Whiteflies commonly infest plants that are growing under cover. But they can also be a big problem in vegetable gardens where onions and peas are growing, and flower gardens where susceptible plants like fuchsias, chrysanthemums, and hibiscus are planted. Even though they are called flies, whiteflies are more like tiny moths, and they can be lethal to plants.
Unfortunately, whiteflies are very difficult to control or kill. The best defense is to have a management strategy that prevents them from infesting your plants. Nevertheless, we do have a few tips that will help you to control and kill whiteflies if they infest your plants.
What Are Whiteflies?
Whiteflies are tiny, white, winged insects that infest our plants. Closely related to mealybugs and aphids, they have wings and can fly, but they aren’t flies.
There are hundreds of different whitefly species, with the Silverleaf whitefly attacking more than 500 plant species. Other types, including greenhouse whiteflies generally only affect a limited number of host plants.
Banded winged whiteflies feed on ornamental plants and some weeds, but they reproduce on very few of them.
Needless to say, they are all very difficult to control.
Heavy infestations often cause leaves to drop or become stunted. They may turn yellow and you will see a general decline in the health of your plants.
In colder climates, they won’t survive the winter outdoors. But in warm climates and greenhouses, whiteflies commonly reproduce throughout the year.
Adult whiteflies are only about 1/10 to 1/16 of an inch long. They have broad bodies and delicate wings.
Females lay 200-400 eggs and they commonly continue feeding during this process.
Bruce A. Barret of the Division of Plant Sciences at the University of Missouri Extension explains that they keep their sharp mouthparts in the host plant. Then they move only their abdomens while they lay their eggs in a circular pattern.
Where Are Whiteflies Found?
In the US, whiteflies are usually found in USDA zones 8 and hotter. They won’t survive in USDA zone 7 and colder areas unless they get into greenhouse environments or prey on indoor plants.
You are most likely to spot them when you water your plants or handle them. The females lay eggs on the underside of leaves.
It takes about 5-7 days for them to hatch in nymphs, which are pale green and tiny. Known as crawlers, the nymphs move over the plant feeding for 2-3 weeks.
When the nymphs lose their antennae and legs they attach themselves to the underside of the leaves. This results in them looking like small, white, oval-shaped scale insects.
Nymphs stay attached to leaves feeding for about 4 weeks. Eventually, after a pupa stage, the adults emerge.
Adult whiteflies only live for about a month. During this time they feed on plants and the females lay hundreds of eggs.
Both nymph and adult whiteflies have short, needle-sharp mouthparts. They stick these into the foliage and suck out the sap.
Whiteflies don’t go far from the plants they infest. They will flutter about if disturbed but then return to the plant to continue feeding.
They are generally active during the day, especially when the weather is warm.
What causes whitefly infestation?
It’s difficult to know what causes whitefly infestations. They thrive in a greenhouse environment and so quite often infest plants that are grown for resale.
In fact, one of the most common causes of whitefly infestations is the introduction of infested plants! Even a very minor infestation will quickly spread to other plants in the near vicinity.
Because all life stages of whiteflies are present at the same time, infestations continue at every level. You will have eggs, nymphs, pupae, and adults co-existing with generations of the pests often overlapping.
How to identify whiteflies on plants
It’s not always easy to identify whiteflies on plants because they are tiny. Unfortunately, it is easier to identify them when plants are heavily infested.
If you don’t spot eggs, larvae, or adult whiteflies on your plants, you might notice a sticky residue on the leaves. Bruce Barrett says these tiny flying pests suck out more plant juices than they can digest.
So, they excrete the excess sap in the form of honeydew, which is a sweet, sticky substance. When the honeydew covers leaf surfaces, it becomes a growing medium for a black, sooty mold.
The mold and loss of plant juices interfere with photosynthesis, damaging the plants.
How to kill whiteflies on plants
Unfortunately, the combination of whiteflies and houseplants makes them very difficult to kill and control. But there is a natural parasite of the greenhouse whitefly that has been used to kill them for decades.
Encarsia Formosa is a tiny wasp that feeds on whiteflies. They are widely used for whitefly control in greenhouses and commercial grow houses.
The female wasps lay their eggs inside the whitefly nymphs. Then, when they hatch, the wasp larvae feed on the nymphs.
Spiders, lacewings, and lady beetles will also kill them, but unlike the wasps, they aren’t usually for sale commercially! But they do occur naturally, which is a good reason not to use insecticides to kill whiteflies.
The problem is that the only other sure way to kill whiteflies is to use insecticides.
And if you do opt for commercial insecticides, remember that you are dealing with toxic chemicals. Because of this, it’s important to only use insecticides that are manufactured for use on houseplants.
Over-the-counter insecticides for whitefly control
Horticultural oil is often used to control whiteflies. Mix 2-5 tablespoons of oil with about one gallon of water. Pour into a spray bottle and apply on a dry day when the temperature is below 90 ℉.
Horticultural oil is a contact pesticide so be sure to spray the whiteflies themselves. It’s also a good idea to spray both sides of the leaves and drench the plant stems.
Insecticidal soap is also a contact pesticide. You can use a spray bottle to apply this too, but be sure to follow the instructions on the label.
Horticultural oils and soaps help to control whiteflies but are unlikely to kill entire populations.
Neem oil is another good home remedy, though, like the products mentioned above, it will usually control whiteflies but not eliminate them.
It is a fungicide that has insecticidal properties. According to the Environmental Protection Agency (EPA), it is safe to use on vegetables and other food plants as well as ornamentals.
There are also several over-the-counter chemical sprays that contain pyrethrins, permethrin, malathion, and imidacloprid.
How to prevent whiteflies from coming back
A good management plan will usually prevent whiteflies from coming back. But if you do spot any eggs or whiteflies on your plants it’s vital to start control measures right away.
Also, remove any affected plants and isolate them until you are sure you have killed all the insect pests. If you can’t kill them, remove infested leaves and destroy them by burning them.
It is also good practice to remove all host plants from greenhouses and indoor environments from time to time. This helps to break the reproductive cycle of whiteflies.
Other alternatives for killing whiteflies
A growing number of people prefer to avoid potentially harmful insecticides. There are several control measures you can use, though most are relatively labor intensive.
For example, you can wash the eggs off the undersides of houseplant leaves using a sponge or damp cloth. A good home remedy is to add a dash of dish soap to the water. This makes washing infected plants more effective.
There is another home remedy that you can try. It will keep in the fridge for up to a week.
Process a garlic bulb, onion, and a teaspoon of cayenne pepper in a blender with a quart of water. Leave it to steep for an hour and then strain through muslin or cheesecloth.
Add a tablespoon of dish soap and mix well. Pour into a spray bottle.
There are also a few mechanical controls that you can try. They are particularly effective in a greenhouse environment where there are large numbers of potted plants.
Yellow sticky traps can be effective, though Bruce Barrett maintains you need one trap for every two vegetable plants. This means that you will need a lot of sticky traps.
The way yellow sticky traps work is that the bright color attracts the whiteflies and they fly into them, getting trapped. But a caveat is that, like insecticides, sticky traps can also kill the whitefly’s natural enemies.
The advice here is to use sticky traps when you first spot whiteflies. When they decrease in numbers, stop using them.
Another mechanical technique people use is a small, hand-held vacuum cleaner. These work best early in the morning when the weather is cool and the adults are moving slowly.
A good way to then kill them is to remove the vacuum bag that contains the insects and freeze it for 24 hours. Then discard safely.
Prevention is the best strategy when it comes to getting rid of whiteflies. If you find that plants are heavily infested and you can’t kill whiteflies, it’s best to discard them. Otherwise, they are likely to spread to other plants.
You are only likely to control these pests if you take action straight away. They multiply quickly and will spread rapidly.
If you are growing in a greenhouse, it is essential to monitor for whiteflies regularly. Then take whatever action possible to kill them.