Expected benefit of treating crape myrtles for new scale

Two ‘Natchez’ crape myrtles demonstrate the potential impact of bark scale on the size and number of blooms in early summer. The tree on the left was treated seven weeks earlier with the insecticide dinotefuran, the one on the right was left untreated. Note the smaller blooms and mold-covered bark on the untreated tree. Click on the image for a closer view. Photo by Jim Robbins, U of Arkansas.

Earlier this year I posted some information about a new scale pest that is attacking crape myrtle trees in Texas and other parts of the south. It is called the crape myrtle bark scale, Eriococcus lagerstroemiae, and its range continues to expand. This year the scale has jumped from north Texas to College Station and, more recently, Sugarland in the Houston area.

We do not see this scale killing crape myrtle; but like many sap-feeding scale insects, these little scales can stress and reduce the appearance of the trees. They also produce large amounts of sticky "honeydew" that can coat the leaves and anything under the tree (including freshly washed cars). Thanks to Drs. John Hopkins and Jim Robbins of the University of Arkansas Cooperative Extension Service, we can now show you what we believe is likely to be another impact of these scales on trees--namely, smaller flower clusters and reduced blooming.

The above picture was taken of two trees at a church in Little Rock, Arkansas.  The tree on the left was treated with Zylam^® Systemic Insecticide on the 28th of  May, and the one on the right was left untreated.  Zylam^® (active ingredient dinotefuran) was applied as a drench in 5 gallons of water between the trunk and a circle three feet away from the trunk.  The picture was taken seven weeks after the treatment was made. Note that this picture is not the same as a scientific trial, which would involve more trees to ensure that the differences seen here were not accidental.  Nevertheless, according to Dr. Hopkins, scale numbers and honeydew were noticeably less on the treated tree.  And there was a difference in the average bloom size between the treated and untreated tree, with blooms being noticeably larger on the treated tree.

John estimated that the cost to treat the tree on the left with Zylam would be approximately $39--not cheap, especially with multiple trees to treat.  But at least you, and your customer, can see what the expected benefit from a tree treatment might look like.  For a consumer-oriented discussion of the scale, clip and use this link: http://citybugs.tamu.edu/2014/08/14/crape-myrtle-bark-scale-reduces-bloom/

What about the bees?
To date the most promising treatments for crape myrtle bark scale have been the neonicotinoid insecticides,  Readers of this blog should know about the growing concern about the impact of soil-applied neonicotinoid insecticides on honey bee and pollinator health.  So should we be using these products on a flowering tree like crape myrtle? Although date on pollination rates on crape myrtle seem to be lacking, these trees do not appear to be highly attractive to bees (entomophilic). Currently I don't believe that a properly applied soil insecticide (following label directions) will have any significant impact on foraging bees.  But if anything changes in that formula, I'll be sure to let you know. And be sure to read the labels on these neonicotinoid products carefully.  New, pollinator-friendly labels are coming to the market this year.  In the meantime, Extension will continue to look for less susceptible varieties of crape myrtle and possibly safer, less costly treatments for this scale.

New pest on crape myrtle

Texans (and many other southerners) love their crape myrtles! And why not? Crape myrtle (Lagerstroemia species) is one of the few trees that bear colorful flower displays through much of the summer, come in a variety of stunning colors, is easy to grow, and until now has been relatively pest free. Unfortunately, the pest-free reputation is changing with the advent of a new exotic scale pest.

Crape myrtle trees infested with CMBS in Richardson, TX.  Black sooty mold
 deposits on the branches and trunks of infested trees are often the first
sign of an infestation.  Note the white, scale encrusted upper branches. 

In 2004 I received a call from a local lawn maintenance company that was having a difficult time controlling an unusual scale on crape myrtle at a commercial property in Richardson, TX. After examining it I thought this scale might be something new, and sent samples of the insect off to experts for identification. Under the microscope it appeared to be a Eriococcid scale called azalea bark scale, Eriococcus azaleae.  Azalea bark scale is a common pest of azaleas in the eastern U.S.,  but had never been recorded as occurring on crape mrytle before. A suggestion by a retired USDA scale specialist led us to change our minds and believe that the scale might actually be a closely related scale known as Eriococcus lagerstroemia. This scale is an important pest of crape myrtles in China, Japan and Korea.  The difficulty was that no one knew how to tell these two scales apart.  And because the only place that the scale occurred was in the Dallas and Fort worth metroplex, it was difficult to interest others in investigating the problem without some financial backing.

After several years of slow spread through several north Texas counties, last year the scale made its move. In 2013 the scale appeared in several locations in Arkansas, Louisiana and Tennesee.  As a result, this winter researchers from the University of Arkansas and the USDA's Animal and Plant Health Inspection service agreed to lend a hand and take a closer look at these tiny insects.  

Molecular and new microscopic examinations now appear to confirm that our new scale is likely to be a recent import from Asia, probably Eriococcus lagerstroemia.  Last year Dr. Mengmeng Gu, Extension Horticulture specialist with Texas A&M AgriLife, had a chance to see these scales in their native Asian environment in a trip to China. The messy crape myrtles she saw there stimulated her interest in the scale, and led us to coauthor two new publications with the University of Arkansas. Both Texas and Arkansas now have fact sheets on this pest.

Crape myrtle branch with a heavy infestation of scale.

A scale-encrusted crape myrtle branch. Note
the pink blood from crushed scales where a
finger has been dragged across the infested
stem. Photo by M. Gu.

Many pest control professionals provide IPM services for landscape and turf pests, so it will be important to be able to recognize and identify it when you see it. Crape myrtle bark scale (CMBS) is a small white scale that bleeds pink when crushed. No other insects found on crape myrtle share these characters.  They can be found on the trunks branches and twigs of crape myrtle. Don't be surprised to find this scale in Arkansas, Louisiana, southern Oklahoma, Germantown, Tennessee, and possibly South Carolina. In Texas the Dallas/Fort Worth area is widely infested and it may also be found in Tyler, Longview, and College Station. Based on the current rapid spread, within the next ten years this scale will likely be common in many communities throughout the South.

If you are interested in learning more about CMBS, Dr. Gu asked me to talk about this pest this week in a Webinar, which is now posted on YouTube. In the webinar I discuss the appearance and damage caused by CMBS, how it is spread, and what is known about control.

The best control methods that we currently have involve soil injections of neonictinoid insecticides. Control recommendations are also listed here and in the fact sheets mentioned above. If you encounter this scale in areas outside Texas, or in areas of Texas I did not list, I would be interested in knowing about it. Leave comment on this site or drop me an email.  We hope to eventually have a website where sightings can be more easily reported.