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