P&PDL Picture of the Week for
July 7, 2008

Maple Bladder Galls

Tim Gibb, Insect Diagnostician, Department of Entomology, Purdue University

You may see samples or receive photos similar to the above in the coming weeks. These small black or red wart-like growths that appear on silver maple leaves at this time of year are maple bladder galls. Other maples are also attacked by the tiny mites that cause these galls. These growths first appear as red, then turn green, and finally black. They occur singly or in clusters and may be so abundant that the leaves become crinkled, deformed and drop early.

The important thing to remember about galls is that once formed, they cannot be lifted or removed from the leaves because they are composed of plant tissue and are actually part of the leaf. This also means that they are not going to cause serious injury to the tree even though many homeowners become alarmed when they discover infestations of the maple bladder gall and often fear that their trees might die unless control measures are taken. Rest assured that although they can become unsightly, the galls never cause permanent injury and have little effect on tree health and vigor.

Maple bladder gall mites overwinter in cracks and crevices of the bark. As the buds swell in the early spring, they migrate out on the bud scales. This is when mites are most susceptible to dormant applications of oil spray. When buds open, the mites feed on the newly developing leaves. In response to this feeding, hollow galls are formed. The mites then live, feed, and mate inside the gall where they are protected from predators and also chemicals. Dormant oil sprays are really the only reliable way to knock theses back. However, doing nothing will not hurt the tree and even though some will occur in the following years, the populations most often cycle down to tolerable levels on their own.

Click image to enlarge

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service