If you’ve spent any amount of time admiring
’s fine selection of oaks, you may have noticed strange growths on various parts of the trees. Bulbous tan lumps, tiny spiny projections, and bloated stems riddled with holes all embody galls – tumor-like growths induced by parasitic organisms wishing to make themselves a home. While many different plants are hosts to galls, California ’s oak trees contain the largest assortment. And though a variety of organisms can initiate gall development, one family in the class Insecta stands wings and thorax above the rest – the gall wasps. California
Oak gall wasps belong to the Cynipidae family, which makes up over fifty percent of
’s gall-inducing insects. Unlike the vespid wasps, best known for terrorizing picnic areas and barbeques, cynipid wasps are very small and relatively harmless. Usually no bigger than a housefly (with some species much smaller), they are nigh impossible to spot during their short adult life. These fascinating insects and their unusual life cycle are known almost entirely from the galls where they spend most of their life. California
|Tiny cynipid wasps (from Forestry Images).|
Depending on the species and the generation, the female oak gall wasp chooses a host oak and lays her eggs inside a specific region of the tree’s meristematic tissue (the undifferentiated tissue that has yet to become a particular part) via a long, penetrating ovipositor. Some species oviposit in bud tissue, others in leaf tissue, and yet others right through the bark into the cambium. Once inside the host, the egg takes control and gall initiation begins.
|"Oak apples" - galls of Andricus californicus on valley oak stem, Coloma.|
The egg secretes various plant hormone replicas, causing a chamber to form around it. Further hijacking the host tree, the larval chamber then produces its own vascular system, tapping the vascular system of the plant and creating nutritive cells which feed the larval wasp as it grows. On the outside of this alien incubator, a hard layer of cells form which become the shell of the gall. Gall shape and size varies widely by the host species, wasp species, and the location on the tree. The oak apple gall, caused by Andricus californicus, can be up to 15cm across; while the diminutive dot of Dryocosmus minisculus reaches only 1mm in width.
|Tiny galls of Dryocosmus minisculus on live oak leaf, Oakland.|
While developing, the gall wasp larvae take care not to bespoil their oaken palace – their intestine remains disconnected from the anus, and only hooks up for deposit immediately prior to pupation. Don’t defecate where you eat, the old adage goes. Larvae may remain in the gall for weeks, months, or even years, depending on the climate and species of wasp.
|Asexual generation galls of Antron douglasii on valley oak leaf, Union City.|
Upon maturation, the larvae pupate into adult wasps, bore a hole through their gall, and emerge into the outside world. Here is where the cynipid wasp life cycle gets even more complicated (as if egg-injecting and meristematic hijacking wasn’t enough) – many cynipids have an alternating-generation lifecycle – a sexual generation featuring a male and female wasp mating and the female laying eggs, and an asexual generation in which the female wasp creates an offspring with no male involved, also known as parthenogenesis. Typically the sexual generation will occur in the spring (when love is in the air) and the asexual female generation in the fall, often creating overwintering galls.
|Asexual generation galls of Andricus kingi on valley oak leaf, Fremont.|
And now for the capstone of weirdness – many cynipid wasps, including the aforementioned oak apple gall wasp (A. californicus) are known only from the asexual generation, and there are no recorded males of the species. These ladies have found a way to carry on life with no guys involved, and the only cost is a slight loss of genetic diversity. Luckily this has not yet happened naturally among Homo sapiens, for if it did I fear the men of our species would face a swift demise.
|Asexual generation gall of Callirhytis quercusagrifoliae on live oak stem, Fremont. This gall contains a female.|
Life isn’t all fun and galls for the cynipid wasps, however. Other wasp species have discovered their ingenious evolutionary strategy, and developed their own methods to capitalize on these cozy homes. Parasitoid wasps, also members of the cynipid family, invade the gall with their own eggs, their larvae consuming the original gall-former. Yet other hyper-parasitoid wasps prey upon the larvae of the parasitoid wasps. In response, some oak galls secrete nectar which recruits ants, which then defend the gall against potential invaders. One study from the
found 17 different wasp species developing inside a single gall. The resulting trophic levels in this closed community are enough to make even a hardened naturalist’s head spin. U.K.
|Gall of Callirhytis quercussuttoni showing multiple exit holes from many different wasps.|
Persistent oak galls can be found year-round, but fall is the time when many of the asexual generation galls are forming with the female larvae inside. So take a close look on a
oak the next time you’re out on a stroll, and you’re just liable to discover an entire microcosm growing right before your eyes. California