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The Gall of it All

The youngster rolled the woody ball between his fingers trying to figure out what it was.  Every guess so far had resulted in a “no” response.  He scratched his head and pondered.  He thought he’d try one more time.  “It’s an acorn!”  We told our young friend that some wasp had the gall to lay her eggs on a live oak twig, and this is what happened.

Our young Honey Creek pal was perplexed by the strange and curious world of galls – specifically an oak gall.  We’ve seen then lots of times both in the park and at Honey Creek.  Scientists also sometimes call galls hypertrophies.  They are tumorous (neoplasmic) outgrowths from rapid mitosis or morphogenesis of plant tissue.  Ok --enough of the complicated science stuff!

There are a lot of plants that produce galls, usually as a reaction to some irritation caused by some insect, fly, wasp, moths, caterpillar, spider, mite, aphid, or even a nematode.  The plant’s tissue swells and causes a protective shell to be formed over the irritation, often completely enclosing the cause of the irritation.  When that happens it both protects itself and the irritant organism at the same time – or at least it’s eggs.  The gall provides the residing insect with food and shelter during the developmental stage.  Once hatched, the insect can bore out of the gall to freedom.  Galls are not thought to harm the growth or health of the plant itself.

Galls can be pretty and decorative.  But it can be a pest too – especially on fruit, grain, and vegetables.  Most of the ones we see at Honey Creek are Oak Bullet Galls, or mealy oak galls formed by a tiny wasp called Disholcaspis cinerosa.   Disholcaspis larva secretes chemicals that stimulate the plant to produce a spherical shaped gall on leaves or stems of the host plant.  From late summer to early fall the larva eat plant tissues within the gall itself.  From about mid-August to October, the internal gall tissue is moist and yellow-green.  By November the tissue turns brown and begins to dry out.  During November pupation occurs and shortly after that transformation to the adult stage takes place.  During December and early January, the adults emerge by chewing holes in the gall – usually at the base. 

The emergent wasps from stem galls are all asexual females that do not need to be fertilized, and they promptly seek out swollen leaf buds on live oak branches in which to deposit one or two eggs per bud.  They live 2 – 6 weeks and can lay up to 20 viable eggs during this period.  Harsh weather appears to have little limiting effect on their activities.  The eggs remain dormant during the rest of the winter.  The wasps that emerge from leaf galls are the second generation and are sexual – producing males as well as females.

Old galls from the season before are left on the tree and begin to weather and dry hard.  It is no longer a living part of the tree.  The gall actually dies about a month before the adult wasp emerges.  If left undisturbed galls can remain on the host tree for several years.  The gall itself has completed its task, and new galls are formed by the continuing life cycle of the wasp. 

One interesting fact, which might be considered a form of poetic justice, is that at certain times the gall wasp is itself attached by other parasitic wasps.  These parasites lay their eggs in the gall wasp larva, and benefit from the gall.  These parasitic wasps do not feed on the plant like the gall wasp, but only on the larva inside the gall.  There is also evidence that the oak tree gains some immunity after producing galls for a period of time.

More information about this strange and interesting insect and the galls that are produced from them can be found at http://entowww.tamu.edu/extension/bulletins/mp-1315.html .  There are some pretty good pictures there too of galls and the wasp.  You can also see a real good picture of a Live Oak Gall Wasp at eNature.com

Our little Honey Creek hiker was still perplexed and eventually decided that the gall was kind of like a little hotel for the baby wasp.  I couldn’t have explained it any better.  Talk about room service – we could eat the walls down at the Motel 6!  That would take a lot of gall. 

Ernie Lee, Texas Master Naturalist