Sunday, March 8, 2015

Galls are a good thing*

*Or at least, mostly harmless.
Galls on an oak tree. Probably mealy oak gall (Disholcapsis cinerosa).
Lately, I feel like my blog's tagline could be "It's just aesthetics: leave well enough alone." First I defended mistletoe, then tent caterpillars, now gall wasps. I have noticed more oak galls in Texas than anywhere else I've lived. When I visited Dinosaur Valley State Park on Saturday, some trees were hung so thickly with them they looked like persimmon fruits. Others, which had supported leaf galls instead of twig galls, instead looked like they had odd round red acorns scattered about their trunks. You can get an idea for which critter created the gall by its appearance.

Gall wasps (Cnipidae) are the creatures that create the little brown orbs hanging in your oak trees. These cryptic creatures spend most of their lives as grubs. Adults are the size of a fruit fly and live about a week. They emerge, mate (every other generation), and then oviposit their eggs in stems, leaves, and twigs of oak trees all within that brief window. Note I didn't say "eat". Adult gall wasps don't eat- they live entirely of the plant matter they consumed as grubs inside their gall. They alternate generations between asexual females (who live in one type of gall), and sexually reproducing males and females (who live in another).
Another type of gall. Possibly Belonocnema kinseyi.
The galls themselves are an abnormal plant growth. They are not directly created by the wasps, but instead are similar to an immune effect in the host tree. There is consensus that this gall-making does not seem to hurt the host. The impact is so minuscule when weighed against the mass of the tree, it does not do measurable damage. I suppose it's like when you knick yourself shaving. You might bleed a little, but beyond the initial cut you don't suffer for it. I could not find a single source suggesting that galls harmed trees or required intervention. Most sources suggested doing nothing at all in response to an "infestation". The galls are not a danger, and they are only a problem if their untidy looks bother the homeowner.
A closer look at possible B. kinseyi galls.
Bizarrely, gall wasps are prevented from becoming super abundant because there are more trophic levels above them: they themselves are parasitized by a different kind of wasp.  Then, the wasp which parasitized the gall wasp is parasitized by ANOTHER kind of wasp. Nature just doesn't quit. It seems that being born a wasp is an awful tough way to make a living. An interesting effect this hyperparasitism has on the casual naturalist is that you have no idea if the bug which emerges from the gall is the one that built it. You could say that the gall structures the first wasps create serve as protective homes for many other insects. At a minimum, they provide an example of intense competition within the animal kingdom.

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