Whiteflies are tiny, sap-sucking insects that can be major pests of ornamental plants. They are most common during periods of warm to hot weather and can build up to extremely high populations on susceptible plants in greenhouses and nurseries.
When actively feeding, whiteflies cause leaves to turn yellow and eventually brown as the leaf tissue dies. Large colonies typically develop on the undersides of leaves where they normally lay tiny, oblong eggs that range from yellow to white in color. After the eggs hatch, the young whiteflies go through four nymphal stages called instars. Winged adults emerge from the last nymphal stage. All stages feed by sucking plant juices from leaves and excrete excess liquid as drops of honeydew when they feed. Whiteflies are extremely difficult to control when populations are high, so preventative management is crucial.
Ornamental production areas with warmer climates such as Florida and California provide an opportunity for these pests to breed year-round. They move from one host to another as plants go through the production cycle. In addition to the destructive feeding damage whiteflies cause on plants, the honeydew excreted on the affected leaves provides a source of nutrition for sooty mold fungi. Sooty mold fungi are not plant pathogens, but they readily colonize plant tissues as they feed on the honeydew excreted by the whiteflies. The fungus causes any affected surfaces (plants, concrete, etc.) to blacken from an abundance of spores and subsequently reduces overall plant quality. The best way to eliminate sooty mold is to control the whiteflies that are feeding on the plant.
Best Practices for Control
Effective whitefly management in greenhouse and nursery production requires a preventative approach. Identify and closely monitor plants known to host high populations of whiteflies, because infestations will likely start on the most susceptible plants. Plants that flower throughout the summer and those with yellow flowers are often attractive to whiteflies. This includes many types of weeds, so you will need to remove them and control them with herbicides. Yellow sticky traps can be used to aid in monitoring whiteflies and, under high populations, even helps to reduce their numbers. The use of biological controls (live predators) can also be effective, but they limit the choice of chemical insecticides and should be used with this consideration in mind.
Whitefly colonies occur on the underside of leaves, making them more difficult to target, especially when using contact insecticides.
Rotate Pesticides to Reduce Chance of Resistance
Over the years, whiteflies have also developed resistant populations that can withstand triple the amount of insect control products that were successful a decade ago. This makes it important to rotate insecticides with different modes of action. Bayer currently offers three insecticides for controlling whiteflies affecting ornamentals including Altus, Kontos, and Savate. Both Altus and Kontos are systemic and can be applied as a drench with minimal to no impact on beneficial insects and mites used for biological control of whiteflies. Savate works on contact with excellent translaminar movement that protects both the upper and lower leaf surface and provides knockdown activity and control of all developmental stages of whiteflies. Regardless of the products being used, it is good stewardship not to apply more than two sequential applications of any insecticide within the same Insecticide Resistance Action Committee (IRAC) group.