How to Control Whiteflies
If you need to control whiteflies, visit a garden supply store and purchase beneficial insects to add to your garden. Choose an insect that lacewings, minute pirate bugs, big-eyed bugs, some ladybugs, and some spiders. You can also purchase parasitic wasps that prey on whiteflies. However, most whiteflies are resistant to insecticides, but these predators are not, so avoid using chemicals in your garden, as it can actually increase whiteflies.
Control whiteflies on outdoor plants by introducing the whitefly's natural predators. One of the most effective ways to get rid of whitefly infestations is to introduce animals to your garden that will feed on the whiteflies, but not your plants. Lacewings, minute pirate bugs, big-eyed bugs, some species of ladybugs, and some spiders all eat whiteflies. Garden supply stores often sell these bugs, and should say whether the insect they are selling will eat whiteflies. Purchase whitefly parasites. Encarsia formosa or other Encarsia species are tiny parasitic wasps, which can be introduced to your garden to invade the whiteflies' bodies and disrupt their ability to reproduce. They are commercially available, but most can only thrive indoors, in greenhouse environments, and in tropics. Avoid insecticides. Many strains of whitefly are resistant to insecticides, while their predators and parasites are not. Thus, using insecticides can sometimes increase the amount of whiteflies in your garden. Get rid of ants. Ants are not predators of the whitefly, despite their frequent appearance near infested plants. Some species of ants will actually feed on the "honeydew" secretions of the whitefly nymphs, and could defend or discourage actual predators of whiteflies. Purchase whitefly traps (optional). Whitefly traps are available commercially. They are usually bright yellow strips of sticky cardboard, staked or hung above plants. As you might have guessed from this description, they are easy to make yourself as described below. Note that these traps will only catch adult whiteflies, and young wingless nymphs will still remain on the plants and damage leaves. Combine this with other treatments for best results. Cut your own traps instead. If you'd rather make your own traps, start by cutting plywood or masonite board into strips roughly 12 inches (30 centimeters) long and 6 in. (15 cm) wide. This size should be suitable for most plants, but you may use smaller ones if you only have a small flowerpot or two to protect. Paint your traps bright yellow. Use any type of paint to make your traps bright yellow, on both sides. Adult whiteflies are attracted to this color. Coat both sides of the trap with a sticky substance. Petroleum jelly mixed with an equal amount of mineral oil or detergent creates a long-lasting sticky adhesive. Alternatively, use a heavy-grade motor oil or a commercial product such as Tanglefoot. Whichever substance you choose, be sure to coat both sides of the trap generously with a paintbrush, so any whitefly landing on the trap will be stuck. Place them above plants. Hang them above your plants, or stake them in the ground near your plants. The stakes should be at the same level as the leaves you see the whiteflies feeding on. One trap for every two large vegetable plants or flowerpots is a good guideline. Clean the traps periodically. Hopefully, there'll be plenty of dead whiteflies on the trap, along with dead leaves or other debris that gets stuck to them. Wash with soap and water or scrape the traps regularly to remove these, and reapply the sticky substance so you can catch more pests. Remove the traps when the whiteflies are mostly gone. After the whitefly population is mostly dead, and you only see a few on the traps, take down the traps. Because the traps also have the potential to kill whitefly predators, they are not recommended for a low level infestation, unless the predators fail to keep the whiteflies in control. Remove badly infested leaves by hand. Check daily during a severe whitefly infestation, and pluck off badly infested leaves. These have many white eggs and/or wingless nymphs ("crawlers") on the underside of the leaf. If severely infested, the leaf may be coated with a sticky or waxy substance produced by the feeding nymphs, or it may turn yellow and spotted. This method works best for small gardens where you can check each plant regularly. Spray or vacuum whiteflies off the leaves. You can dislodge young whiteflies from the underside of leaves by spraying them with a hose. To catch the winged adults as well, use a hand-held vacuum cleaner to catch whiteflies when they are most sluggish, in the early morning or during cool weather. Put the vacuum cleaner bag inside an airtight plastic bag after capturing, and freeze it for 24 hours to kill the whiteflies before emptying in the trash. This method is most helpful when the whiteflies are first noticed. If they have had a chance to lay eggs, these may remain on the leaf and start a new generation of many hundreds or thousands of whiteflies after hatching. If the infestation is minor, this method can help reduce the population of whiteflies to the point where natural predators will be able to keep the population in check. Add a reflective mulch. Add a layer of aluminum foil or reflective plastic mulch covering the ground. This can make it more difficult for adult whiteflies to locate host plants, reducing the amount of eggs successfully laid. This step will require special water considerations. Plants surrounded by plastic mulch will need to have a drip irrigation system. Do not use mulch in hot weather, as it may overheat the plants. Isolate new or infected plants. If a plant is severely infested, transfer it to a separate room or an outdoor area distant from other plants. Isolate all newly acquired plants for several days before introducing them to a location with other plants, and inspect the undersides of the leaves for whiteflies. Use insecticide as a last resort. As explained in the section on natural enemies, insecticide rarely works on whitefly populations, and may even kill the animals that feed on them. However, if the whitefly population in your garden is out of control, you may have nothing to lose. Neem oil, insecticidal soap, and malathion are common options, and you may wish to choose two of them to alternate between, in case your whiteflies are resistant to one. Apply an insecticide once every five to seven days to the undersides of infested leaves. Always check to make sure the insecticide you plan to use will not harm the plant you are applying it to. 
Control whiteflies on outdoor plants by introducing the whitefly's natural predators. Purchase whitefly parasites. Avoid insecticides. Get rid of ants. Purchase whitefly traps (optional). Cut your own traps instead. Paint your traps bright yellow. Coat both sides of the trap with a sticky substance. Place them above plants. Clean the traps periodically. Remove the traps when the whiteflies are mostly gone. Remove badly infested leaves by hand. Spray or vacuum whiteflies off the leaves. Add a reflective mulch. Isolate new or infected plants. Use insecticide as a last resort. 