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November 21, 2004

A lot of galls

wandagalls Hiking today on Mount Wanda, Becky and I found the tree pictured at left, one of a grove of several dozen black oaks infested with cynipid wasp galls. The word "infested" is slight hyperbole. While the galls do injure the tree very slightly, a healthy tree can tolerate a rather large number of such galls.

Yesterday, Matthew and I found other cynipid galls in Castle Rock State Park as well. It's pretty hard to hike in an oak forest anywhere in California - or the Northern Hemisphere, for that matter - without finding such galls, if you know where to look.

Cynipid wasp females lay eggs on healthy growing oak tissue - the meristem, the oak's equivalent of stem cells, found in each bud on the tree. Eggs hatch out, larvae begin eating the meristem tissue, and substances in the insects' saliva stimulate dramatic and bizarre growth - the galls. Galls provide food and shelter for the larvae, which emerge as adults and mate and lay eggs on new oak meristematic tissue.

Except when they don't mate, that is. Some species of cynipid wasps alternate between sexual and nonsexual generations, a phenomenon which any veteran of both the Clinton and Bush administrations will appreciate. Nonsexual cynipid wasps emerge from their solitary gall confinement as pregnant females, ready to fly off to find more oak meristem tissue to "impregnate."

There are about 150 species of cynipid wasps, and
characteristic gall.

Wasps of alternate generations within the same species may produce very different looking galls, which can make identifying species by gall shape challenging.

Just as North America's oaks provide a groaning board for a huge diversity of cynipid wasps, cynipid galls are abundant enough that other organisms have evolved to exploit them. These animals, which typically lay eggs inside a developing gall, are referred to as parasitoids. Brian Inouye, whose photos the last four are, has studied the interactions of invasive Argentine ants with cynipid galls in California: the ants harvest sugars from some of them. Inouye has found that the ants defend some types of cynipid wasp galls against parasitoids, with as yet undetermined ecological effects. As if it weren't bad enough that Argentine ants get into your flour and that they've been implicated in the decline of California's horned lizard population, the notion that they may be tweaking the complex ecology of oak gall formation is reason to fume. Parasitoids may well act to regulate the cynipid wasp population by eating the larvae in the galls. Though the galls are lovely, a four- or fivefold increase in their numbers could do some marginal oaks in. It's not unlike the situation with mistletoe, which Becky and I also found in large amounts today and I don't think how we reacted is really any of your business. A small amount of mistletoe, like this Phoradendron we saw today, can be a good "tenant" in an oak. Too much, and the oak will succumb.

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If I found that many oak galls all together, I'd be tempted to gather them and experiment with them in homebrewing. Galls concentrate tannin, and were a traditional bittering agent in some European folk brews, before hops came into widespread use.

Posted by: Dave at November 21, 2004 05:58 PM
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We have a massive oak behind our house, I knew those galls were caused by insects but never what they were called or the kind of bug that made them grow. We apparently have either beak or wanda galls, if I'm interpreting your picture naming system correctly. There are years when there are many in our yard, and years when it seems like fewer. The tree does seem healthy enough! Thanks for the botany lesson today that relates to my own backyard!

Posted by: susurra at November 22, 2004 09:38 AM
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Have you ever seen the jumping galls of California?

I've been told that the suckers that grow up from the roots of live oak trees (which I wish I could find a way to control)are caused by wasps laying their eggs on the roots. Do you know how to control the suckers without using insecticides? Or if what "I have been told" is true?

Like the way you described meristem cells!

I see that Rita is blogging now also. Happy Thanksgiving to you and yours. dr

Posted by: David Roycroft at November 23, 2004 08:02 AM
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