| There are over 1200 known species of whiteflies,
most of which feed on only one or a few species of
plants each. Some are pests on valuable crops such
as the sweet potato whitefly (Bemisia tabaci).
Others, like the greenhouse whitefly (Trialeurodes
vaporariorum), feed on a broad range of herbaceous
plants both inside the greenhouse and outside in the
garden. Some of the host plants that the greenhouse
whitefly attacks are the tomato, fuchsia, lantana,
and coleus.
The silverleaf whitefly (Bemisia argentifolii) has been
found on as many as 500 hosts such as canna lilies,
bearded iris, crepe myrtle, lantana, petunia, and rose.
Whiteflies are one of the most difficult insect pests
to control. They are not true flies but are more
closely related to aphids, mealybugs, and scale,
which are also sap-sucking insects. In some parts of
the country, some species of whitefly can transmit
several plant viruses.
Symptoms and Diagnosis
Identification of the whitefly is easy as a white cloud
of insects rise from foliage when disturbed. The
adults are about 1/16 to 1/10 inch in length, wedgeshaped,
and appear powdery. Infested leaves may
show little if any symptoms or may be mottled and
yellow. Leaves will also become coated with a sticky
substance called honeydew, which is secreted by
feeding whitefly nymphs and adults. A black, sooty
mold may sometimes grow on the honeydew. Sooty
mold does not damage the leaf. The feeding,
however, removes nutrients from the plant, which
results in stunting, poor growth, defoliation, reduced
yields, and sometimes death. On certain plants the
silverleaf whitefly causes specific damage symptoms
such as silvering of leaves on squash, irregular
ripening in tomato, whitestalk in broccoli and
cauliflower, light root in carrots, and white stem
in poinsettia.
Life Cycle
The whitefly spends its winters in weeds and
ornamental plants, migrating to crops and gardens in
spring and summer. Once temperatures warm up in
the summer, populations can build rapidly with the
highest populations probably occurring in late
summer. They lay their tiny eggs on the undersides
of leaves. Adult females usually lay between 200 to
400 eggs. Sometimes the eggs are deposited in a
circular pattern (not true in all species) in groups of
30 to 40 as the female will often keep her mouth
part in the plant to feed while moving in a circle to
deposit eggs. Within about a week the eggs hatch
into flattened nymphs, called crawlers that wander
about the plant. Soon, they insert their mouth part
and begin to feed. The first stage has legs and
antennae, but these are lost after the first molt and
the flattened, oval-shaped larvae stay fixed at one
feeding site. The last part of the fourth instar is the
pupa. Adults emerge from the pupa through a Tshaped
slit and soon mate and reproduce. The adults
live about one month. Within a population, all life
stages are present and generations
often overlap.
Integrated Pest Management Strategies
1. Inspection. Inspect all new plants planted in the
garden for whiteflies. Inspect regularly and
remove by hand older leaves that are heavily
infested with whiteflies in a non-mobile
nymphal stage.
2. Biological control. Whiteflies have many natural
enemies in the garden setting including spiders,
lady beetles, and lacewings. However, these
predators are not effective if there is a frequent
use of insecticides.
3. Mechanical control. The use of traps can be very
helpful in controlling light infestations. Make
yellow sticky traps about 12 x 6 inches and coat
them with a sticky substance such as Tanglefoot,
petroleum jelly, or heavy grade motor oil. Hang
the traps vertically near the affected plants. The
adults are attracted to yellow. Traps are also
commercially available. Use one trap per plant.
4. Chemical control. Whiteflies have developed
resistance to some chemicals. Because of this
resistance, a certain product may work well in
one area but not in another. Resistance may be
delayed by alternating the types of chemicals
used. In addition, the egg and non-feeding pupa
stages are generally not as susceptible to
insecticides as are the adults and nymphs.
Consequently, eradication of a whitefly
population usually requires four to five
applications of a registered insecticide at five to
seven day intervals. Be sure the applications are
made to the lower leaf surface, and apply as soon
as whiteflies are detected. Do not wait until
populations become severe.
Before using any insecticide for whitefly control,
make sure that the site and target plant are both
listed on the label. Follow label directions
carefully.
5. Chemicals. Some of the over-the-counter
chemicals which could be used are malathion,
imidicloprid, permethrin, or pyrethrins. Insecticidal soap,
petroleum oil, and insect growth regulators can
also be effective. |
