rarely noticed eggs give rise to semitransparent nymphs that quickly settle on a leaf underside and then become immobile, incapable of moving when even touched. They take the form of small, almost transparent scales glued to the leaf, usually in the presence of adults.
After 16 to 18 days, the adults emerge from the pupal shell, which remains on the leaf, transparent and empty. The female can sometimes already start laying eggs only 24 hours after hatching.
Whiteflies damage plants in three ways.
First, the nymphs (and, to a lesser extent, the adults) suck sap from the leaves, leaving them weak, wilted and often mottled and distorted. If there are many, the whole plant loses so much sap it wilts.
Whiteflies can also transmit diseases to plants, such as mosaic virus, that can weaken them and cause abnormal leaf and stem deformations and coloration.
Finally, whiteflies emit a sweet transparent liquid called honeydew that drips onto lower leaves and the floor. Eventually, the honeydew blackens when a fungus called sooty mold settles on it. Sooty mold is not directly harmful to plants, but because of its black color, it reduces photosynthesis and the plant can no longer absorb solar energy. Plus, a plant covered in black gunk is really not very attractive.
In extreme cases, plants infected with whiteflies weaken so seriously they even die.
Controlling the White Horde
Whiteflies are extremely prolific and are essentially generalists: more than 700 plant species are known to be susceptible to them, including many houseplants, annuals overwintering indoors and seedlings. However, they have a clear preference for fuchsias, pelargoniums, poinsettias, herbs and tomato plants and, in general, plants with thin leaves.
The problem will more or less correct itself if and when you place your plants outdoors for the summer. There wind, rain and natural predators often reduce the whitefly population to an acceptable level. On the other hand, between the time whiteflies awaken from their dormancy in February-March and time you’re able to get your infested plants outdoors in May or June, they’ll have plenty of time to turn your plants into toast.
As soon as you notice the presence of whiteflies, immediately isolate the affected plants. Move them at night, because whiteflies are diurnal and won’t budge in the dark. That way, the normally very flighty adults will stay put on the infested plants as you move them. If you don’t have a room where you can quarantine the plants, seal them in a clear plastic bag.
Insecticides to the Rescue
Whiteflies can be controlled by spraying the plants with just about any insecticide. I prefer to recommend ones that will kill them, but are fairly innocuous to humans, such as insecticidal soap and neem oil. Apply them on the leaves of affected plants, your goal obviously being to completely cover the undersides with spray. That’s hard to do effectively, which is why insecticides may seriously reduce an infestation, but rarely seem to cure it. A few whiteflies always seem to escape. That means you’ll have to repeat the treatments weekly, at least on seriously affected plants, until you’re able to put the plants outdoors.
When Whiteflies See Yellow
Whiteflies have an interesting weakness, one they in fact share with many other flying insects (aphids, flies, etc.). They’re highly attracted to the color yellow, which represents, in their eyes, a weakened plant and therefore an easy prey. So, why not turn this inherent weakness to your advantage?
Place yellow sticky traps (available in garden centers and online, or you can make your own) near the affected plants and whiteflies will stick to them in droves. Unfortunately, there are usually enough left on the plants to continue causing at least some damage.
It’s even more effective to paint the tip of a hand vacuum yellow and run it through the leaves of infected plants: whiteflies, frightened by the sound, will take off as they usually do, flutter about for a second or two, then look for a haven. And doesn’t that yellow object right there look good! So, off they head, drawn to the yellow-dipped vacuum cleaner like flies to honey … and—tragically for them, happily for you—they’ll be sucked in.
Obviously, this treatment will only reach adults; the immobile nymphs remain stuck under the leaves and no passing vacuum is going to dislodge them. You’ll therefore have to do a bit of vacuuming every four to six days, collecting the new adults as they hatch from their pupal shell. After about three or four treatments, though, the last adults will have been hoovered up and the problem will be solved.
A Light Trap
Another solution, more expensive, but requiring much less effort, is to attract whiteflies with a light trap. Such traps consist of a lamp with one or more blacklight-type fluorescent tubes specially designed to attract insects and a glue board (a sheet of sticky cardboard or plastic) set at the back.
Flying insects, including whiteflies, are irresistibly attracted to blacklight and will flock to the trap, then remain stuck on the glue board. You only have to change the latter occasionally, as it will eventually become covered in insects. With such a device, whitefly problems can be reduced to such an extent that, if there are a few present, they’ll be so in such small numbers they’ll be no need for any other treatment.
You can find more information on light traps in the article Let Light Traps Eliminate Your Indoor Pests.
On a Personal Note
Whiteflies used to be a huge problem for me. Every spring I struggled trying to keep my plants alive, especially my seedlings, until I could get them outdoors where the whiteflies soon dissipated. … until I discovered light traps. I’ve been using them for nearly 20 years now and whiteflies are no longer a problem, nor are any flying insects. The light trap gets them all! Just make sure you get an insect trap that uses a glue board. The popular bug zappers that electrocute mosquitoes aren’t effective against whiteflies.
Predators to the Rescue … in the Greenhouse, At Least
Yes, there are many predatory insects you can use to control whiteflies … in a greenhouse environment. Releasing predatory insects in a typical house won’t be nearly as effective, largely because usually the air is too dry for them. The result is that the predators struggle to survive and little headway is made on whitefly control.
In a greenhouse, though, predators can be very effective. You could try parasitic wasps (Encarsia formosa, Eretmocerus eremicus or Eretmocerus mundus) or even the small black ladybug known as Delphastus catalinae. Do note you can’t spray pesticides if you will be using predatory insects. They’re just as susceptible to pesticides are whiteflies; often even more so.
If you’re reading this in March, the whitefly hunting season has unfortunately already begun. If you want to have beautiful, insect-free plants, therefore, keep your eyes peeled and get ready to react immediately.