As the name whitefly suggests, they are white, and they do fly, but they aren’t technically flies! Whiteflies are closely related to aphids, cicadas, and scale insects–the true bugs, or Hemipterans–not flies. These tiny pests are usually around 1/16 to 1/10 of an inch long and have solid or banded white wings (Figure 1). Much like leafhoppers, whiteflies will scatter and fly away when disturbed. As true bugs, whiteflies are sap-sucking insects, and will use a needle-like mouthpart to suck the life out of your crops without intervention.
As savvy sap-suckers, they also make honeydew, like aphids. Problems from honeydew include: covering leaves and resulting in sooty mold fungal growth (Figure 2), attracting ants or other pests, and blocking out sun on parts of the leaf used for photosynthesis. In addition to these issues, whiteflies can also transmit disease.
When you see whiteflies on crops, it is likely you are seeing adults. Adults are the only stage in which these pests can fly. After adult females lay eggs on the underside of leaves, small translucent “crawlers” hatch and walk around until they find a spot to feed. Once a feeding spot is found, the insect will lose all six of its legs. It will then remain in one spot under the leaf, sucking sap and progressing through its nymphal life stages until a few weeks later when the adult can emerge. Casings are left on leaves from where the adults emerge.
Whiteflies are one of the most widespread pests out there, yet it is difficult to see the numbers that you may be dealing with. Due to their small size and obscure lifestyle, they may only be noticed as adults. Usually, whiteflies are seen in mating pairs, and females are always larger than males. You can find them on leaves or flying above the plants.
Since they are so small, whiteflies may be hiding on crop starts or other introduced plants. Precautions can be taken to avoid the spread of whiteflies before transplanting and when introducing new crops. Some ways to keep areas free of whiteflies include keeping the area weed free (since whiteflies will also feed on many weed species), not over-fertilizing (this can attract whiteflies), and thoroughly checking plants for pests before planting them. Some research has also shown reflective plastic mulching helps to repel whiteflies and other pests!
Whiteflies are difficult to see on sticky cards but can be found with patience! Using blue cards makes them easier to see on a dark background, while on yellow cards they easily blend in with the light-yellow color and appear as miniscule orange dots. If you do choose to use sticky cards to check for whiteflies, use a hand lens to get a closer look.
While many traditional insecticides can be used for whiteflies, an easy and organic solution is a neem oil spray. If neem oil does not work, Malathion (an organophosphate) can be used. Insecticidal soaps can be used to kill whiteflies on contact.
Natural enemies can be released if the grower is interested in utilizing biological control. This can be relatively more expensive than insecticides but can help if the grower is willing to commit/has the funds for it. A common biocontrol agent used is the ladybug larvae Delphastus. Other biocontrol agents that can be used for whiteflies include lacewing larvae, predatory mites Amblyseius swirskii, and parasitic wasps (in which wasp species depends on whitefly species). If winter is approaching, the easiest fix is to let them die once temperatures reach freezing; whiteflies cannot survive a freeze. However, greenhouses and high tunnels should continuously be monitored since plants and insects inside are often kept protected from freezing temperatures.
By Katie Grubb, Entomology Graduate Research Assistant and Jonathan L. Larson, Entomology Extension specialist