My mother's yard used to contain a magnificent pin oak tree which gave abundant shade to the south side of her house. Unfortunately, the century-plus-old tree had to be removed earlier this week, because several of its branches were cracking apart under their own weight, threatening to fall on the house. The entire tree was heavily weighted by oak galls, and it had been parasitised by gall wasps for much longer than we'd been living in that house. The oak galls had already been a dangerous nuisance for many years, dropping smaller limbs all over the yard and putting dents in the roof of my mother's tool shed. Fallen galls would damage her lawn mower or trip her as she walked through the yard, injuring her ankles more than once. Bare feet in her yard are simply not an option; fallen galls are covered in sharp spikes and spars of damaged wood.
Gall wasps are a topic of endless love-hate fascination for me. As a person who has enjoyed her share of calligraphy and the use of fountain pens, I appreciate wasps' contributions to the art of ink-making: iron gall ink originally required wasp-induced oak galls for a major portion of their pigments and tannin fixatives. The ink company Rohrer & Klingner produce two incredibly beautiful inks, "Salix" and "Scabiosa," some of the only iron gall inks still widely in use by pen enthusiasts.
As a person who appreciates a properly ancient oak tree and the shade and bird nesting sites it provided, I grieve the loss of this tree. I dislike that in summer my mother will have steeper electrical bills, needing to cool her house without the benefit of a tree blocking direct sunlight on her kitchen windows. I dislike that her yard will be less attractive to songbirds, because it has less cover from the local hawks and stray cats.
As an armchair scholar of all things Hymenoptera, these wasps are altogether diverting for me. They are part of a complex web of parasites who parasitise other parasites: the gall wasp has its larval stage safe inside galls on an oak tree; the Eurytoma wasp family deposits its own larva in the same galls to prey upon the Neuroterus gall wasp; hyperparasites like the chalcid wasp lay eggs on the Eurytoma wasps. The Synerginae family of insects are inquilines of gall wasps, inhabiting the galls alongside the wasp larvae but not causing harm to the larvae with which they cohabit. Due to the exceedingly longer lifespan of oak trees compared to wasps, the phrase "The parasite of my parasite is my friend" does not entirely apply; hyperparasites do not prevent the galls themselves from forming, and it is the weight of the galls, not the wasps themselves, which create problems for oak trees. The galls are the tree's own inflammatory response, not terribly different from how human skin reacts to a mosquito bite, but the galls don't simply subside after the wasps hatch. One successful gall wasp breeding season can set in motion the gradual decimation of an entire oak grove, and in this region, the problem is severe enough that many landscapers in southern Illinois will refuse to plant two oaks within a city block of each other.
Gall wasp reproductive cycles alternate generations, with both types of generation occurring within the same year; one generation has a conventional male-female breeding cycle, and the second generation has pure parthenogenesis, which omits male wasp participation entirely. Both generations produce galls, and entomologists refer to the type of gall created to indicate the specific wasp species present, since the differences between galls are more apparent than the differences between the wasps themselves. Galls from parthenogenic generations look different from galls of sexual generations, as well. Different wasp species target different plants, with roses and maples also vulnerable to some wasp species. Galls vary significantly in size and appearance; they can look like large woody and spiky potatoes encasing oak twigs, as on the tree at my mother's house. They can look very similar to walnut fruits or hedge apples; they can look like a hardened piece of pea-green taffy, and they can even be a deceptively fluffy-looking mass of slender thorny tendrils on a rose stem.
Iron Noder Challenge 2014, 2/30