Good afternoon Katherine, 

It appears that this may be a type of Wooly Aphid, which, according to University of Maryland can happen to Serviceberries. Here's some more information: https://extension.umd.edu/resource/woolly-aphids-trees/

In the outdoors, wooly aphids are typically controlled by parasitic wasps, however since your problem is significant, it is recommended to use either horticulture oil or insecticidal soap to reduce the infestation. A commercial brand of insecticidal soap would be the safest insecticide to use. 

However given the very hot weather this week I want to flag that insecticidal soap should not be applied when temperatures are greater than 90 degrees, so it would be best to apply in early morning on a cooler day -- it looks like next Thursday temperatures will drop somewhat so that might be a wise option. Here's more information from Cornell Extension on applying insecticidal soap.

All my best, 

Rebecca

These look like they could be one of the woolly aphids, which often have two host plants, i.e. elm and serviceberry. They overwinter and begin their breeding cycle on the primary host, raising a generation with wings. This winged generation flies to the secondary host plant, such as your serviceberry, to feed for the summer, ending that phase by raising another generation with wings to fly back to a primary host plant.

The good news is that trees generally withstand woolly aphids fairly well. The bad news is that WA multiply very rapidly. 

Good that you spotted the problem early! Suggest literally knocking down the population by hosing all of the tree you can reach with a firm spray. Monitor the serviceberry and spray treatments can be repeated as needed, if needed.

Although their waxy coat that gives them a “woolly” appearance protects WA from horticultural oils, soaps and even permethrin, a jet of water can dislodge them.

WA have many natural predators in the garden, which can keep the reduced population in check.  

How to boost the natural predator population in your garden:

A practice to attract natural predators to your garden is to include flowering native and herb plants in your garden. Another is to draw in small insectivorous birds with a shallow birdbath of fresh water (changed every other day & kept clean) and nesting spots. Hummingbirds might be more attracted to a bath with a drip, spray or small fountain.

Unfortunately, some sucking insects can sometimes carry plant pathogens from a plant that is already affected to another plant. So continue to monitor, as you have been, to reduce that risk by keeping the populations down. 

More details in the link below:

https://extension.umn.edu/yard-and-garden-insects/aphids#use-a-high-pressure-water-spray-344411

Please keep us posted on your serviceberry!

Kate