Friday, November 18, 2016

What's Wrong With This Tree?

If you've spent any time in an oak woodland (and I sincerely hope that you have!!) you may have noticed something odd: strange growths on the branches and leaves of oak trees.  You might wonder if the trees are sick or diseased, or plagued with a dangerous pest.

But not to worry; these fascinating growths are a normal part of a healthy oak community. 
Spined Turban Gall on Valley Oak, with a bonus gall: the tiny orange sphere to the left of the larger gall might
be a gall of the California Jumping Gall Wasp (inconclusive)
They are in fact galls, caused by a variety of wasps.  Galls form when an insect lays eggs in a plant's tissue, releasing chemicals which stimulate the plant to use its own tissue to form protective structures around the eggs.  As the insect larvae develops, it feeds on plant tissue from within the cozy gall home.  Galls form on a variety of plants, including alders, poplars, willows and even sagebrush.  But the best known and most diverse galls in California are found on oak stems and leaves.  Oak stem and leaf galls are caused by more than 150 species of gall wasps in the family Cynipidae, of the order Hymenoptera, which is the order of insects including wasps, bees and ants.

However, today I only have examples of three types of galls (four, I suppose if you paid close attention to the first photo above).

The first gall is a leaf gall, fascinating in shape, which is found on Blue Oak (Quercus douglasii) and Valley Oak (Q. lobata).  The photo below is an example of the gall of a Spined Turban Gall Wasp (Antron douglasii) on the leaf of a Valley Oak.
Galls of the cynipid wasp Antron douglasii on Valley Oak leaves

The second type of gall, also pictured here on the leaf of a Valley Oak, is formed by the Red Cone Gall Wasp, Andricus kingi.  These little galls remind me of tiny pink chocolate chips!

Galls from the wasp Andricus kingi, found on Valley Oak.
The third type of gall is an oak stem gall, and one you might be more familiar with.  These large tan-colored galls are sometimes called "oak apples" and range in size from less than golf ball-sized to baseball-sized (up to several inches in diameter).  They are also formed, or more accurately induced, by a cynipid wasp, the California Oak Gall Wasp (Andricus quercuscalifornicus).

In the photo below, the oak galls have aged to a darker color.  The astute observer of nature will notice something extra special about this photo as well: not only a female bushtit (Psaltriparus minimus) but her beautifully camouflaged nest as well, tucked in amongst the galls!
California Oak Galls, with a female bushtit (Psaltriparus minimus) and her nest.

California Oak Gall, also called "Oak Apples"
 

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