Whitefly Management
Authors: Gugulotu Laxman
Division of Entomology, ICAR-Indian Agricultural Research Institute, New Delhi-110012
Corresponding author: laxmanagrico@gmail.com

Whiteflies are tiny, sap-sucking insects that may become abundant in vegetable and ornamental plantings, especially during warm weather. They excrete sticky honeydew and cause yellowing or death of leaves. Outbreaks often occur when the natural biological control is disrupted. Management is difficult once populations are high.

Identification and life cycle

Whiteflies are not true flies (in the insect order Diptera) but are in the order Hemiptera, related to aphids, scales and mealybugs. They derive their name from the mealy white wax covering the adult’s wings and body. Adults are tiny insects with yellowish bodies and four whitish wings. Although adults of some species have distinctive wing markings, many species are most readily distinguished in the last nymphal (immature) stage, which is wingless and lacks visible legs. Depending on species, whitefly nymphs vary in color from almost transparent yellow or whitish to black with a white fringe.

Whiteflies develop rapidly in warm weather, and populations can build up quickly in situations where natural enemies are ineffective and when weather and host plants favor outbreaks. Large colonies often develop on the undersides of leaves. The most common pest species­—such as greenhouse whitefly, Trialeurodes vaporariorum and sweetpotato whitefly, Bemisia tabaci have a wide host range that includes many crops and weeds. These species breed all year round in warmer parts, moving from one host to another as plants are harvested or dry up.

Whiteflies normally lay their tiny oblong eggs on the undersides of leaves. The eggs hatch, and the young whiteflies gradually increase in size through four nymphal stages called instars. The first nymphal stage (crawler) is barely visible even with a hand lens. The crawlers move around for several hours before settling to begin feeding. Later nymphal stages are immobile, oval, and flattened, with greatly reduced legs and antennae, like small scale insects. The winged adult emerges from the last nymphal stage (sometimes called a pupa, although whiteflies don’t have a true complete metamorphosis). All stages feed by sucking plant juices from leaves and excreting excess liquid as drops of honeydew as they feed.

Some common whiteflies and their hosts:

1. Sweetpotato whitefly (Bemisia tabaci)

Host plants: very broad including many herbaceous and some woody plants including cole crops, cotton, cucurbits, tomatoes, peppers, crape myrtle, lantana, roses, and hibiscus.

Characteristics:Fourth-instar nymphs have no waxy filaments or marginal fringe. Adults have white wings and yellow body; they hold their wings slightly tilted to surface or substrate.

1. Greenhouse whitefly (Trialeurodes vaporariorum)

Host plants: very broad including most vegetables and herbaceous ornamentals. Also may occur on avocado, fuchsia, gardenia, lantana and redbud.

Characteristics:Fourth-instar nymphs have very long waxy filaments and a marginal fringe. Adults have white wings and a yellow surface or substrate.

1. Citrus whitefly (Dialeurodes citri)

Host plants: citrus, gardenia, ash, ficus, pomegranate.

Characteristics: Fourth-instar nymphs have no fringe around their edges but have a distinctive Y-shape on their backs. Adults are white

1. Ash whitefly (Siphoninus phillyreae)

Host plants: many broadleaved trees and shrubs including ash, citrus, Bradford pear and other flowering fruit trees, pomegranate, redbud, toyon

1. Bandedwinged whitefly (Trialeurodes abutilonea)

Host plants: very broad including cotton, cucurbits, other vegetables.

1. Iris whitefly (Aleyrodes spiraeoides)

Host plants: iris, gladiolus, many vegetables, cotton and other herbaceous plants.

1. Mulberry whitefly (Tetraleurodes mori)

Host plants:avocado, citrus, mulberry, other trees.

1. Crown whitefly (Aleuroplatus coronata)

Host plants: oak, chestnut.

1. Giant whitefly (Aleurodicus dugesii)

Host plants: avocado, begonia, hibiscus, giant bird of paradise, orchid tree, banana, mulberry, vegetables, and many ornamentals.

1. Woolly whitefly (Aleurothrixus floccosus)

Host plants:citrus, eugenia.

DAMAGE

Whiteflies use their piercing, needlelike mouthparts to suck sap from phloem, the food-conducting tissues in plant stems and leaves. Large populations can cause leaves to turn yellow, appear dry, or fall off plants. Like aphids, whiteflies excrete a sugary liquid called honeydew, so leaves may be sticky or covered with black sooty mold that grows on honeydew. The honeydew attracts ants, which interfere with the activities of natural enemies that may control whiteflies and other pests.

Feeding by the immature sweetpotato whitefly, Bemisia tabaci, can cause plant distortion, discoloration, or silvering of leaves, and may cause serious losses in some vegetable crops. Some whiteflies transmit viruses to certain vegetable crops. Whiteflies are not normally a problem in fruit trees although their populations can build up in citrus, pomegranate and avocado.

Several whitefly species occur on ornamental trees and shrubs, but most are uncommon because of natural controls such as parasites and predators. Most whiteflies on trees have limited host ranges. Low levels of whiteflies are not usually damaging. Adults by themselves will not cause significant damage unless they are transmitting a plant pathogen. Generally, plant losses do not occur unless there is a significant population of whitefly nymphs.

MANAGEMENT

Management of heavy whitefly infestations is difficult. The best strategy is to prevent problems from developing in your garden or landscape. In many situations, natural enemies will provide adequate control of whiteflies; outbreaks often occur when natural enemies are disrupted by insecticide applications, dusty conditions, or interference by ants. Avoid or remove plants that repeatedly host high populations of whiteflies.

In crops, whitefly populations in the early stages of population development can be held down by a vigilant program of removing infested leaves or hosing down with water sprays. Reflective mulches can repel whiteflies from vegetable gardens, and yellow sticky traps can be used to monitor or, at high levels, reduce whitefly numbers. If you choose to use insecticides, insecticidal soaps or oils such as neem oil may reduce but not eliminate populations. Systemic insecticides may be more effective but can have negative impacts on beneficial insects and pollinators.

Cultural Control

The best control for whiteflies is to maximize the distance and time interval between host crops.

• When possible, plant tomatoes at least one-half mile upwind from key sweetpotato whitefly hosts such as melons, cole crops, and cotton.
• Maintain good sanitation in areas of winter and spring host crops and weeds by destroying and removing all crop residues as soon as possible. Control weeds in noncrop areas including head rows (headland areas) and fallow fields.
• Harvest alfalfa on as short a schedule as possible.
• Allow the maximum time between harvest and subsequent planting of whitefly host crops.
• Grow vegetables and melons in the shortest time-span possible.

• Avoid injudicious application of nitrogenous manure and fertilizers. Also ensure that the plants are not lacking in magnesium and phosphorous

Mechanical control

1. Hand removal of leaves or plants heavily infested with the nonmobile nymphal and pupal stages may reduce populations to levels that natural enemies can contain.
2. Remove and destroy whitefly-infested vegetable plants after harvest. Always inspect new plants for whiteflies and nymphs before introducing them in the greenhouse or garden.
3. If you have high populations of whiteflies in a greenhouse, removing all host plants from the greenhouse for at least 2 weeks (and assuring that no whiteflies can enter from outside) may eliminate problems.
4. Water sprays (syringing) may also be useful in dislodging adults. Watering can also reduce the hot, dry dusty conditions that favor whiteflies and inhibit their natural enemies.

Physical control

1. Reflective Mulches

• Adult whiteflies are repelled by silver- or aluminum-colored mulches. Place reflective polyethylene mulches on planting beds before seeding or transplanting to significantly reduce rate of colonization by whiteflies and delay the buildup of damaging numbers of whiteflies by 4 to 6 weeks.
• This delay in infestation can be especially important if virus transmission is a major concern.
• The mulches lose their effectiveness when more than 60% of the surface is covered by foliage. Therefore, they are effective only for the first few weeks after seedling emergence or transplanting of either spring or fall tomatoes.
• In addition to repelling whiteflies, aphids, and leafhoppers, the mulch will enhance crop growth and control weeds.
• Reflective mulches have been shown to deter pests that transmit viruses in commercial vegetable crops, perhaps helping to reduce disease incidence and crop loss.
• When summertime temperatures get high, remove mulches to prevent overheating plants.

1. Traps

Sticky yellow straps : These can be set around plants to help detect an infestation early enough. The reason why this works is because of the whiteflies attachment to the color yellow. In case one taps or disturbs a plant that is infested, the flies are likely to move up to the trap. Other than early identification, they also help in catching the whiteflies especially where they are in large populations.

In vegetable gardens, yellow sticky traps can be posted around the garden to trap adults. Traps are most useful for monitoring and detecting whiteflies rather than controlling them.

Biological Control

Whiteflies have many natural enemies, including parasitic wasps, and predators, such as plant bugs, mites, ladybird beetles, lacewings and others. Outbreaks of whiteflies frequently occur when these natural enemies have been disturbed or destroyed by pesticides, dust buildup, or other factors.

The predaceous mite Amblyseius swirskii is widely used to control whiteflies in non-tomato crops, while in tomato, the parasitic waspEretmocerus spp. and predaceous plant bugs, such as Nesidi­ochorus tenuis, are used alone or in combination to control whiteflies.

Whiteflies have a number of naturally occurring parasites that can be very important in controlling some species. Encarsia spp. parasites are commercially available for release in greenhouse situations; however, they are not generally recommended for outdoor use because they are not well adapted for survival in temperate zones.

Avoiding the use of insecticides that kill natural enemies is a very important aspect of whitefly management. Products containing carbaryl, pyrethroids, or imidaclo­prid (especially as a foliar application) can be particularly disruptive. Control of dust and ants, which protect whiteflies from their natural enemies, can also be important, especially in citrus or other trees.



Chemical control

Whiteflies can be difficult to control with insecticides. Most less-toxic products such as insecticidal soaps, neem oil, or petroleum-based oils control only those whiteflies that are directly sprayed. Therefore, plants must be thoroughly covered with the spray solution, and repeat applications may be necessary. Be sure to cover undersides of all infested leaves; usually these are the lowest leaves and the most difficult to reach. Use soaps or oils when plants are not drought-stressed and when temperatures are under 90°F to prevent possible “burn” damage to plants. Early evening, when there is enough light to safely apply products but when the sun is not shining directly on plants, may be a good time to spray.

The soil-applied systemic insecticide imidacloprid can control whitefly nymphs. Imidacloprid can have negative impacts on natural enemies, honey bees and other pollinators in the garden, especially when applied as a foliar spray or as a soil application to plants that are flowering or soon to be flowering. It can also cause outbreaks of spider mites. Reserve its use for special situations where these problems can be avoided. Avoid using other pesticides (other than soaps and oils) to control whiteflies; not only do most of them kill natural enemies, whiteflies quickly build up resistance to them, and most are not very effective in garden situations.




About Author / Additional Info:
Pursuing Ph D in Entomology in Indian Agricultural Research Institute