Despite their abundance, Spotted Lanternfly does not cause serious damage to trees, and would not contribute to wood decay. (If wood decay is present, an unrelated issue is the cause, and was likely already occurring well before the lanternfly appeared.) Since they suck tree sap, they produce a sugar-rich waste liquid called honeydew, which can be sticky and, if it ferments when abundant, can have a foul smell. Sooty mold, a fungus which does not infect plants, often grows on top of honeydew because it uses the sugars as food. That is what gives honeydew residue beneath pest insects like these a blackened color. Although a nuisance, it does not harm anything, especially tree bark.

If you have concerns about tree health, have a certified arborist or licensed tree expert assess the tree. They cannot cure every ailment or reverse wood decay, but they can help with diagnosis and look for sources of tree stress that might be able to be alleviated. The extremely close proximity of the fence in the photo may not be helping, and whatever conditions exist on the other side of the fence (such as if a neighbor has cut into roots by digging out a garden bed, covered roots with a patio, mown-over shallow surface roots and injured them, etc.) need to be taken into account since a significant portion of the tree's root system is on the other side.

We do not recommend the use of insecticide to kill the lanternflies; it's not necessary, and won't prevent more adults from flying into the area to recolonize any areas the prior insects were present. If necessary, a low-toxicity topical spray like horticultural oil or insecticidal soap may work, but it must contact the insects directly (dried-on spray residues won't have any impact), and since these insects jump or fly away readily, it probably won't work very well, or for very long. (Blasting them off with a strong jet of water from a garden hose would probably work just as well, and in this drought, the extra water for the tree's roots would probably benefit it.)

Miri