Friday, June 3, 2016

Elm Leaf Finger Galls

Early Elm Finger Gall

Jeff Birchler, the new Watershed Center Coordinator has been finding these on field trips, causing some excitement among the students.  He sent this picture for identification.  The small teeth below the larger teeth suggests this an elm leaf.  Many galls have a simple descriptive name and this is as simple as it gets, the Elm Leaf Finger Gall.

Full bloom with a baby starting
These are caused by Eriophyid mites, tiny creatures less than a half millimeter long.  There are over 3,600 species described with many more out there. Being mites they are in the same Arachnid class of 8-legged critters as ticks, chiggers, and spiders.  They are the exception to the rule, having only two pair of legs on an elongated carrot-shaped body .  They are usually specialists, concentrating on a specific species of plant.

The mites overwinter as adults, nestled in bark cracks and under bud scales.  When they emerge in the spring they lay eggs and their larvae look like little adults.  They are so small that their method of spreading is by blowing in the wind.

Shriveled after picking
These mites crawl out onto leaves, sucking at the sap and causing the release of chemicals that cause the scaring and producing the galls.  The different species can make a wide variety of galls in color and shape.  Some of these mite species have become major agricultural pests while a few have been used to attack weeds or invasive plant species.  Other species can cause scarring, bronzing and other damage as the plants react to their nibbling.

Hackberry Witches' Broom
Eriophyid mites also cause witches' broom, a dramatic proliferation of branches on the hackberry tree.  In this case the mite damages a tree bud, infecting it with a powder fungus and the tree reacts by producing a cluster of deformed branches.  It doesn't cause any significant tree damage although it can worry a homeowner.

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