Whiteflies come not by dozens, but by thousands! Photo: http://www.selbst.de
Question: In my beautiful garden there are lots of tiny flies, smaller than a mosquito. They are white and appear as soon as I move a plant or shake its leaves. They then fly about in a small cloud. What are they? Are they harmful? If so, what should I do?
Answer: They’re whiteflies. Probably greenhouse whiteflies (Trialeurodes vaporariorum) which, in spite of their name, are not at all limited to greenhouses. Another species, the tobacco whitefly (Bemisia tabaci), is another possibility. There are more than 1,500 species of whitefly found especially in the tropics, but the above two are the most common in our gardens, although there is also the giant whitefly (Aleurodicus dugesii), twice as big as the others, although it is presently limited to California and similar mild climates.
They’re indeed tiny: most are less than 2 mm long and white (at least their wings are), but they’re not true flies, but rather closely related to aphids. As you noticed, they take off in panic when you disturb their host plant, then quickly return to it. People often say they look like flying dandruff.
Indoors and Out
Whiteflies are very cold sensitive. Few species tolerate any frost at all. So how could they possibly be turning up in gardens in colder climates?
That’s where greenhouses come in. Typically, whiteflies overwinter in greenhouses or on houseplants, often in a state of diapause (a state close to hibernation), then start to multiply massively, indoors, in the spring. When windows are first opened on a spring day, out they go … and a new generation reaches our outdoor gardens.
In the fall, they’re carried back indoors on plants brought in for the winter, on clothing or, again, come in through an open window, and the cycle repeats.
You may find whiteflies particularly abundant in dry summers, as regular rains interfere with their feeding and reproduction and help keep them in check.
There are multiple generations per season and generations overlap, so at any point, there’ll be eggs, nymphs and adults. Populations increase over the summer, then drop off in the fall.
The adult female lays eggs on the underside of plant leaves, usually in a circle. 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 legged 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 affect a whole range of plants, but seem to prefer those with soft, thin leaves. They attack vegetables (tomatoes, cabbage, eggplant, peppers, cucumbers, squash, okra, sweet potatoes, etc.), herbs and fruit trees as well as ornamental plants (coleus, fuchsias, lantana, pelargoniums, poinsettias, etc.). The giant whitefly seems to prefer trees and shrubs.
Both nymphs and adults feed on sap, piercing plant tissues on the underside of leaves. Oddly, they rarely take enough sap to really harm the plant. In fact, many plants show few symptoms of their sucking action other than the cloud of whiteflies that shimmer around them when they’re disturbed. On hot days, though, so much sap may be taken from heavily infested leaves that they may wilt due to decreased turgidity. Also, on some plants, especially when the infestation is serious, leaves may become mottled or yellow. Also, even at low rates, growth can be stunted and yields reduced.
Often lower leaves become coated with honeydew, a sticky, sugary substance given off by both nymphs and adults. Sooty mold may eventually form on the sticky leaves. It is harmless per se, but does block photosynthesis. It can simply be washed off.
The most serious problem due to whiteflies is that they can transmit plant diseases, including viruses, and there is no cure for plant viruses.
Note that treatments will be most effective early in the season (nipping the pest in the bud). It’s hard to entirely eliminate whiteflies if you start late in the season, when the infestation is already severe, but their numbers can at least be seriously reduced.
Also, apply treatments at night or early in the morning, when whiteflies aren’t active and adults won’t be able fly away and avoid them.
Here are some solutions.
1. Tap plants before purchase. If you see flying dandruff, leave them in the store. And definitely look gift plants in the mouth … or, more realistically, check under their leaves.
2. Install yellow sticky traps. Whiteflies are attracted to the color yellow and become stuck on yellow sticky traps (widely available commercially). You can also make your own sticky traps by coating a yellow index card with something sticky, like petroleum jelly.
Install sticky traps early in the season to (hopefully) catch the first marauders before they attack your plants. When the first trap fills in, you may need to replace it with a fresh one.
Whether traps seriously bring down the population of an already serious infestation is debatable, but it can still be immensely satisfying to the gardener to see thousands of tiny enemies stuck to a trap!
3. Vacuum them to oblivion. Paint the tip of a hand vacuum yellow and run it through the leaves of infected plants: whiteflies, startled by the vibration, will take off and, attracted by the yellow tip, be sucked into the device. Obviously, this treatment will only reach adults; the immobile nymphs remain stuck under the leaves. You’ll therefore have to repeat every four to six days under the population collapses.
4. Spray leaves with water. Yes, surprisingly, hitting the undersides of the leaves with a strong blast of water knocks whiteflies off and chases them away while wet leaves discourage egg laying. You’ll need to repeat daily for a week or so, but you’ll be surprised at how well this simple technique works. Spray early in the morning so leaves have time to dry off, as wet leaves at night tend to encourage diseases and you won’t want to substitute one problem for another..
5. Spray insecticidal soap, neem or horticultural oil. Even more effective! These are fairly safe biological pesticides you can spray on the undersides of leaves where they’ll kill both adults and nymphs. Thoroughly coat the leaf for best protection. You’ll need to repeat for full control. Don’t spray these in hot weather (read the product label for details).
6. Try biological controls. Whiteflies have lots of natural enemies: lacewings, ladybugs, pirate bugs, spiders, even dragonflies and hummingbirds, and they may offer some control. Certainly, you should hold off on spraying insecticides when beneficial insects and animals are present. Unfortunately, they rarely offer 100% control.
There are beneficial insects you can order and release (parasitic wasps such as Encarsia formosa, Eretmocerus eremicus or Eretmocerus mundus, a black ladybug called Delphastus catalinae, etc.), but there’s no guarantee they’ll stick around if you release them.
If you start noticing certain nymph cases under a leaf have turned black, that usually means parasitic wasps are at work. You’ll want to stop other treatments to let the wasps do their job.
7. Spray chemical insecticides. Save this for use as a last resort, as they can harm bees and other pollinators. Pyrethrum is fairly safe if used carefully. Neonics (dinotefuran, imidacloprid, etc.) will work, but do you really want to responsible for applying a pesticide despised by pretty much anyone with environmental concerns?
Whiteflies: they’re tiny, but numerous and hard to control. Swing into action with an appropriate control as soon as you see them; otherwise it may be too late.