Whitefly is a small insect that infests a diverse array of plants. More than 1,500 species of whitefly have been discovered worldwide. Whiteflies comprise the family Aleyrodidae. The nymphs and adults are injurious to plants. They feed by extracting sap, and other plant liquids from new growth. The persistent feeding causes leaf discoloration, while stunting growth, and reducing crop and flower yield. Dense whitefly populations will gradually weaken host plants, rendering them more susceptible to disease pathogens, and other environmental stressors. Small plants may eventually succumb to infestation.
Distribution & Habitat
Whitefly has a global distribution. It may be found wherever susceptible plants are present. Whitefly is most prevalent in North America, Asia, and southern Africa.
Whitefly infests more than 250 ornamental and vegetable plants. Common hosts include astilbe, cabbage, chenille, citrus, columbine, cucumber, chrysanthemum, dicentra, eggplant, flowering maple, gerbera daisy, glory bower, grape, hibiscus, lavender, laventa, malva, monarda, mint, Martha Washington geranium, okra, poinsettia, potato, primula, salvia, scaevola, tomato, rosemary, sweet potato, verbena, and zinnia.
The young nymphs, also called crawlers, are small, flattened insects that lack wings. The immature stages are nearly translucent, enabling them to blend in with the color of the leaf that they are attached to. Both sexes have functional mouthparts. The adults are moth-like insects with four powdery white wings, a pair of short antennae, and legs that are adapted to crawling.
Whitefly produces several generations each year, with each generation reaching maturity within approximately 25 days. The young nymphs overwinter on the leaves of host plants. In late spring, the nymphs mature, and mate. The females deposit 200 to 400 eggs in circular clusters on the underside of leaves. The eggs hatch in 4 to 12 days, revealing masses of nymphs. The first instar nymphs resemble small mealybugs or aphids. Upon emerging, the nymphs navigate away from the remnants of the eggs, before flattening their bodies against the leaves to feed. They commence feeding by piercing the leaf cells with their mouthparts, and consuming the plant’s sap. As they feed, the nymphs pass through three more instars. Within six weeks, the nymphs mature, and enter the pupal stage. They pupate for a week, before emerging as young adults to begin the cycle anew. Adults may live for one to two months.
Symptoms of Infestation
Infested plants develop mottled leaves that turn yellow before dropping prematurely. Honeydew, an ooze that is excreted by the crawlers as they feed, may be observed on the branches and main trunk. Honeydew can encourage the development of sooty mold on the plant, which can smother growth, and inhibit the photosynthetic process. Ants are attracted to the honeydew, and can often be seen crawling on infested plants.
- Crawlers and adults can be dislodged from infested plants using a strong stream of water.
- Whitefly has several natural predators, which are available for commercial use. Ladybugs and lacewing larvae feed on the eggs. Encarsia formosa and Eremocerus sp. are small parasitic wasps that eradicate the nymphs and pupae. The adult wasps lay eggs in the whitefly nymphs. As the eggs hatch, the larvae consume the nymphs from within, effectively killing them.
- Beauvaria bassiania is a common fungus that can be employed to control whitefly populations. The fungal spores penetrate the insect’s cuticle, causing it to become infected. Once the fungus has become established, it begins to expand within the insect, producing toxins that disrupt the insect’s immune system. Eventually, the insect will succumb to the infection. Do not apply fungicides in areas where Beauvaria bassiania is being used, as this can reduce the efficacy of the treatment.
- When ants are lured to a plant by the presence of honeydew, they will gather to protect the nymphs from their natural predators. This enables the nymphs to feed, and excrete more honeydew, which the ants readily consume. To effectively combat the nymphs, it is recommended that the ants be removed as well. There are several techniques that can be utilized to remove ants from infested plants. Infested plants can be doused with water, which will cause the ants to tumble to the ground. Ants are also repelled by citrus and vinegar, which can be combined with water, and sprayed onto infested plants. The solution will mask the scent trails created by the ants, and act as a deterrent when honeydew is present. A mixture composed of boric acid powder, sugar, and water can be placed in small containers, and distributed throughout the environment. The sugar will lure the ants to the tainted solution, which they will consume, and deliver to their queen. Once the queen has been exposed to the solution, she will expire, and the colony will collapse.
- Organic pesticides can be applied to infested plants to limit whitefly populations. Applications should begin as soon as the insects have been detected. If whitefly populations are dense, repeated applications may be required to establish control. Applications should be performed at seven to ten day intervals, as needed.
- Organic neem oil is safe for use on vegetables, fruit trees, and flowers. It can be administered during all stages of the insect’s life cycle. To ensure that the treatment is effective, thoroughly drench all leaf or flower surfaces.
- Horticultural oils can be used to smother the insects. They are effective at suppressing the eggs, nymphs, and adults.
- Botanical insecticides may be utilized to reduce whitefly populations. Applications should begin in early spring, during leaf expansion. Additional applications can be performed at seven to ten day intervals.
Photo courtesy of Scot Nelson CC-by-2.0