Saturday, April 2, 2011

Ash Flower Gall

These brown, dried-up balls can cover an Ash tree (Fraxinus spp).  I would notice them in the winter and wonder what kind of tree I was looking at.  I used to think they were the dried up flowers or seed pods of the tree.  They look like they belong on the tree.  But they are actually caused by a microscopic mite, the Ash Flower Gall Mite (Aceria fraxiniflora), feeding on the male flowers of the Ash tree.  A gall is any structure on a plant caused by an insect, mite, etc, that causes a part of the plant to grow "unnaturally".  Many plants have galls, and galls are as diverse in their shapes and structures as the plants that host them and the organisms that cause them.  From what I've read, whether the insect induces the gall, or whether the gall is formed by the plant as a reaction is unknown.  In this case, the feeding of the mite doesn't harm the tree. 

They are so universal that I wonder if sometimes the plant doesn't gain some benefit from the galls.  For example, Hackberry tree (Celtis occidentalis) leaves are almost always covered in galls caused by psyllids, or jumping plant lice (Pachypsylla spp).  Maybe the galls deter other herbivores, and the feeding of the Psyllid cause less damage to the tree than other potential herbivores (the Hackberry trees don't seem to be harmed).  It's just a thought that crossed my mind. 

Something we can't see, causing very noticeable changes to something we can easily see.