Trialeurodes vaporariorum Westwood Homoptera Aleyrodidae

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Natural History

Distribution. Greenhouse whitefly is found widely around the world, including most of the temperate and subtropical regions of North America, South America, Europe, Central Asia and India, northern and eastern Africa, New Zealand and southern Australia. It does not thrive in most tropical locations, and occurs in colder regions only by virtue of its ability to survive in winter in greenhouses. In the northern United States and Canada, it overwinters only in such protected locations, but in mild-winter areas of the southern states and in Hawaii and Puerto Rico, it survives outdoors throughout the year. The origin of this species is not clear, but is thought to be Mexico or the southwestern United States.

Host Plants. This species has a very wide host range, with over 300 species recorded as hosts. However, some hosts are more suitable. Vegetable plants often serving as good hosts are bean, cantaloupe, cucumber, lettuce, squash, tomato, eggplant, and sometime cabbage, sweet potato, pepper, and potato. Among greenhouse-grown vegetables, the most common hosts are tomato, eggplant, and cucumber. When adults land on favored host plants such as eggplant, they almost always remain to feed and oviposit; on a less preferred host such as pepper, they usually take flight after tasting the plant. Many ornamental plants serve as good hosts, including ageratum, aster, chrysanthemum, coleus, gardenia, gerbera, lantana, poin-settia, salvia, verbena, zinnia, and many others.

Natural Enemies. Natural enemies of greenhouse whitefly are numerous, but few are consistently effective, especially under greenhouse conditions (Gerling, 1990; Onillon, 1990). Greenhouse whitefly is attacked by the common predators of small insects, including minute pirate bugs (Hemiptera: Anthocori-dae), some plant bugs (Hemiptera: Miridae), green lacewings (Neuroptera: Chrysopidae), brown lacew-ings (Neuroptera: Hemerobiidae), and lady beetles (Coleoptera: Coccinellidae). Parasitic wasps attacking greenhouse whitefly are largely confined to the family Aphelinidae, but many species are involved and they vary regionally. Those known from North America, including Hawaii, are Encarsia formosa Gahan, Aleuro-dophilus pergandiella (Howard), Eretmocerus haldemani Howard, Prospaltella transvena Timberlake, and Aphi-dencyrtus aphidivorus (Mayr). Although these agents exercise considerable control on whitefly populations in weedy areas or on crops where insecticide use is minimal or absent, they do not survive well in the presence of most insecticides. Encarsia formosa is used successfully under greenhouse conditions, and to a lesser extent field conditions, to affect biological suppression. (For more information, see the section on biological control.)

The pathogens of greenhouse whitefly are principally fungi, particularly Aschersonia aleyrodis, Paecilomyces fumosoroseus, and Verticillium lecanii. All occur naturally and can cause epizootics in greenhouses and fields, and also are promoted for use in greenhouses as bioinsecticides. Aschersonia is specific to whiteflies, Verticillium has a moderately wide host range, and Paecilomyces has a broad host range (Fransen, 1990; Osborne and Landa, 1992). For optimal development of disease, high humidity is required. Aschersonia is spread principally by rain fall, so often it fares poorly in greenhouse environments.

Life Cycle and Description. The development period from egg to adult requires about 25-30 days at 21°C, and 22-25 days at 24°C. Thus, because the pre-oviposition period of adults also is short, less than two days above 20° C, a complete life cycle is possible within a month. Greenhouse whitefly can live for months, and oviposition time can exceed the development time of immatures, resulting in overlapping generations. Optimal relative humidity is about 75-80%. The developmental threshold for all stages is about 8.5°C.

  1. The eggs are oval, and suspended from the leaf by a short, narrow stalk. The eggs initially are green and dusted with white powdery wax, but turn brown or black as they mature. The eggs measure about 0.24mm long and 0.07mm wide. They are deposited on the youngest plant tissue, usually on the underside of leaves in an incomplete circular pattern. Up to 15 eggs may be deposited in a circle that measures about 1.5 mm in diameter. This pattern results from the female moving in a circle while she remains with her mouthparts inserted into the plant. This pattern is less likely on plants with a high density of trichomes, because plant hairs interfere with the oviposition behavior. Duration of the eggs stage is often 10-12 days, but eggs may persist for over 100 days under cool conditions. When cultured at 18°, 22.5°, and 27°C, egg development requires an average of 15, 9.8, and 7.6 days, respectively. Maximum fecundity varies according to temperature; optimal temperature is 20-25°C regardless of host plant. When feeding on eggplant, greenhouse whitefly produces over 500 eggs, and on cucumber and tomato about 175-200 eggs (Drost et al, 1998).
  2. The newly hatched whitefly nymph is flattened, oval in outline, and bears functional legs and antennae. The perimeter is equipped with waxy filaments. The first instar measures about 0.3 mm long. It is translucent, usually appearing to be pale-green but with red eyes. After crawling one centimeter or so from the egg, it settles to feed and molt. Development of the first instar requires 6.5,4.2, and 2.9 days, respectively, when cultured at 18°, 22.5°, and 27°C. The second and third nymphal stages are similar in form and larger in size, though the legs and antennae become reduced and nonfunctional. They measure about 0.38 and 0.52 mm long, respectively. Duration of the second instar requires about 4.3, 3.2, and 1.9 days, whereas third instars require 4.5,3.2, and 2.5 days, respectively when cultured at 18°, 22.5°, and 27°C. The fourth nym-

Greenhouse whitefly egg.

Greenhouse whitefly egg.

phal stage, which is usually called the "pupa," differs in appearance from the preceding stages. The fourth instar measures about 0.75 mm long, is thicker and more opaque in appearance, and is equipped with long waxy filaments. The pupal stage actually consists of the fourth nymphal instar period, which is a period of feeding, plus the period of pupation, which is a time of transformation to the adult stage. Thus, pupation occurs within the cuticle of the fourth instar. Duration of the fourth instar and pupal periods are 8.7 and 5.9, 5.9 and 4.0, and 4.5 and 2.8 days, respectively, when cultured at 18°, 22.5°, and 27°C.

The form of the pupa is used to distinguish among whitefly species, and can be used to separate greenhouse whitefly from the similar-appearing silverleaf whitefly, Bemisia argentifolii Bellows and Perring, and sweetpotato whitefly, B. tabaci (Grennadius). Greenhouse whitefly is straight-sided when viewed laterally, ovoid, and lacks a distinct groove near the anal end of the body. In contrast, the Bemisia spp. are oblique-sided, irregularly oval, and possess a distinct groove in the anal region.

Individuals of greenhouse whitefly that develop on lightly or moderately pubescent leaves tend to be relatively large and to have four pairs of well-developed dorsal waxy filaments. In contrast, whiteflies developing on densely pubescent leaves tend to be smaller, and they bear more than four pairs of dorsal filaments. These morphological variations are not entirely consistent, and have led to considerable taxonomic confusion.

Adult. The adults are small, measuring 1.0-2.0 mm long. They are white, with the color derived from the presence of white waxy or mealy material, and have reddish eyes. They bear four wings, with the hind wings nearly as long as the forewings. The antennae are evident. In general form, viewed from above, this insect is triangular in shape, because the distal por-

Greenhouse whitefly nymph.
Life Cycle Greenhouse Whitefly Martin
Greenhouse whitefly "pupa" (top view).
Aleyrodidae Wing
Greenhouse whitefly "pupa" (side view).

tions of the wings are wider than the basal sections. The wings are held horizontally when at rest; this characteristic is useful for distinguishing this species from the similar-appearing silverleaf whitefly and sweetpotato whitefly, which hold their wings angled or roof-like when at rest. Mating may occur repeatedly, though females can also produce eggs without mating.

Detailed description of greenhouse whitefly was provided by Hargreaves (1914) and life history characteristics by van Roermund and van Lenteren (1992). Madueke and Coaker (1984) described temperature relations. A key to the common whitefly pests was published by Martin (1987).

Aleyrodidae
Adult greenhouse whitefly.

Damage

The adult and nymphal whiteflies use their piercing-sucking mouthparts to feed on the phloem of host plants. This results in direct damage, resulting in localized spotting, yellowing, or leaf drop. Under heavy feeding pressure wilting and severe growth reduction may occur. Whiteflies also secrete a large amount of sugary honeydew, which coats the plants with sticky material, and must be removed from fruit before it is marketed. The honeydew also provides a substrate for growth of sooty mold—a black fungus that interferes with the photosynthesis and transpiration of plants.

Greenhouse whitefly is, as its common name suggests, primarily a pest in greenhouses, and causes a serious limitation to the production of vegetables grown in such structures. However, it can also be a field pest, often in warmer climates but also in cool climates when seedlings contaminated with whiteflies are transplanted into the field. For example, in Hawaii field-grown tomatoes suffered a 5% reduction in fruit weight with just 0.7 whiteflies per square centimeter of leaf tissue, and a 5% reduction in grade-A fruit owing to contamination with honeydew at densities of about 8.3 whiteflies per square centimeter of leaf tissue (Johnson et al., 1992).

Greenhouse whitefly is capable of transmitting viruses to plants, but is not considered to be a serious vector, particularly relative to the Bemisia spp. However, greenhouse whitefly transmits beet pseudo-yellow virus to cucumber in greenhouse culture.

Management

  1. Although whitefly nymphs and adults can be detected readily by visual examination of foliage, most monitoring systems take advantage of the attraction of adults to yellow, and use yellow sticky traps to capture flying insects. Sticky cards or ribbons are suspended at about the height of the crop for optimal monitoring. Traps must be placed close to plants or to the ground, or population densities will be underestimated (Gillespie and Quiring, 1992). An important aspect is to have traps dispersed widely, because whitefly distribution is not uniform within a crop. Whitefly flight peaks at about noon, but under greenhouse conditions it is independent of temperature if the basal flight temperature of 16-17° C is exceeded (Liu et al., 1994). Heinz et al. (1992) suggested that sticky card traps could be subsampled by counting only the central vertical portion, thereby reducing labor and time for population estimation.
  2. Applications of insecticides are often made to minimize the effects of whitefly feeding on crops in greenhouses. Greenhouse whitefly feeds on the lower surface of foliage and is sessile throughout most of its life, habits that minimize contact with insecticides, and resulting in frequent applications and effectiveness mostly against the adult stage. In greenhouse culture, application intervals of only 4-5 days are common, and sytemic insecticides are often used to increase the likelihood of insect contact with toxins. Thus, whitefly resistance to nearly all classes of insecticides is known, and rotation of insecticide classes is encouraged. Mixtures of insecticides are often used, which is indicative of high levels of resistance among whiteflies to insecticides. Field populations of greenhouse whitefly invariably are derived from greenhouse populations, and possess similar resistance to many insecticides. Applications of petroleum oils (Larew and Locke, 1990) and biological control agents (van Lenteren et al., 1996) help to avoid difficulties with insecticide resistance.

Some insecticidal materials can be integrated into biologically based whitefly management systems. Selective materials that affect only adult and nymphal whiteflies, insect growth regulators, and insecticidal soaps are somewhat compatible with parasitoids and can be used when parasitoids are failing (Dowell, 1990).

Cultural Practices. Few cultural practices are available, but disruption of the whitefly population with host-free periods is important. Continuous culture of plants allows whiteflies to move from older to younger plants. Similarly, weeds may allow white-flies to bridge crop-free periods, and should be eliminated. Culture of plants over white reflective mulch also decreases whitefly densities (Kelly et al., 1989). Yellow sticky traps can be hung in greenhouses to capture adult whiteflies, thereby reducing whitefly density.

Biological Control. Seasonal inoculative release of the parasitoid Encarsia formosa Gahan into crops infested with greenhouse whitefly has been used extensively for suppression of whiteflies on greenhouse-grown vegetable crops. Excellent suppression of whiteflies is attainable, but on host plants such as cucumber and eggplant, which are very favorable for whitefly reproduction and have hairy leaves that interfere with parasitoid searching, frequent releases must be made. Alternatively, cucumber varieties with reduced trichome density have been developed which favor parasitism. Another critical factor is temperature, because low greenhouse temperatures are more suitable for whitefly activity than parasitoid activity. Daytime temperature of about 24° C seems to be optimal; temperature of 18°C or less suppresses parasitoid searching. A cold-tolerant Encarsia strain that is active at 13-17° C has also been used to overcome this temperature problem. Interference from pesticides can markedly affect parasitoid survival, so other pests such as mites must be managed biologically also. Lastly, release rates are important because if too many parasitoids are released the host whiteflies are driven nearly to extinction, leading to disappearance of the parasitoids; this is most likely to occur in small greenhouses. Alternatively, parasitoid releases can be made throughout the season, irrespective of whitefly presence. Use of Encarsia formosa for whitefly suppression was reviewed by van Lenteren et al. (1996). Although the protocols and technologies for whitefly management using E. formosa have been perfected for use in greenhouses, management under outdoor conditions awaits further research.

The fungus Verticillium lecanii is sometimes used commercially in Europe for whitefly and thrips suppression in greenhouses, though its success is strongly affected by humidity. Where humidity can be raised to a high level, epizootics can be induced in 1-2 weeks. Both young and adult stages are susceptible to infection.

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Responses

  • kaija
    Can Aleyrodidae survive in the winter?
    7 years ago

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