Surrey365; Day 41. A Galling Tale.
A bit more about galls today. Galls are growths on plants caused by parasites; wasps or flies, often, but also fungi and even bacteria. The galls are actually formed by the plant itself, influenced to do so by chemicals released by the parasite. The mighty oak tree supports over 280 species of insects, including 79 gall-forming species. Some of these are so tiny that you wouldn’t notice them unless you knew what to look for, and others are noticeable by their weird forms and colours. The thing that fascinates me about galls is that a huge tree, with a lifespan of centuries, can be controlled and manipulated by a microscopic larva with a lifespan of a few months. As an example, the wasp that causes the Marble galls I showed yesterday lays her egg in the bud of a Sessile or Pendunculate oak (our two native species). When the egg hatches, the larva releases chemical messengers which instruct the tree to form a structure around the larva, providing it with food and protection. This structure is nothing like what the tree would naturally produce; the larvae rewrite the tree’s blueprint for its own tissues, co-opting the tree’s resources to provide the perfect nursery. The gall shape depends on the species of parasitic insect, but is always the same for that species, varying from tiny, doughnut-shaped Silk Buttons, or the spaceship-shaped Spangle galls, right up to the gall in today’s picture, the multi-celled Oak Apple.
The other interesting thing is that oak gall wasps have alternate generations, one sexual (ie male and female mate, then the female lays eggs) and one asexual, or agamic, in which all the insects are female, and don’t require a male to fertilise their eggs. These generations look completely different to each other, and have different behaviours and sometimes host plants. In the case of the Marble gall wasp, the male and female wasps emerge from small galls on the non-native Turkey oak, and set off to find a native oak tree. There they mate, and the female injects her eggs into the buds. The larvae grow inside their gall nursery, eventually drilling their way out (leaving the tiny holes you can see in vacated galls) and emerging as the asexual female generation. She flies off in search of a Turkey oak, lays her eggs there, and the cycle begins again. For the Oak Apple gall wasp, the sexual generation of Biorhiza pallida emerge from the gall, and the mated female lays her eggs in the roots of the oak. These form little marble-like galls, from which emerge an asexual generation of females, which lay their eggs on the twigs, and it starts again.
There’s another chapter to this strange tale… the many species of insect which parasitise these parasites! There are inquilines, who are basically squatters; moving in without persmission and often hogging the resources of the gall larvae. Then there are the parasitoids, who are the really scary ones; their eggs are laid directly into the larvae, and they eat their host from the inside out. But maybe I’ll save that for another day, and leave you with a picture. 😉