Tuesday, June 12, 2007

Red Nodules on My Maple in New York City

I have a very lovely Maple tree on my terrace. It is growing very well, beautiful leaves. However on the leaves there are tiny red nodules that are some sort of blight or insect. They don't move, but by seasons end most of the leaves have holes in them. The little red guys turn brown toward seasons end. The tree is healthy otherwise, and I would like to save it, but I cannot find out what this is or how to rid it from the maple. I have a fifty or so foot terrace, and this insect has not infested any other of the plants. If you can give me an idea of what this is and how to rid the tree of these "things" it would be greatly appreciated.

Unsightly galls can be the work of many different kinds of flies, wasps, and mites. Because many insects only feed on specific hosts, knowing that it is a maple helps us narrow the options of what the insect is. Your description of the galls helps narrow the list down even further. The damage you are describing sounds like that of a gall mite. Eriophyd mites are microscopic mites that live and feed on many different kinds of trees and shrubs. Of the gall-forming mites, there are at least two that prefer to feed and live on maples, maple bladdergall mites (Vasates quadripedes) and maple spindlegall mites (Vasates aceriscrumena). The galls were describes to me as being more rounded than spindly, so I am going to guess that this damage is the work of the maple bladdergall mite.

As I usually do with insect queries, let me describe the life cycle and then recommended control. Adult mites over-winter on the bark of the maples. As the leaves emerge in spring the mites migrate to the fragile new leaf tissue and begin feeding. As they feed, the mites deform the plant cells and cause these galls to form. The adults burrow within these galls and lay their eggs. The larvae emerge, feed on the leaf tissue, and molt into adults in a few weeks time. The adults move on to feed and continue cycle. The galls are no longer used and usually turn brown.

The bad news is that these galls look unsightly; the good news is that this damage is not likely to kill your maple. Regarding control at this time, the only option is to remove the leaves that are heavily infested with galls. Physically removing the galls before the larvae can emerge will help control the populations. The next step is to take action the following spring. Use a dormant horticultural oil to coat the bark of the tree in spring before the leaves emerge. This oil will coat and suffocate the over-wintering mites and will help to diminish their populations before they get a chance to feed on the new leaf tissue. I would not necessarily count on complete eradication of the mites, but after two years of treatment you should find the populations to be more controllable and the damage to your maple less of an eyesore.

For this and other insect questions I refer to Garden Insects of North America by Whitney Cranshaw, available in our reference section here at the HSNY Library at 148 West 37th Street, 13th Floor.

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