These are Spotted Lanternfly (SLF) and they should not put the tree at risk. These insects have been in our region for a decade now, and research and observations have not indicated that any mature trees or other landscape plants were lost due to SLF feeding on their sap. Early management recommendations from the MD Dept. of Agriculture to squish the insects were made primarily to slow or stop the spread before they reached other counties and states, but now they occur in most Maryland areas and even in several surrounding states. No control measure in a home yard will eradicate them or put much of a dent on the local population as a whole, at least for counties in which they are established. Many SLF reside in treetops and will be well out of reach.

We do not recommend the use of any insecticide to treat SLF, in part because it won't be effective for very long, and in part because there is no chemical that will not risk harm to other insects (pollinators, beneficial predators, etc.) or other wildlife. Most insecticides will involve contact-type ingredients, meaning that they have to direct contact the pest in order to work. Since SLF usually jumps, walks, or flies away at the slightest disturbance, few would get enough of a dose of spray to be killed by it.

Next season, if they are too bothersome to leave alone, you can try to squish SLF by using a plain water spray from a garden hose to knock them off of a tree trunk in the morning, when they are still chilled and need to warm up more in order to move normally. Once knocked down, they could be squished or tossed into a pail of soapy water to drown. Once we experience a few overnight frosts/freezes, all SLF adults will be dead for the season. If they are still hanging onto the tree trunks they are most likely all dead and will fall off eventually. 

Let us know if you have further questions. 

Emily