Uvalde, Texas
September 6, 2005
In the past, they've blanketed windshields,
caused farm equipment to overheat and damaged various crops. Are
they bio-terrorists?
Not exactly. They're silverleaf whiteflies, and this year there
is a sizeable increase in their Central Texas population.
‘We've seen a significant increase in the whitefly population
already this year, especially in cotton," said Noel Troxclair,
Texas Cooperative Extension entomologist in Uvalde.
"And we are expecting more people – from farmers to homeowners –
to call us about these insects as the year progresses," he said.
Troxclair noted that large quantities of whiteflies have been
seen primarily in the cotton-producing counties of Central
Texas, including Uvalde, Medina, Frio, Zavala, Atascosa, Bexar
and Wilson.
About 1,200 species of whiteflies exist worldwide, but the
silverleaf whitefly is the most economically significant species
in Texas. This is because they use several crops as "hosts" and
feed on a variety of ornamental plants.
"The whiteflies suck the sap from the leaves and deposit a
sticky substance we call ‘honeydew' on them," Troxclair said. "A
sooty mold then grows on the honeydew, reducing the plant's
ability to photosynthesize."
Large numbers of silverleaf whiteflies can affect squash color
and growth or even kill the plant, he said. They can also stunt
the growth of tomatoes, peppers and similar crops, as well as
affect the quality and yield of cotton.
"If whiteflies appear late in the cotton-growing season, their
honeydew can cause ‘sticky cotton,' which causes problems during
the ginning process," he said. "This reduces the value of the
cotton."
Whiteflies can also transmit viruses to plants and are carriers
for broad mites, which can have an adverse effect on tomato and
pepper quality, he added.
Troxclair said the last notable silverleaf whitefly infestation
in the region occurred in the early 1990s.
"There were ‘clouds of whiteflies' in the lower Rio Grande
Valley causing problems for people in both urban and rural
areas," he said. "They were so thick that it was dangerous to
drive the highways, and farmers had to cover their mouths as
they worked."
The whiteflies then migrated north into Central Texas crops.
"There hasn't been a significant, widespread whitefly problem in
Central Texas from then until this year," Troxclair said. "We're
keeping an eye on them and want to make sure they don't grow to
levels that could have an impact on commercial vegetable
production."
Whiteflies are also troublesome to homeowners.
"The silverleaf whitefly feeds on a large variety of ornamental
plants including lantana, hibiscus, verbena, Gerber daisies,
poinsettia and Mandevilla," says Carlos Bográn, Extension
entomology specialist at Texas A&M University in College
Station. "And they'll attack the same sort of plants -- tomato,
squash, cucumber and the like -- in home gardens as they would
in large commercial fields."
Increases in silverleaf whitefly populations may be due to
several factors, he added.
"The causes include agronomic practices, such as cotton
defoliation in surrounding areas, insecticide applications
against other insects that help control whiteflies and favorable
environmental conditions for the whiteflies," he said. Bográn
said several insecticides are registered and available for
silverleaf whitefly control in commercial crops and home
landscapes and vegetable gardens, but the decision to treat also
depends on a various factors.
"While large populations of this insect can cause significant
damage to crops, growers need to consider the cost of control
versus its benefits,"
he said. "They should also consider crop value, estimated yield
and commodity price against the cost of whitefly control."
The sooty mold produced on silverleaf whitefly honeydew can
discolor ornamentals, usually causing leaves to yellow, as well
as stunt plant growth.
Bográn said ornamental plants can generally survive moderate
whitefly infestations, but homeowners may want to treat for
them, particularly if the health of an expensive plant is at
stake.
For more information about whiteflies, go to
http://insects.tamu.edu/
.
For Extension publications on whitefly biology and management,
visit http://tcebookstore.org |