Nine years ago a blast of straight-line winds from a passing thunderstorm knocked down eight, fifty year old spruce trees on the west side of our house. The loss of these trees took away most of our afternoon shade out on our deck and also exposed a great deal of ongoing ecological activity about which I had not been aware.
Under several of the spruce trees were small oak seedlings. Their acorns had probably been inadvertently deposited under these long standing, heavily shading spruces by passing blue jays or crows (there are no oaks within a half a mile of my house, so these acorns had to have had very mobile carriers to get here!). These acorns had germinated in the protected micro-environment under the spruces and had been growing very slowly for possibly a decade or more waiting for some ecological opportunity. Their opportunity came in the sudden removal of the spruces whose dense shading had long ago become much more inhibiting than protective.
Over the next nine years the oaks (a mixture of white oaks, scarlet oaks, and northern red oaks) grew at an incredible rate. They are now between 15 and 25 feet tall and are rapidly spreading their branches into and over the remaining spruces undoubtedly setting to return the ecological “favor” of strangling out the spruces’ sunlight and sending them into successional oblivion.
All that drama and action right out here in my yard!
I was looking one of the white oak trees yesterday and noticed two remarkable things: 1. There was an odd, fluffy, spherical growth on one of the small branches of the tree (an oak gall! Specifically, a wool sower’s gall (also called an “oak seed gall”)), and 2. This small white oak (it is barely 15 feet tall and very spindly) with just nine years of open, sunlit growth had several clusters of little green acorns on its middle branches!
A gall is structure in which the larvae of an insect (called a “gall former”) develop. In the United States there are 2000 described, gall forming insects. Most of these gall formers are tiny wasps or even tinier midges (a type of fly). Something really remarkable to me is the fact that seventy percent of the known gall wasps make their larvae protective galls on oak trees! There are so many things to talk about here!
Galls can form on leaves, on bark, on twigs and branches, and even on roots. The gall is a structure made by the host plant out of plant cells and tissues possibly under the stimulation of chemicals that are released by the parent gall former when it lays its eggs, or from chemicals that are found in the saliva of the larvae after they hatch out and start to feed. There is also some speculation that these “chemicals” might actually be specialized, symbiotic viruses or even RNA molecules. Whatever these agents are and from whatever source they actually arise their impact is to change the composition and activity of the host plant’s growth hormones which then causes the very specific structure of the gall to form.
The gall is not only the incubator space for the larvae, it is also a food source! Gall tissues are rich in proteins and triglycerides and provide the rich nutrition needed by the larva as they pass through their immature growth phases and eventually metamorphose into adults.
This particular “wool sower” gall is caused by a small, parasitic wasp called Callirhytis seminator. It is only found on white oaks and forms in the spring. I have searched for more information about this wasp (what are its other relationships in its ecosystem? what are its other roles?) but I have not yet been able to find any more information other than it makes these galls. There are many statements about the lack of taxonomic and ecological information about the whole group of gall formers. It seems like a wide open area for study!
The gall, then, is part of the oak tree and also an extension of the gall formers’ physiology.
Most authorities feel that galls on an oak tree have very little effect on the heath or vigor of the tree. Few management or control steps are recommended to remove or to prevent the formation of these galls. But, the question that kept coming to me as I read about these galls and observed the gall on my little white oak was, “why do oak trees have so many galls?”
An answer to this question was not easy to find. After some digging around I did come across a paper by Taper and Case published in the journal Oecologia in 1987. They found that the tannic acids which are abundant in oak leaves, bark, wood, acorns and also their galls acted to help to protect the gall former’s larvae from fungal infections and, thus, greatly increased their survival and facilitated their development into adults. Now galls do not ONLY form on oaks, but possibly those insects that utilized oaks for their galls had better success because of higher rates of reproduction. In evolution even a very slight edge in reproductive success leads to selection of a trait or characteristic. This tannin-generated survival edge may explain the incredible prevalence of present day oak galls.
White oak acorns:
I went to my silvics book to read up on white oaks. I had a feeling that they were very slow in coming into reproductive maturity and, sure enough, the first thing under reproduction was the statement that white oaks start to make acorns at about 50 years of age. A few sentences down there was a qualifying statement that some white oaks might begin flowering and reproducing (i.e. making acorns) as early as twenty years of age. I apparently had one of these precocious reproducers! Maybe I need to put it on a curfew?
I checked the other oaks in the yard but found no other acorns. I don’t know whether to collect the acorns when they ripen (white oak acorns mature in 120 days and drop off the tree 25 days later, so I would look for these acorns in late September) or let the squirrels and jays and crows have them. They planted the trees after all. Thank goodness I have a few months to think about it.