Silverleaf Whitefly
Bemisia argentifolii Bellows and Perring, Aleyrodidae, HOMOPTERA


DESCRIPTION

Adult - The silverleaf whitefly is slightly smaller (about 0.96 mm in the female and 0.82 mm in the male) and slightly yellow than other whiteflies. The head is broad at the antennae and narrow toward the moth parts. The wings are held roof-like at about a 45 degree angle, whereas other whiteflies usually hold the wings nearly flat over the body. Hence the silverleaf whitefly appears more slender than other common whiteflies.

Egg - The eggs are inserted on end in the undersides of new leaves. The eggs are whitish to light beige with the apex tending to be slightly darker.

Nymph - The nymphal stage appear glassy to opaque yellowish and may or may not have dorsal spines, depending on leaf characteristics. The body is flattened and scale-like with the margin relatively near the leaf surface. There is not a marginal palisade of waxy spines.

Pupa - The pupa or fourth nymphal instar will be somewhat darker beigeish-yellow and opaque. Pupae are relatively more plump compared to previous nymphal stages. The apex of anterior and caudal spiracular furrows have a small amount of white wax deposits. The caudal setae are prominent, and the caudal end is somewhat acute. Dorsal spines are present when the host leaf is hairy and absent when the host leaf is smooth.


BIOLOGY

Distribution - Silverleaf whitefly occurs around the world in tropical and subtropical areas and in greenhouses in temperate areas. It has been reported from all southeastern states. Additionally, it has been reported from Arizona, California, District of Columbia, Maryland, and Texas.

Host Plants - The number of host plants is extensive; however, the plant families most often reported are Leguminosae, Compositae, Malvaceae, Solanaceae, Euphorbiaceae, Convolvulaceae, Cucurbitaceae, Labiatae, Verbenaceae, Cruciferae, Amaranthaceae, Rosaceae, and Moraceae. The most frequently reported hosts in the southeastern U.S. are poinsettia, gerbera daisy, and tomato.

Damage - Direct damage is caused by the removal of sap, and indirect damage as a disease vector. The silverleaf whitefly is a vector for several important virus diseases of lettuce and melons in the southwestern U.S. Both the adult and nymphal stages contribute to direct damage. Chlorotic spots sometimes appear at the feeding sites on leaves, and heavy infestations cause leaf wilting. The excretion of honeydew and the subsequent development of sooty mold fungi also may reduce photosynthesis and other physiological functions of the plant. Even though the silverleaf whitefly is considered an economic pest, little information is known about the damage it causes or the economic thresholds.

Life History - Developmental times from egg deposition to adult emergence appears to be primarily controlled by temperature, humidity, and host plant. These times will vary from 16 to 38 days depending on these factors. The number of eggs laid by each female over her lifetime varies considerably, but appears to be around 80 to 100. There have been reports (in Israel) that repeated applications of insecticides have produced a highly fecund (300 eggs/females) strain of silverleaf whitefly. Apparently at temperatures above 36 degrees C, eggs fail to hatch. "Crawlers" hatch from the eggs and crawl about until they insert thread-like mouthparts into the underside of the leaf to feed. They tuck their legs and antennae underneath and settle down closely to the leaf surface.

Crawlers molt into scale-like nymphs that also suck out sap. Nymphs molt a second and third time. The fourth stage eventually becomes a nonfeeding pupa. The adult whitefly develops within the pupa. Adults emerge from the pupa through a T-shaped slit about a month from the time the egg was laid. Females live about 2 weeks.


CONTROL

Control of silverleaf whiteflies is difficult because the eggs and older immature forms are resistant to many aerosol and insecticide sprays (in addition, the adults are extremely resistant to dry pesticide residue). For good control, the pesticide mixture must be directed to the lower leaf surface where all stages of the whiteflies naturally occur. One must make regular applications of pesticides to control crawlers and second stage nymphs until the last of a whole generation of immature whiteflies has hatched. However, some of the pyrethroid pesticides are somewhat more effective and need not be applied as often. Neem seed extract is not as acutely toxic as some of the synthetic pesticides, but has the advantage of being toxic to young nymphs, inhibiting growth and development of older nymphs, and reducing oviposition by adults. For specific chemical control recommendations, see Cooperative Extension publications on ornamental plant and vegetable pests.

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