Answer: If you've examined the leaves and cannot find evidence of insects, then the problem is probably cultural in nature. Japanese maples grow best where they receive morning or evening sun but will not be in direct sunlight at the height of summer. The delicate foliage can "burn" if exposed to direct summer sun. It's also important to water regularly (one deep soaking per week during the growing season. Apply water in early morning or late afternoon to prevent the soil from drying out.
In addition to burning due to hot summer sunshine, maple leaves seem to be sensitive to the presence of dissolved minerals in the water supply. These minerals build up in the leaves, making them more susceptible to browning and curling when exposed to strong sunlight.
I would try to leach the soil of mineral build-up by soaking deeply once each week for the rest of the growing season. When your tree leafs out next spring, the foliage should look healthy and should not develop crispy margins.
Best wishes with your landscape!
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