Oak Gall Wasps
Oak Gall on Pedunculate Oak (Quercus robur). Photo by Kim Lake
On our most recent visit to Cloudforests One, we noticed some of the Oak trees had again attracted the attention of Oak Gall Wasps, who were busy making galls on the young stalks.
Oak galls are caused by gall wasps, also called cynipid wasps. Between 40 and 50 of the 70+ species of cynipid wasps known to be present in Ireland are associated with Oak trees.
The gall wasps themselves are usually very small - between 1 mm and 8 mm in size - and seem to be very under-reported here in Ireland. In fact, the gall itself is probably better known than the wasp inside it!
The wasp lays its eggs into the tree - where the eggs are laid can depend on the species or even the time of year! It could be the leaf axil, acorn, or stem - in our case, it looks to be at the axil where a new leaf would have developed.
Once it hatches, the larvae secretes chemicals that prompt the plant to grow a gall around it. The larvae then feeds on the nutritious tissue that develops within the gall, pupates, and eventually emerges from the gall as an adult wasp.
So the gall serves not only as a food source, but also as a protective home to keep the wasp larvae and pupae safe from predators.
The galls are green and a living part of the tree while the wasp larvae are living in there and secreting the growth chemicals. When the wasp leaves, the gall become brittle and turns brown, like in our photo. If you look closely, you can see a tiny hole on the top gall where the wasp emerged from its childhood home!
So do these wasps harm the trees or other animals?
Well, they are parasitic in that they live off the host plant - however, they don’t appear to have a huge impact on the overall health of the tree. Species that cause galls to form on the acorn itself may impact the overall number of healthy acorns produced, so there may be some impact if trees become hugely infested and acorns weren’t available to serve other animals as a food source.
From what we’ve seen so far in Cloudforest One, the number of galls on any particular oak is limited, and as the gall only affects the area it’s growing in, the rest of the tree carries on with its business as normal. Of course, we’re open to correction on that last part, but for now, we’re just going to enjoy these little oak apples, and the knowledge that it’s providing a home and food for the little critter inside!
