Whiteflies

How to Identify Whiteflies

Whiteflies of various growth stages live on the undersides of leaves.

Whiteflies are present in many growing systems, including greenhouses. They are small but easily seen on leaves. As the name suggests, they are white to whiteish yellow. Simply shake a leaf and they will fly rapidly in small circles before settling once more on the leaves.

On the left Bemisia argentifolii and on the right Trialeurodes vaporiorum

Two species of whitefly are troublesome pests in the greenhouse, the silverleaf whitefly (Bemisia argentifolii) and the greenhouse whitefly (Trialeurodes vaporariorum). The adults are very similiar in appearance except for the the way they hold their wings at rest. The silverleaf whitefly holds its wings folded like the peaked roof of a house, while the greenhouse whitefly lays its wings flatly over its back.

The pupal stages of B. argentifolii (left) and T. vaporarium (right).

Photo courtey of UC Statewide IPM Program

A more accurate method of telling the difference between the two species is to examine the immature stages of the insect. The pupal stage of the greenhouse whitefly is round with flat raised sides like a cake, and has waxy filaments protruding from the top edges. The same growth stage in the silverleaf whitefly is oblong, has no filaments, and resembles a flattened turtle shell. It is advisable to seek an expert's opinion by e-mailing a high resolution photo to the county agricultural commisioner's office or university extensions. If you need to take a sample in to a local office, do so by carrying the pest in a sealed plastic container.

How to Identify Damage Caused by Whiteflies

Sooty mold is growing in honeydew on these avocado leaves.

Photo courtesy of UC Statewide IPM Program.

Whichever species of whitefly, the damage caused is the same. While the whitefly does feed directly on the contents of plant cells, the economic losses are usually due to the pests producing huge amounts of honeydew. Honeydew is a waste product that is high in sugar. The pest flicks it onto leaf surfaces and this sticky substance fosters rapid development of sooty mold on plant tissue. The sooty mold makes greenhouse plants unsalable. Plants covered in sticky honeydew, grey fungus, and flying insects are not readily accepted into many retail outlets, let alone by consumers.

Greenhouse Conditions

The greenhouse is not just a place where growers produce crops. It is an environment that attracts, supports, and produces plant-eating pests. Keeping this fact in mind, we can realize and deal with the fact that our management of the greenhouse can control pests.

Some greenhouse conditions are known to encourage whitefly populations. The easiest and most effective thing we can do to control whiteflies is to exclude them. If you do not already have whiteflies, quarantine and inspect all incoming stock carefully for adults, immature stages, and eggs on the undersides of leaves. Discard any infected material. Exclusion of the winged adults can be accomplished by covering greenhouse openings (vents, windows, rooftops, etc.) with insect meshes of 405 microns or smaller. Since whiteflies feed on a variety of host plants, keeping the greenhouse clear of weeds can reduce the potential for pest problems to begin there and spread to the crop. Like many insects, whitefly reproduction cycles are sped up by warmer temperatures. Keeping greenhouse temperatures at the minimum requirement for the intended crop will help to slow insect infestations and save money on heating.

The immature whiteflies on the underside of this leaf do not have working legs and are stationary.

Insecticide applications are not generally recommended to combat whitefly infestations. These insects live much of their lives as immobile immature stages on the undersides of leaves. Oil sprays that are effective against them must make contact with them on the underside of the leaf to kill them. This makes applying sprays very time consuming and difficult. In addition to this, the eggs and pupal stages of whiteflies are tolerant of many insectides. This leaves a substantial population alive even after a succesful spray.

Whitefly populations are well known to build up resistance to pesticides very quickly. When an insect population is subjected to pesticide, only insects with resistance to the poison live. These survivors reproduce in an environment that is free of competition and predators. Their offspring are also resistant to the pesticide. Whiteflies go through many cycles of reproduction quickly, and this is why many chemicals are simply ineffective in controlling them. In fact, their numbers are known to increase after in pesticide application, mainly because their natural enemies have been eliminated.

What Are the Natural Enemies of Whiteflies?

Left E. eremicus, natural enemy of silverleaf whitefly and right E. formosa, natural enemy of greenhouse whitefly.

Photos courtesy of Koppert Biological Systems and UC Statewide IPM Project

Both the silverleaf whitefly and the greenhouse whitefly have very effective natural enemies. These enemies are parasitoids that lay their eggs into the whitefly pupa. Each one is very specific to its whitefly host. Eretmocerus eremicus is the parasitoid of the silverleaf whitefly and Encarsia formosa is the parasitoid of the greenhouse whitefly. Each will only go after their specific host, so it is vital that you have the correct identification of your whitefly pest. Your local agricultural commisioner's office or university extension should be able to help. A high resolution photo sent by e-mail is often effective, or if you need to you can usually bring the pest in to a local office in a sealed plastic container.

The E. formosa adult lays her egg into the greenhouse whitefly pupa. The young wasp develps in the safety of the pupa case, then emerges as an adult ready to lay her own eggs.

Image courtesy of UC Statewide IPM Project

These natural enemies are tiny wasps. Their life cycles depend on the presence of whitefly, so they are highly motivated to seek out whitefly pupa in order to lay eggs in them. The wasps are winged insects that move easily from plant to plant. The real beauty of this is that each whitefly that is destroyed in this process then hatches another wasp that repeats the cycle. Application of pesticides can do great harm to populations of these beneficial insects. However, if an application is absolutely necessary, these wasps are relatively cheap and easy to replace. These little insects can greatly reduce whitefly populations and continue to work well when the pest populations are very low. Keeping a working amount beneficials in the greenhouse keeps pests in check for the long-term.

How Can I Get These Natural Enemies?

This easily purchased card will release parastic wasps into the greenhouse.

Photo courtesy of UC Statewide IPM Project

Both E. eremicus (enemy of silverleaf whitefly) and E. formosa (enemy of greenhouse whitefly) are well established as control methods for whiteflies. They are readily available online from reputable companies, or in some cases from local insectaries. Keeping track of pest populations using sticky traps will help you determine when it is appropriate to order natural enemies and schedule releases. More information on using sticky traps to monitor whitefly populations can be found on the UC Statewide IPM Project website (http://www.ipm.ucdavis.edu/PMG/r280390411.html)

These natural enemies are like any other pest control method in that they will come in easy-to-use packaging and have instructions on how to use them.