Whiteflies are tiny, sap-sucking pest insects that infest many ornamental and vegetable plants. Although there are more than 1,200 whitefly species, the pests cause the same general damage and require the same control measures. Managing whitefly infestations can be difficult because most traditional insecticides just don't work well. The best plan involves combining a less toxic pesticide treatment with biological and mechanical control methods.
Attract Beneficial Insects
Gardens with a variety of plants make poor habitats for pesky whiteflies because the flowers attract beneficial insects. Natural whitefly predators include lady beetles, lacewing larvae, predatory beetles, parasitic wasps and spiders. You can attract beneficials to your yard by planting annuals, including cornflowers (Centaurea cyanus) and borage plants (Borago officinalis). Beneficial-attracting perennials include anise hyssop (Agastache foeniculum) and fennel (Foeniculum vulgare), both of which grow in U.S. Department of Agriculture plant hardiness zones 6 through 9.
Protective, sugar-feeding ants, dust buildup and using broad-spectrum insecticides can all prevent beneficial predators from feeding on whiteflies. Get rid of ants by placing sugar-based baits near whitefly-infested plants, but only do so if you don't have curious kids or pets in your household. Hosing off plants with a strong stream of water from a garden hose rinses away both ants and dust, giving the beneficial insects easier access to the whiteflies.
Pick off heavily infested leaves to get rid of stationary nymphs and pupae. Place the infested debris into a plastic garbage bag and toss it into a trash can. Hosing down plants with water, a process known as syringing, rinses adult whiteflies and their ant-attracting excrement from the foliage. In the cool, early hours of the morning, take a handheld vacuum cleaner out to infested plants and suck the sluggish adults off the foliage. Place the vacuum on its lowest setting to avoid injuring delicate plant leaves. Before throwing out the vacuum bag, place it in the freezer for about 24 hours to kill the whiteflies. The vacuuming process works best if you catch infestations early enough that the females haven't had a chance to lay their eggs.
The color yellow attracts whiteflies, so use yellow sticky traps to reduce pest populations. Buy a commercial trap at a garden center or make your own using pieces of 12- by 6-inch poster board or cardboard. Paint the material bright yellow and coat it with petroleum jelly or mineral oil. Support the traps on stakes that you drive into the soil near the infestation site, making sure the sticky sides face toward the plants. Use one trap for every one or two two large plants. Wipe off any trapped insects once a week and apply a fresh coat of your chosen sticky substance. Unfortunately, beneficial insects can also be caught in sticky traps. Reduce the chances of catching natural predators by setting out the traps early in the growing season and removing them once you reduce whitefly numbers.
Using a less toxic insecticide can help get rid of a severe whitefly infestation while minimizing the negative effects on beneficial insects. Neem oil is one alternative that can be used on all types of plants, including ornamentals, fruit and nut trees, flowers, herbs and vegetables. Carefully read and follow the manufacturer's mixing and application instructions as well as the safety precautions. Although not toxic to humans and pets, reduce the risk of exposure by wearing protective clothing and goggles and keeping everybody out of the treatment area until the oil solution dries.
Neem oil pesticides come in various formats, including a ready-to spray form. Simply attach the container to your garden hose, turn on the water and thoroughly coat the plants with the solution. Neem oil only kills whiteflies on contact, so you must spray the pests directly. Apply neem oil at the first sign of whiteflies and repeat once a week until you get rid of the pests. Apply neem oil sprays only when no rain is expected for the following 24 hours, and reapply the oil if it does rain.