Exotic pest wreaks havoc on crape myrtles

Crape myrtle bark scale show up as felt-like white or gray encrustations. Photo by Kathie Rowell.

Crape myrtle bark scale shows up as felt-like white or gray encrustations. Photo by Kathie Rowell.

Are your crape myrtles under attack?

So far, mine are free of crape myrtle bark scale, a new foreign pest that is disfiguring our classic summer-flowering tree.

The tiny pest was first identified in the Dallas area about 10 years ago and is believed to have entered the country from Asia. Since then, it’s been slowly making its way across the South, arriving in Shreveport-Bossier City about four years ago. Infestations have also been verified in Oklahoma, Arkansas, Tennessee and Georgia.

“Ground-zero for crape myrtle bark scale in Louisiana is the Shreveport and Bossier City areas,” said LSU AgCenter horticulturist Allen Owings. It’s also been found in Minden, Monroe, Rayville, Houma and possibly Alexandria.

Movement has been slow because female scales don’t fly and crawl only in the nymph stage, rarely very far from their host tree. Humans are primarily responsible for their spread, either by planting infested trees or transporting the pest from one tree to another on lawn equipment such as pruners.

AgCenter forester Ricky Kilpatrick, based in Bossier City, says he gets calls almost daily from homeowners seeking information about the pest, which sucks nutrients from trees. “It’s popping up everywhere,” he said.

And it’s likely to get worse.

“I think it’s going to be bad because there’s still a lot of unaffected trees that are prime bait,” Kilpatrick said. “Once they are in the vicinity, they spread everywhere.”

LSU AgCenter horticulturist Allen Owings calls crape myrtle bark scale “a troubling and ever-expanding exotic pest issue in Louisiana and surrounding states. … Home gardeners, master gardeners, retail garden center employees, professional horticulturists and landscapers need to be on high alert and should be aggressively monitoring for this insect around the state.”

Symptoms include white or gray felt-like encrustations and extensive sooty mold on leaves, limbs and trunks. Heavy infestations can result in trees so black they appear to have been burned. Both the number and size of blooms on infested trees will be decreased.

Jimmy Dillard, LSU-Shreveport assistant director of facility services, noticed signs of the pest in at least 60 percent of the approximately 430 crape myrtles on campus this spring.

“Everybody was saying what’s going on with the crape myrtles, what’s wrong with the crape myrtles,” he said of the campus’ signature trees.

After identifying the problem with help from LSU-S entomology professor Beverly Burden’s contacts in Texas, treatment was started. So far, the 200 trees near the front of the campus have been treated and are showing signs of improvement. Other trees are in line for treatment.

“About a month ago, there were little white bugs everywhere. Now there are not so many,” Dillard said.

Jimmy Dillard checks out infested trees at LSU-Shreveport. Blackened limbs and trunks are one of the symptoms of crape myrtle bark scale. Photo by Kathie Rowell.

Jimmy Dillard checks out infested trees at LSU-Shreveport. Blackened limbs and trunks are one of the symptoms of crape myrtle bark scale. Photo by Kathie Rowell.

Trees that were heavily infested still show the signs through unsightly blackened bark that is beginning to split and slough off.

South Highlands resident Lynn Eddy noticed the pest on her crape myrtles about a month ago, but didn’t know what it was until recently. She is still deciding how to treat them.

Choices are limited, according to LSU AgCenter extension entomologist Dennis Ring.

Systemic products containing imidacloprid or dinotefuran and contact pesticides with acephate or imidacloprid have been shown to be somewhat effective. Systemic products are soluble in water so they can be taken up into the plant’s vascular system and can last as long as 12 months. Contact pesticides are applied directly to infestations.

Organic gardening options are even more limited.

“I would be skeptical of an organic method to hold the number of these scales to an acceptable level,” Ring said. “You’re really stuck with mechanical methods, such as a brush and soapy water and if the tree is very tall that’s not an option.”

What the future holds is still unclear.

“This is a new insect and it’s an ornamental insect and there’s not a lot of research on it,” Ring said. “Some entomologists are doing some work on it, but it’s very different from, say, soybeans, where there are acres and acres of it. This is on an ornamental plant in our landscape. The plant is important and it produces very nice flowers, but it’s not food. Certainly in this time period when we are reducing inputs, it’s going to be difficult to get anything done on something like this.”

And that could be bad news for our Southern summer landscapes.

“People who have had consistent CMBS issues over the years are tiring of fighting the problem,” Owings said. “This pest seriously threatens future consumer acceptability of our most popular summer flowering tree.”

Ring agrees.

“We can’t eradicate it,” he said. “We have to live with it and decide do I want this crape myrtle and, if I do, am I going to live with it looking crappy or treat it, knowing if I do it, I’m going to have to do it every year.”

Dr. Mike Merchant, a Texas AgriLife Extension urban entomologist in Dallas, says he knows of at least one Texas community that is considering abandoning plans to install crape myrtles in its medians because of the ongoing maintenance issue.

He’s not seeing homeowners eliminating trees from their landscapes yet.

“Like fire ants, you kind of get used to them after a while, but it doesn’t make the trees look any better.”

 

LSU AgCenter treatment recommendations:

  • Wash the trunk and reachable limbs of heavily infested plants with a soft brush and mild solution of dishwashing soap. The egg masses and female scales will be washed off, resulting in improved effectiveness of insecticides. Also, the black mold building up on the bark of infested trees will be removed by washing. The scales and sooty mold may be removed by using water pressure. Removing the loose bark is important because the protected areas where the scales hide are removed. This removes the areas that the scales may use for protection from unfavorable weather in winter.
  • Management of this insect has not been shown using horticultural oil. Applications of dormant oil in the winter may reduce the numbers of these scales. It is very important to thoroughly cover the tree.
  • Applications of systemic insecticides may be made to the plant root zone. Application should be made during May and July. Systemic insecticides include dinotefuran (Greenlight Tree and Shrub Insect Control with Safari) and imidacloprid (Merit or Bayer Advanced Garden Tree and Shrub Insect Control). Allow several weeks for these insecticides to work because they must spread through the plant. Time insecticide applications by examining the cases containing the scale eggs to determine when the nymphs emerge from the eggs. The time that the nymphs emerge depends on weather conditions and plant locations.
  • Infested branches may be treated with applications to the terminals using Orthene (acephate) or Merit (imidaclorpid) in a slurry (4-to-1 ratio of wettable powders to water). Wipe clean the terminals or areas of branches above the infestation. Apply the slurry around the branches or terminals about 6 inches wide. The phloem tissue directly takes up the insecticide, and the materials move through the plant.

–Source: LSU AgCenter

 

 

More info:

Crape Myrtle Bark Scale Fact Sheet: http://www.lsuagcenter.com/MCMS/RelatedFiles/%7B84F4324A-45E7-4D09-8F2A-A51D127149F5%7D/Crape-Myrtle-Bark-Scale-Fact-Sheet.pdf

Crape Myrtle Bark Scale: A New Exotic Pest: http://www.agrilifebookstore.org/product-p/eht-049.htm

 

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