Mold, black leaves and flies

Asked June 22, 2018, 7:59 AM EDT

I have a large shrub like tree near my deck. It has fragile white blooms in the Spring and lots of beautiful foliage. Within the past 2 months it has been literally overcome with flies. It has scaly whitish grey mold on the underside of many of its branches and black coating on many of the leaves.

Marion County Ohio magnolia scale

3 Responses

Hi. Can you send me some photos? That would help me identify both the issue and perhaps also the shrub. That being said, I suspect you have an infestation of some type of soft scale. These small insects look like little bumps stuck to a leaf or branch and often congregate on the undersides of leaves, particularly near the leaf veins. This may be scaly mold you refer to. These insects have mouthparts that they use to pierce the vascular tissue (phloem) of the plant and suck out the sap within. The excess sap is discharged from their anus and falls to leaves and other structures underneath. This sugary liquid is then often colonized by sooty mold, which does no harm but it unsightly (that is probably the black coating you are seeing). The sugary liquid can also attracts a variety of flying insects (wasps, flies). Scale can be a difficult insect to manage, but treatments are available. However, the first step is correctly identifying the type of scale (assuming it is scale in the first place). If you send me high quality photos, I should be able to provide an identification and recommend a treatment. In the mean time, I suggest you read this article from last month’s Buckeye Yard and Garden Online newsletter, which provides a detailed discussion of one type of soft scale known as calico scale (which may or may not be what you have). Virginia Tech also has a nice fact sheet here that discusses scale insects in general and provides many examples of both soft and hard scales.

Here are some photos. I appreciate your response as well as the articles and websites.

Thanks for these. That looks like a type of magnolia, and it definitely is infested with a soft scale, probably magnolia scale based on its very large size and its white, powdery covering. Michigan State U. has a good article on magnolia scale and how to treat it here. You should consult that article for details, but you essentially have two options, given that much of the tree appears to be infested: (1) use a systemic insecticide (soil drench) in August that has imidacloprid or dinoterfuran as its active ingredient; (2) target the “crawlers” (the young scale that move to other parts of the tree) in late summer/early fall with a horticultural oil (“summer” oil). Note that the first option can also affect (kill) pollinators depending on the timing of the application because the active ingredient also goes to the flowers. This article from OSU’s BYGL newsletter suggests application should be made after flowering and before the fall to avoid this issue. It and an earlier article from last year also provide more information about Magnolia scale. If you do use a pesticide, please be sure to read the label and carefully follow label instructions.