Nature Notes April 2024

Sometimes something really interesting appears from the most unpromising material. Last month I picked up an old, dead Oak leaf whilst I was out walking in Normanton Woods. My eye was caught by an old spherical gall attached to the leaf, a sure sign that the gall was formed in response to parasitism by a Cherry Gall Wasp. When the wasp lays an egg on the leaf of an Oak tree in autumn, the tree forms a 25mm gall, usually yellow-green and suffused with pink or red. During the winter, the galls turn dark red and then brown as the leaves fall in the winter.

The wasps that cause the galls are of the genus Andricus. I haven’t ever seen the species that causes Cherry Galls, but the one shown here is the closely relaed causer of Oak Marble galls. I kept my old brown Cherry Gall until the middle of April when I noticed that a small wasp had emerged from the gall. However, unlike the dull brown Andricus galls, this one was bright, metallic green. After a lot of searching and a visit to a Facebook page specialising in galls the green wasp was identified as Torymus auratus. This is a parasitic wasp that infests Andricus wasps. When it identifies a Cherry Gall, the adult female Torymus uses her long ovipositor to drill down into the gall and lays an egg in the central cavity where the Andricus larva is located. The Torymus gall emerges and then kills and eats the larva. It takes over possession of the gall for the rest of the winter, eventually developing into an adult. This emerges from the gall in the spring, as the one that I collected did, appearing as a tiny adult wasp (only 3.5mm in length, with a 4mm ovipositor). It then searches for a male before scanning Oak trees for a Cherry Wasp Gall – it then completes its life cycle.

All very confusing, but an example of the remarkable lifestyles of our smaller invertebrates. A tiny wasp lays an egg on an Oak leaf; the tree forms a gall around the parasite; a second species of wasp locates the gall and parasitises the first wasp larva; the adult parasitic wasp then emerges to continue its life cycle. All of this is going on right under our noses and we would never suspect what is happening, with even the galls often escaping notice. Some Oak galls have been used in the distant past for preparing a dark ink after being crushed and mixed with water and honey, sulphuric acid and gum arabic, together with iron shavings from the bottom of cauldrons to make a blue/black ink. Many medieval manuscripts (including the Magna Carta) were prepared using ink of this type. A little bit of history and lots of biology, all in an inconspicuous gall.