A great abundance of cynipid galls have been seen in recent weeks on blue oaks (
Quercus douglasii) along much of the length of Del Puerto Canyon Rd...from east of Minniear Day Use Area, west to San Antonio Valley Rd, and a good ways north along Mines Rd. About 60 air miles to the south (still in the Diablo Range) near Panoche Pass, Gary McDonald reported a similarly unusual abundance of cynipid galls on blue oaks in his comment with the post below:
It would be very interesting to learn the total geographic extent of this abnormally high cynipid gall production year. How far does it extend throughout the blue oak woodlands of California? And are cynipid galls also more abundant this year on species other than
Quercus douglasii as well?
In the area we visited, by far the dominant gall species seen on blue oak (in terms of numbers and surface cover) was
Andricus crystallinus, as illustrated above in the 1st image of my series here. However, many others gall species were also present in lesser numbers and cover, e.g.:
Andricus gigas,
Cynips quercusechinus,
Andricus atrimentus,
Neuroterus saltatorius,
Cynips multipunctatus,
Cynips canescens, and more. I would guess Joyce may have noted yet other species. Instances of the first 5 species mentioned above can be seen in the 1st thumbnail below, photographed on a tree a few miles east of the one seen above. The last two species mentioned above, and at least one more, occurred on the tree here...as seen in the last three thumbnails below:
A mast year for oaks is a year in which large geographical groupings of oaks synchronously produce unusually large numbers of acorns, usually followed by a period of low acorn output years. One of the leading explanations put forth to explain the mast year phenomenon is the "predator satiation" hypothesis. It posits that acorn production in a mast year is meant to exceed the maximum consumption capabilities of acorn feeding organisms...thus yielding a higher percentage of undamaged acorns that can proceed intact to the seedling stage and beyond. In addition, the low acorn production years which typically follow a mast year would tend to reduce populations of acorn consuming species through relative starvation, helping to relieve overall acorn predation pressure.
I wonder if the same model can be used to describe the phenomenon of prodigious gall production observed here...with gall inducers replacing the role of acorns, and gall inducer parasitoids replacing the role of acorn consumers? A relevant question might then be...how long can a bumper crop of gall inducer larvae or pupae remain dormant before emerging as adults? More than a year? And how about their chalcidoid and other parasitoids?
Even if the "predator satiation" model does not transfer well here, the notion of a "mast year-like" phenomenon for oak galls is quite intriguing...assuming what we witnessed here was not simply a one-time fluke. It gives rise to all sorts of interesting questions for further study.
