What explains the drop in bark scale on North Texas crape myrtles this summer?

They’re arguably the premier summer-flowering shrubs in America, or at least certainly in the South. Crape myrtles bloom three or four times from June into September (provided we don’t butcher them by senseless “topping” in winter). They come in mature heights ranging from three to 33 feet and with colors that include exciting reds, soothing pinks, lovely lavenders, rich purples and sparkling whites. Add in their colorful bark and frequently bright fall colors and you have a plant that needs to be in everyone’s landscape.

However, a fly flew into the ointment back in 2004. Somehow a new (to America) insect showed up in Richardson, and it made itself right at home on our crape myrtles. It looked like an overgrown mealybug, and it had us all puzzled. Initially the entomologists thought it might be a mutation of a scale that we’d known from azaleas, but gradually that was ruled out. The University of Florida, I found out, is a center for scale identification, as is the Smithsonian Institute, and it was critical whether this was a mutation of a previously known domestic type of scale insect or something newly imported.

Then along came 2007. It was a very wet, cool spring. Apparently, that was very much to the scales’ liking, because they proliferated all across Dallas and Collin counties. Gardeners in Tarrant County still didn’t know much about them. By June that year they had inundated crape myrtles in the eastern half of the Metroplex. They plastered themselves to the twigs, stems and leaves of most of the plants, and as they fed, they secreted a sticky “honeydew” sap that coated leaves, stems and trunks of the plants.

Within a few weeks a black fungus started to grow in that honeydew substrate. It’s the same process you see beneath oaks, elms and other large shade trees that have summertime aphids that give off honeydew drippage.

To this day gardeners notice the “black mold” much more than they do the bark scale or honeydew. In reality, to get rid of the sooty mold you must get rid of the honeydew, and to get rid of the honeydew you must first get rid of the insects that let it get started. But let’s back up just a bit.

Crape myrtle bark scale probably came into America on a bootlegged plant. It’s native to China. Still not knowing what it was, horticulturists from Texas saw it and recognized the unknown pest while on an educational excursion to China.

From its beginnings in Richardson, it started to spread slowly. First it moved to nurseries and growers in East Texas, then west to Fort Worth and beyond. Subsequently it has moved east to the Atlantic Coast and north to Virginia and Delaware. Essentially, except for Arizona and California, wherever crape myrtles are grown the bark scale has now been observed.

This pest spreads in several different ways. Obviously, when short distances are involved, it can move via plant-to-plant contact. It can also be carried on maintenance tools and even hanging onto birds’ feet. The sudden spread of crape myrtle bark scale across the South has probably been due to transport of infected plants from one nursery to another, almost assuredly unintentionally.

I am far from an accomplished entomologist. However, one thing that I’ve observed over the years is that populations of certain pests rise and fall with some regularity. A new pest can come along and seem to mark the beginning of the end of a popular species, only to disappear off the radar a few years later.

That’s what I’ve been seeing the past several years, notably so far this year with crape myrtle bark scale. After church many Sundays my wife and I park in front of a Natchez crape myrtle that has been covered with the scale in past summers. This past Sunday, no scale at all. Not one speck. Nor have I seen many plants with scales in major plantings with which I’m working where we are assembling the entire “World Collection” (all known varieties) of crape myrtles for side-by-side comparisons. That’s in a park and in many of the street medians in McKinney, and so far this summer, almost no bark scale to be found.

Natural predators may be at work. An unusual species of lady beetle showed up in the Metroplex back in 2007. Entomologists referred to it as the “twice-stabbed” ladybug because of its two orange-red spots highlighting its shiny black wings. The nymphs of that beneficial insect would gobble up bark scales like tiny vacuum cleaners. They are terrifically efficient, and until bark scale showed up many of us didn’t even know that ladybug existed.

Exhaustive research done by Drs. Mike Merchant (now retired) and Mengmeng Gu, and then updated by Erfan Vafaie of Texas A&M found that a soil drench of Imidacloprid made in April as buds are breaking will be the best means of preventing crape myrtle bark scale. Trunk sprays, to be effective, must be made when crawlers are active (mid-April through early May).

Finally, a couple of important notes. Crape myrtle bark scale and sooty mold are unsightly and disfiguring, but they do not seriously weaken crape myrtles. They certainly will not kill an otherwise healthy plant. And crape myrtle bark sloughs off as the trunk enlarges. Sooty mold will be falling to the ground with it in the process.