What are those one-inch Christmas trees growing on the oaks?

1 Oct 2014   Idea of the day  “Psychiatric drugs send just under 90,000 individuals to emergency rooms each year.”  Journal of the American Medical Association

I was out in the field today evaluating the trees at a mid-size condominium complex when I noticed the strangest looking growths on a few sand live oaks.

They were growing right at the meristems at the tips of stems where you would expect to find acorns this time of year.   But this oak had very few acorns and lots and lots of these one-inch long miniature “Christmas trees” growing in those spots instead!  It was down right weird looking.

Meristems are those portions of a tree where plant tissue responsible for growth differentiates and divides to form the various organs of the tree.   But instead of differentiating from twig to acorn, these magical trees were growing dwarf spruce trees.

My first thought was that these were “acorns gone bad”, like deformed miscarriages.  I started doing some research and could not find exactly what I had seen.   But finally I found something close to it.  It was growing from the bud stem in between three leaves.  The arborist who spotted it on a sand live oak posted it for a year as a “female oak flower” because that’s what it looks like!   A year later she was corrected by a Master Naturalist.

It’s a parasitic wasps’ gall !    Turns out scientists are still learning about these unusual creatures which number in the hundreds of different varieties.  And what a strange life cycle they have!

I had thought all galls were round and hard, and in fact I had seen a couple of those types on this same oak.  But there were so many more of the Christmas tree shaped ones with nothing hard about them that I thought they had to be something different. The have soft “needles” similar to what you’d find on a pine.

A gall is an abnormal plant growth on a leaf or, in this case, at the meristems where you would expect acorns to be growing that parasitic insects, wasps or flies, use for their reproductive cycle.   Here is how it works.

In July or August adult wasps hatch from the galls, mate and then fall to the ground (apparently from post-coital exhaustion? 🙂    The females then burrow into the ground and inject eggs into the roots.

The larvae hang around and munch on the roots for up to a year until they reach the immobile resting transformation stage called pupae.   Only wingless females hatch from these pupae.   In early spring they crawl out of the soil, climb up the tree (!), find a meristem and inject their eggs.

The larvae hatch inside the twig’s reproductive tissue.   The larvae emit a chemical that causes a gall to grow around them, in the case of this variety, a miniature Christmas tree.  They use some parts of the gall for nourishment.  When they mature and hatch to start the cycle all over, they leave their bushy “miniature pine trees” hanging in the sand live oaks in autumn looking like they should be acorns.  As parasites, it’s not unlikely that they do use the tree’s energy that otherwise would be producing more acorns.

In the meantime they are not unattractive autumn ornaments that have probably befuddled many an arborist.  As long as they don’t appear in large volume, they won’t hurt the trees.



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