Bright red bladder-shaped growths on the top of maple leaves are evidence of the Maple Bladder Gall mite. Adult mites that have overwintered beneath bark and bud scales on host trees move to newly developing leaves and begin feeding. The pouch like galls develop in response to this feeding activity. Initially, the galls are green but gradually turn red and then black.
Adults deposit eggs in the galls as they feed.
Eventually, the adults and their progeny will leave the gall in search of newly forming leaves so that they may continue the cycle. Mite activities drop off as summer heat arrives. The galls may be unsightly but do not damage the maple trees. Therefore the need for control is questionable. Also, spraying must start before the galls are formed to be effective.