Whiteflies are a common plant pest that feed on more than five hundred species of host plants. Greenhouse-grown ornamentals such as poinsettia, hibiscus, ivy, gerbera daisy, lantana, verbena, garden chrysanthemum, salvia, and mandevilla are especially susceptible to whitefly damage.
Whiteflies feed on plant phloem by injecting enzymes and removing the sap, reducing the vigor of the plant. Honeydew secretions from whiteflies promote the growth of sooty mold, which can significantly reduce a plant's ability to photosynthesize.
The most obvious indications of whitefly feeding damage are stem blanching, chlorotic spots, leaf yellowing and shedding, and at high population levels, plant death. In many crops, the damage caused by certain species of whiteflies is indirect, caused by the transmission of devastating viruses and not necessarily by the act of feeding itself.
Despite their name and appearance, whiteflies are neither moths nor flies but true bugs more closely related to aphids, scale insects, and mealybugs. They derive their name from the white wax covering found on the wings and bodies of adult insects.
Adults are moth-like in appearance and covered with a white, waxy powder. They are small insects and range anywhere 1-5+mm depending on the species. Nymphs vary in color from nearly transparent to black with a white fringe. Whitefly eggs are pale yellow when laid though they darken when they are about to hatch. These insects have piercing mouthparts with which they ingest plant juices and produce a substance known as honeydew (similar to aphids). This honeydew excretion, not always visible, can be detected by the feeling of stickiness on the underside of leaves. The presence of honeydew is an indication that whiteflies have been feeding for several days.
In greenhouses, whiteflies typically feed on foliage on the under-surface, particularly near the veins. They tend to prefer new growth and can often be found on newly unfurled leaves. Whiteflies can be found harbouring on a variety of plants, from flowering ornamentals to warm-weather vegetables. Some species have been known to feed on sweet potatoes, plants from the cabbage family, and even citrus trees. Indoors, soft-leaved houseplants are particularly attractive to Whiteflies.
The whitefly life cycle (egg to egg) consists of 4 instar phases before finally emerging as an adult. Whitefly eggs are small and oval in appearance, typically laid on the underside of leaves, often in a circular pattern. The transition from one stage to the next is somewhat temperature-dependent. However, at 21C eggs typically take 6-10 days to hatch. Upon hatching, the whitefly emerges as its first instar referred to as crawlers. These crawlers wander over leaves for about a week in search of feeding sites. Instar phases 2-4 resemble small-scale insects that are immobile and lack legs. With each phase lasting between 3-5 days. At instar phase 4, typically referred to as the pupal stage, whiteflies cease feeding and remain like that for 6-10 days before emerging as adults. Adult whiteflies can live 30-40 days. However, without plant material to feed on, they will succumb to starvation in as little as 3 days.
Are whiteflies hazardous to humans?
Whiteflies are not hazardous to humans but can be dangerous to plants through the propagation of disease, mold, and overfeeding (if populations are high enough).
How can I detect whitefly infestation and how bad it is?
You can monitor whitefly population levels by trapping winged adults on sticky cards and inspecting leaves for the presence of feeding immatures.
To provide information about the presence and movement of whiteflies, strategically place yellow sticky cards throughout the greenhouse, especially near doors and among new plants.
To detect whiteflies on plants, randomly select ten plants per thousand square feet of greenhouse space and--hand lens or magnifying glass--thoroughly examine the undersides of plant foliage for the presence of whitefly adults, nymphs, and eggs. Additionally, the presence of honeydew can be detected by a feeling of stickiness on the underside of leaves.
How can I prevent whitefly infestation?
You can reduce your risk by placing new plant stock in a secure place to prevent whiteflies from escaping. Quarantine all new plant material introduced into your nursery until it has been carefully inspected.
How do I remove whitefly infestation?
Populations can be managed by removing infested leaves, introducing natural predators such as parasitic wasps, hosing down plants to remove adults, or removing infected plants. Additionally, the deployment of reflective mulches has been shown to repel adults and inhibit the growth of associated pathogens.
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