Q: We have just noticed that the leaves of our Autumn Blaze maple tree are covered with little raised, red spots. They look like tiny little red balls. Is this a disease or are they insect eggs of some kind? Is there an "un-poisonous" way to treat it?
A; What you got are what's called Maple bladder galls. These are cause by an insect relative of the mite, also known as Vasates quadripedes (Shimer).
The red bladders are usually first noticed in May; generally when the leaves have become fully expanded. At first the galls are green. They then turn pink, red and eventually black. In some cases the leaves are so infected that they twist and curl and may fall off prematurely.
There is no cause for alarm as these are not harmful to the overall health of the tree. They don't look asthetically pleasing, however. There is no significant damage to the tree. The tree will produce new leaves to account for the food production loss of the damaged leaf.
"The maple bladder gall mites overwinter as free living mites under loosened bark and around the callous growth of wounds, scars and pruned branches. These overwintered forms produce the gall forming stage in early spring. When the maple leaves first appear, the mites migrate to expanding buds and begin to feed on the undersurface of leaf buds. This causes the formation of a blister which expands into a hollow bladder or spindle as the leaf expands. The mites enter the cavity and continue to feed within its protective walls. This stage reproduces asexually within the galls and the new mites mature by late June to mid-July. At this time the galls dry out and the tiny entrance hole opens up to allow escape of the mites. These mites then seek out overwintering sites." ~ David J. Shetlar
Since these leaf galls of maple do not cause any real harm to the trees, control measures are not generally needed. Tree owners and tree managers are encouraged to learn about the life cycles of these pests and learn that no lasting damage will result.
Use Resistant Maples - Norway maples and some of the named cultivars of maples with outstanding red or yellow leaf color appear to be resistant to these gall mites and midges. Talk to the plant supplier to see if the tree cultivar has a history of leaf gall problems or not.
Dormant Oils - Use of dormant oils on maples is discouraged because leaf and twig damage can result unless the tree is truly dormant. However, some reports of success have been made where the trunk has been drenched with dormant oil to kill the overwintering stages of the bladder gall and spindle gall mites.
Standard Insecticide/Miticide Sprays - Several insecticides and miticides are registered for control of gall mites (eriophyid mites) and gall midges. If these products are to be used, they have to be applied precisely when the new leaf buds are opening. Most sprays have little, if any, effect because the window of opportunity is very short. Once the gall has formed, it is too late to make an application.
Systemic Insecticides/Miticides - Several systemic pesticides (sprayed, soil injected or trunk injected) have been recommended as useful in controlling these gall forming pests. However little evidence of success has been found in the current literature. Once the gall has formed, it is too late to make an application.
No endorsement is intended for products mentioned or criticism for products not mentioned.
David J. Shetlar and the Ohio State University Extension