What is wrong with my Crepe Myrtles?

Does it look something like this?

For the sake of the discussion, let’s ignore the horrible pruning that led to the knots and sprouting growth here and focus on the black sooty mold beginning to move down the stems from the area around the knots.

That’s Crepe Myrtle Bark Scale. Let’s take a closer look at exactly what is going on here.

Scale insects are a diverse species of sap sucking insects. There are about 8,000 different known species of scale insect and they come in a variety of sizes, colors, saps of preference, etc. Their name is derived from the scale, or shell-like, waxy coating that envelops them to the plant and allows them to cling on and do their damage.

They are further categorized by the type of shell they develop, either as a soft scale with a thin protective layer surrounding them or “armored” meaning they develop a dense scale shell that makes them harder to control. The protocol for handling scale insects predominantly depends on what kind of shell is protecting them.

So what do I do about Crepe Myrtle Bark Scale?

I’m glad you asked.

Crepe Myrtle Bark Scale will generally go through 2-4 generations per year, meaning over the course of the common active period for the insects during the year they can be born and reproduce up to four familial generations. If these scale insects were human, that would mean your grandmother was born in February, then birthed your Mom in late May, then you would be born in September and have a child before the end of the year. That’s how fast these insects can reproduce.

The treatment protocol depends on when they end up on your plant, or at least when you notice them.

If the infestation is caught early in the year, between March through June, the treatment protocol will involve a systemic soil drenching around the tree with a chemical insecticide called dinotefuran. When dinotefuran is applied to the soil, it turns the tree itself into an insect trap. A neonicotinoid, it embeds itself in the plant and as the insects continue to suck on the sap, it will attach itself to the bug’s central nervous system, causing any number of horrible sounding effects before killing the bug from the inside in a matter of hours. (Seriously, don’t Google what it does to the bug, it might make you sympathetic but will definitely help explain the reasoning behind applicator certification and licensing to ensure safety with chemicals like these.)

The systemic soil drench approach is good between (approx.) March after the foliage begins to bud and June, just before it gets too hot and the chemical loses efficacy (anything over 80 degrees and dinotefuran is a non-starter). Once the temperatures creep above 80, which they often do even earlier than June if you are in the Southeast where there are more Crepe Myrtles than people in some cities, treatment becomes a little more cumbersome. If you catch the infestation after the temperatures throw the systemic drench protocol out the window, it doesn’t mean there’s nothing you can do about the problem even if it means it’s going to take a bit more time to fully address it.

Later in the year, from those still comfortable days of the early Summer heat through the close of winter, you have a few options that can help curb the spread and growth of the infestation, but not really anything that is an overall solution at all. From Summer through Late Fall, the best solution is an insect growth regulator. It doesn’t do anything about the existing bugs, but it keeps their damage and the mess to a minimum. This is applied via a canopy spray. If you don’t catch it until you’ve moved into the colder months, the best option is an alternative canopy spray of oil to try to slow a particularly heavy infestation from spreading over the cold months. With either of these canopy spray options, the only solution is to use them to slow the spread until Spring when you can apply the systemic soil drench at the appropriate time for maximum efficacy.