Answer: I'm willing to bet you've got some Fall Webworms in your crabapple tree. They usually show up in August and become very evident as they build protective webbing around the branch they're feeding on. As the caterpillars grow, the tent expands, encompassing more and more of the branch. Since they're caterpillars, they're susceptible to Bt (Bacillus thruingiensis), but I usually just cut the infested branch part off the tree and dispose of it. Just in case you're interested, the caterpillars are larvae of the Hyphantria cunea moth, and they only feed on deciduous trees (as opposed to Tent Caterpillars which are indiscriminant in their feeding habits, enjoying both leaves and needles).
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