Knockout roses are a type of rose created by rose breeder Bill Radler. Although they are often sought after because they are easy to grow and disease resistant, they are still susceptible to many of the same common pests found on regular roses. One of these pests is the whitefly. Whiteflies are small winged insects that have a somewhat feathery appearance. They are often found on roses and are particularly hard to control.
Adult whiteflies are sometimes thought to resemble moths. The adult grows to around 1/8-inch long and is whitish or yellowish-white in color with a waxy-like coating. Whiteflies are not true flies. They are actually related to other pests that often enjoy knockout roses. These insects include mealybugs, scales and aphids.
Whitefly eggs are typically laid about a week after the adults emerge. The eggs are quite small and are generally found on the underside of the leaf where they are hard to see. The eggs are cone-shaped and may be seen in several different shades of color from dark gray to almost burnt orange. When conditions are favorable, the eggs can hatch in about a month. However, when conditions are not adequate, the cycle can take up to a year. Regardless of the time it takes, once the young emerge from the eggs, they immediately begin to feed.
Whiteflies damage knockout roses by sucking the juice out of the leaves, stems and buds. When not stopped in time, the rose bush will yellow and eventually die. The first sign of whitefly infestation is usually browning leaves and a plant that does not look healthy. The whitefly also produces a type of sap called "honeydew" that often turns into a sooty mold.
The best way to control whiteflies is to catch them before they have an opportunity to infest any plants. The simplest solution is to hang whitefly traps close to the top of the knockout roses. These yellow sticky traps will lure whiteflies to them and not only catch them, but also alert gardeners that they are in the area. Once the flies are actually on the plants, however, they become much harder to eliminate. An insecticidal or seaweed spray used directly on the plants is somewhat effective. Introducing predators to the garden may also help.
Whiteflies have several natural predators. The ladybug is one of the most effective. A ladybug larvae can consume over 1000 whitefly eggs during its lifetime. Other predators include green lacewing, wasps and predatory mites. Even songbirds such as the swallow and kinglet will feed on whiteflies.