Light blue- grey with spots, not stripes right? So these ARE GYPSY MOTH caterpillars.
Maybe someone else will tell you a different way, but I would just try to get them off of the tree physically, rather than spraying the entire tree and surrounding area hoping to kill them. Even Bt which is caterpillar specific bacteria that will infect them and kill them will kill ALL Lepidoptera (butterflies and moths) in the area for one thing. Stronger poisons will also kill other insects including honeybees and bumblebees that are probably already visiting those blossoms and pollinating them. Not to mention other beneficial insects including those that prey on the gypsy moth caterpillars.
I'm more familiar with tent caterpillars but their activities are similar. They spread out throughout the tree to feed at night, then gather in those webby tents to hide during the day. You will see stragglers on the branches and trunk of the tree mostly early in the morning and dusk in the evening and cloudy overcast days.
So middle of a sunny day is best time to catch them "napping" as it were.
Spread a tarp all under the tree so you don't lose any that fall on the ground. Many caterpillars jump off by reflex when threatened. Get on ladder if you can reach by hand (wear long kitchen gloves) or use long stick or pole to break one side of the webbing off from the branch and rapidly whip the stick around, wrapping and winding the webbing around it and trapping the caterpillars in their own nest.
Have several plastic grocery bags handy and thrust the stick with nest in, grab from outside of the bag and draw/scrape the nest off of the stick inside the bag (I say stick but I always use bamboo poles which is slick and easy to get the stuff off). If the caterpillars are actively moving, take no chances, tie this bag closed and use other bags as many as needed. I squeeze the air out when closing, toss on the ground/tarp, and thoroughly stomp on the bags... and stomp on any loose caterpillars on the tarp.
Wear rubber boots, etc. gardening boots. Oh. Be sure to wear a hat and tuck your hair in if you have long hair. Wear long sleeves, long pants tucked into the boots, etc. maybe safety goggles if you want.
Common sense but in case you are not used to garden/hiking type gear, maybe I should mention it.
If you get most of them and squish them around, the scent will attract the birds and other predators and they will also join the fray. Hopefully pick up ones that you missed. You don't have to get all of the caterpillars but make sure to get all the tents as well as webbing flat against the bark like your photo -- as best you can. Those are their hiding places -- protects them from predators as well as any sprays. Plus they have to manufacture the silk from their own bodies. So if you take that away, they will have to make more, expending energy.
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