Thursday, August 14, 2014

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:

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.

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