Your crepe myrtles likely have a relatively new problem called crepe myrtle bark scale (CMBS). CMBS came over from Asia in a shipping container, possibly through Mexico, first arriving in Texas a few years ago. It has slowly moved East from there. The actual scale insects look like tiny pieces of puffed white rice. Though the male of the species is quite mobile, the female, which causes most of the problems, is relatively immobile, leaving us a little puzzled as to how the disease/pest spreads. Our thinking now is that it might spread by wind or birds but more likely spreads through mulch, bought in bulk from mulch yards, made of limbs and debris dumped at the mulch yard by landscapers, i.e. crepe myrtle bark pruning byproduct (think of the crepe murder that so many practice every year). Another possibility is that it gets on the clothing of landscapers themselves who travel from yard to yard. Regardless, it’s here!
The insect hurts the crepe myrtles in two ways. First, it feeds off of the tree, getting its nutrients from the outer cambium layer, depriving the tree of much needed nutrients, causing it to deteriorate. Second, it excretes a clear sticky substance, euphemistically called ‘honey dew,’ which becomes a perfect host for sooty mold, turning the tree black, coating the leaves and inhibiting photosynthesis, and often coating nearby plants too.
Currently, we are recommending a two-pronged approach, a systemic insecticide in the form of a soil drench, absorbed through the roots and moving throughout the tree, killing the insects when they feed on the tree. This is a slower but more thorough approach, slower because trees have a passive circulatory system and the substance is only pulled up through the tree as moisture transpires from the leaves during the night. The second prong is a contact pesticide or oil to provide a quicker knock-back, but a less complete kill. The important thing to remember is that a single pregnant female insect introduced on a crepe myrtle can have the entire tree covered in CMBS in just 2-3 weeks! So, act with a reasonable degree of haste.
We are currently recommending Fertilome Tree & Shrub Systemic Insect Drench (containing imidacloprid in a very high concentration) as the systemic, which may stay in the tree up to one year, combined with either horticultural oil or the liquid version of Sevin.
Environmental concerns: We prefer organic and natural approaches where possible, but there doesn’t seem to be an effective organic/natural control/kill for CMBS yet. Though this is debatable, with good arguments on both sides, we currently think the systemic, imidacloprid, despite being a neonic, has minimal impact on pollinators and other beneficial insects, because of the difficulty it has traveling into the flowers of the crepe myrtle. We do, however, caution that one should be cautious in using it near other plants that pollinators utilize, like perennial gardens. Additionally, we do know that any contact pesticide will kill any insect it comes in contact with. Therefore, out of respect for the benefits pollinators like butterflies, bees, etc., provide us, we urge you to only use the liquid Sevin, for example, closer to dark, when pollinators are less active. Finally, we urge that you consider cultural changes like switching to pine-straw mulch rather than wood mulch or at least getting the wood mulch in bag form, not planting crepe myrtles or removing and replacing the ones you already have with other trees, asking that your landscapers not come to your yard right after pruning infected crepe myrtles and making sure they are educated about the disease, making sure the trees are adequately watered and fertilized because we know that healthy trees are less attractive to harmful insects, etc.
Unfortunately, though we initially thought the CMBS species would limit itself to crepe myrtles, there is some evidence that it is attacking beauty berries, hawthornes, and maybe even other plants.
Each year, Greg Touliatos gives a seminar on this new disease and other pest problems. To learn more, check our facebook page or email us for a schedule of our free classes and attend them on Saturdays.
Caution! Don’t forget to read the label carefully and follow all procedures. The label is the law. Although we are told that imidacloprid is not easily absorbed transdermally, wear gloves and take other precautions necessary to keep it off of your skin and certainly keep it away from your face, i.e. not allowing it to splash as you are pouring. Further, remember that it is more harmful to pets than to humans. Therefore water it in heavily after applying it to the soil and keep your pets away until it has had a reasonable amount of time to sink below the surface.