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Mar 16

Ask Allen: Wasps in Crape Myrtle...

Posted on March 16, 2023 at 11:05 AM by Aaron Street

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Ask Allen ~ "Wasps on Crape Myrtle black stuff"

Question:  Yvonne Fall: I rent this home and there is a Crape Myrtle outside the window of my office which also happens to be close to the front door. My issue is wasps. I imagine they are there for the black stuff that is on the tree. When the tree blooms, it gets worse. Makes it hard for deliveries?? Any advice on how to get rid of them and keep them away? You mentioned some stuff to pour at the base of the tree but what about something to spray on it also? I’d like to get ahead of all of that this year. I looked for a wasp nest last year but didn’t see anything. I admit, I didn’t try very hard…because of the wasps. Thanks in advance. ?

Allen's Answer: Hey Yvonne, Great question. I believe your problem with the wasps and the black stuff are related, and by getting rid of the sooty mold (black stuff) it will solve your wasp problem. Your Crape Myrtle, more than likely, has an infestation of Crape Myrtle Bark Scale, a small insect that attaches itself to the tree and sucks the fluid out of the tree. As it feeds on the plant, the scale secretes honeydew, a sticky substance that accumulates on the limbs and on anything under the tree. This honeydew ferments and turns black. The wasps, along with other insects, feed on the honeydew, thus attracting the wasp to the tree. So, controlling the bark scale should also keep the wasps away. Here are a couple of the more common methods that are used to control the scale. One is an application of a horticulture oil while your tree is dormant. This method is more labor intensive and requires spraying the tree completely, making sure to get the spray into the crevices at the limb junctions. The second method is to drench the soil with a systemic product that is mixed with water and poured around the base of the tree.  It will be absorbed by the roots of the tree and will move into the tree.  There are also Granular types.  This method is best applied as the tree begins to green up, usually around the first of April, and works for an entire year.  Examples of these products are Bayer Tree and Shrub Concentrate (Imidacloprid) and Ortho Tree and Shrub Insect Control Granules (Dinotefuran).  Always read and follow the label instructions of these products. Hope that helps - Allen 

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Allen Bates is the City of Hot Springs urban forester, and he manages the City’s urban forestry care, preservation and health plan to provide the City with a safe, healthy environment for the City's urban forest.