Sooty Mold on Crape Myrtle - Ask Extension
Have a mature Natchez Crape Myrtle with a scale infestation leading to honey due leading to sooty mold. Visible black on bark.
Is this scale specific...
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Sooty Mold on Crape Myrtle #864264
Asked April 12, 2024, 2:07 PM EDT
Have a mature Natchez Crape Myrtle with a scale infestation leading to honey due leading to sooty mold. Visible black on bark.
Is this scale specific to Crape Myrtle or should I be concerned about other plants in the landscape.
In addition to dormant oil is there a specific pesticide I can use.
thank you
Howard County Maryland
Expert Response
Research is still taking place with Crapemyrtle Bark Scale, since it's only relatively recently been introduced to the mid-Atlantic, and has become widespread here fairly quickly. According to references we searched when creating the page linked above, crapemyrtle is the primary host plant in North America, but like many scale species, it can also feed on other unrelated plants. A short list of those (since it's still being studied) is included on that page.
Scale insects cannot crawl very far on their own, and for most of their lives, they essentially don't move. However, they are so small that crawlers (the newly-hatched juveniles that do wander, as their name implies) can drift onto new plants on the wind or by hitching a ride on wildlife (bird feet, squirrel fur, etc.). Therefore, while potential host plants in the immediate vicinity of an infested crapemyrtle are probably the most at-risk, they would potentially spread anywhere in the yard or neighborhood if other plants are vulnerable. Anecdotally, we've seen a few instances where this scale species appears to have colonized St. Johnswort (Hypericum) and Purple Beautyberry (Callicarpa) shrubs, though it was uncertain whether that happened at a nursery or in the landscape from a nearby crapemyrtle with an outbreak.
Dormant oil is a good starting point because it reduces the risk to beneficial insects and pollinators. A next step would either be an insect growth regulator (IGR) or a systemic (plant-absorbed treatment) applied by a certified pesticide applicator with experience treating scale (so they know how to monitor for a crawler stage, as it's the most vulnerable to pesticides). How often a treatment would be needed will depend on the product used, but once-yearly applications or more could be likely for the life of the plant. There may a risk of systemic ingredients contaminating the next season's blooms (nectar or pollen) and putting pollinators at risk, but we do not have research conclusively demonstrating whether this does or does not occur in this situation.
Miri
Scale insects cannot crawl very far on their own, and for most of their lives, they essentially don't move. However, they are so small that crawlers (the newly-hatched juveniles that do wander, as their name implies) can drift onto new plants on the wind or by hitching a ride on wildlife (bird feet, squirrel fur, etc.). Therefore, while potential host plants in the immediate vicinity of an infested crapemyrtle are probably the most at-risk, they would potentially spread anywhere in the yard or neighborhood if other plants are vulnerable. Anecdotally, we've seen a few instances where this scale species appears to have colonized St. Johnswort (Hypericum) and Purple Beautyberry (Callicarpa) shrubs, though it was uncertain whether that happened at a nursery or in the landscape from a nearby crapemyrtle with an outbreak.
Dormant oil is a good starting point because it reduces the risk to beneficial insects and pollinators. A next step would either be an insect growth regulator (IGR) or a systemic (plant-absorbed treatment) applied by a certified pesticide applicator with experience treating scale (so they know how to monitor for a crawler stage, as it's the most vulnerable to pesticides). How often a treatment would be needed will depend on the product used, but once-yearly applications or more could be likely for the life of the plant. There may a risk of systemic ingredients contaminating the next season's blooms (nectar or pollen) and putting pollinators at risk, but we do not have research conclusively demonstrating whether this does or does not occur in this situation.
Miri
