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Thanks.They're leaf galls. You know that bump you get on your skin when a mosquito bites you? This is a similar response by a tree to insect damage, although bacterial and fungal damage can also cause the response.
By the time you see them, the guilty organism is usually long gone. They're not egg sacks or anything like that, it's just some deformed cells formed as a reaction to being chewed on. They won't harm a healthy tree. On hickory trees, they're usually caused by aphids.
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