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Description
The adult greenhouse whitefly is about 1/16th in. long, with four
wings and a yellowish body. The wings have the appearance of having
been dusted with a very fine white material. Both males and females
are capable of flying. The wings are held roof-like over the body
and are more or less parallel to the leaf surface. Newly emerged
adults may be pale green or yellow, but soon become the more familiar
white. Eggs are laid on short stalks in a circular or crescent-shaped
pattern attached to the lower leaf surface. The eggs are spindled
shaped and creamy white to yellow when young, but then turn gray
prior to hatching.
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The
nymphs or crawlers are usually pale green and scale-like in
appearance. They possess antennas and. have minute hairlike
projections along the edge of their bodies. The pupae are
cake-like in shape with raised sides approximately perpendicular
to the leaf surface. The periphery of the insect is surrounded
by a fringe of white waxy hairlike filaments or appendages.
Several pairs of filaments may project up from the upper surface
of the pupa.
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Click
on image for larger version
Figure 1. Grrenhouse Whitefly Nymphs, Pupae, and Adults
on Poinsettia
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Life
History
Whiteflies have piercing-sucking mouthparts with the adults very
active and easily disturbed. The immature stages are less mobile
and usually found on the undersides of the foliage. Adult whiteflies
feed on the lower leaf surfaces where they lay their eggs, but may
rest on other areas of the host plant. Egg laying commences 1 -
3 days after emergence and during the day. Eggs are attached to
the lower leaf surface with egg hatch occurring 7 - 10 days later.
This stage known as the "crawler" is the only mobile immature
stage. It moves only a short distance, inserts its needle-like mouthparts,
and begins to feed on plant sap.
For
approximately 7 days, the insect remains sessile, but goes through
3 molts. The second and third instars also feed on plant sap with
each stage being 3 - 4 days depending on temperature. The insect
then molts into a fourth stage nymph or resting stage called the
pupa. No feeding takes place during the pupa stage. Approximately
1 week later, the adult whitefly emerges from the pupa, usually
in the morning. Even though adults may fly up to 50 feet in a 24
hour period, most remain within 8 - 10 feet of their hatching site.
The entire whitefly lifecycle may average 32 days at temperatures
of 65° - 75° C and are favored by temperatures averaging
75° C in northern areas, the insect is not capable of overwintering,
but generations may be continuous and/or overlapping under greenhouse
conditions. The greenhouse whitefly is known to attack 249 plant
genera in 84 plant families.
Damage
Plants infested with whiteflies will lack vigor, wilt, turn yellow,
and may die. In addition, heavily infested plants will be coated
with a sticky material called honeydew which reduces the attractiveness
and salability of the plant. Heavy concentrations of honeydew will
promote the growth of a black sooty mold which interferes with photosynthesis.
Sooty mold may also interfere with production or harvest operations.
Most important, some species of whiteflies are known to vector plant
viruses.
Nonchemical
Control
Attempts at relying solely on chemical control have been only moderately
effective with insecticide resistance occurring in certain situations:
Most insecticides are not effective against the egg or pupal stages.
Good sanitation such as weed control in and around the production
area, monitoring, and initial plant inspections are essential for
an effective pest management program. Screening intake vents and
avoiding wearing yellow clothing will diminish the spread of adult
whiteflies. Insecticidal sprays of insect growth regulators will
help reduce immmature stages. The wasp parasite, Encarsia formosa
has proven to be effective against the immature stages.
Chemical
Control
Insecticides may be applied for control of immatures and adults.
Remember, very few insecticides are effective against the egg or
pupal stages.
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