While some invertebrates are generalists, and feed on or live in a wide variety of trees, many can only be found on certain species, or a very small number of species. Sometimes the name is a dead giveaway – oak apple gall wasps, the blue willow beetle etc. But some can be somewhat inaccurate – for instance the oak eggar moth does not feed on oaks, but gets its name due to the cocoon resembling an acorn.
Under the Bark
Peel back any loose bark and you’ll almost certainly find some invertebrates. Often woodlice will be the first thing you find, and usually lots of them. But also under the bark you can find beetles, spiders, centipedes, millipedes and quite likely other kinds of invertebrates as well. Among my favourite invertebrates behind bark are the larvae of cobweb beetles, which steal the remains of dead insects from spider webs. If they are caught by the spiders, the erect their long bristles so that the spiders can’t bite them.
Blue Willow Beetles
On Top of the Leaves
On top of the leaves you will find caterpillars, aphids and other invertebrates feeding on the leaves. Leaf-cutter bees cut pieces out of leaves and use them to make their nests. Leaves are also regularly used by insects for resting on or basking, especially if the leaves have direct sunlight on them.
Inside the Leaves and Buds
If you look at a horse-chestnut trees in late summer/early autumn, you will almost certainly notice dead patches. These are leaf mines caused by the caterpillar of the horse-chestnut leaf miner moth. The damage to the tree is largely superficial, as the caterpillar attacks late in the growing season, and there is no evidence to suggest that an infestation can lead to death. There are many different kinds of leaf-miners, and not all of them are moth caterpillars. The holly leaf-miner is the larvae of a fly. Many leaf-mines can easily be identified by the species of tree and the shape and colour of the mine.
Leaf galls take many different forms, including small and large bumps on the leaf surface, and plate-like spangle galls. Some galls are caused by fungi, while others are caused by the larvae of flies, wasps, sawflies, aphids, psyllids, scale insects and mites. Galls are not confined to the leaf, and can in appear in other parts of the tree such as the bud. The oak apple gall and the oak marble gall are usually very easy to find and are caused by tiny gall wasps. The striking robin’s pincushion on dog-rose is caused by a mite.
Underneath the Leaves
Leaves provide shelter for many invertebrates. Look under large leaves during rain and you will often find insects taking refuge there. Some caterpillars and aphids can also be found feeding underneath the leaves.
In the Canopy
Some butterflies are very difficult to spot because they spend most of their time high up in the canopy. The purple emperor a rare species, and even if you know of a wood where they are present, you are unlikely to see one. However, they do occasionally come to ground for food and water, and there are various methods of luring the males to the ground. The purple hairstreak is a more common species that has similar habitats.
In the Fungus
While not technically in the tree itself, many species of fungi only grow on certain types of tree. Opening up a large, soft bracket fungus (such as dryad’s saddle) will often reveal a whole host invertebrates including several species of woodlice, shining fungus beetles, small tree-fungus beetles, fungus weevil, rove beetles, earwigs and fly larvae. Some species are generalists and can be found on many different species of fungi, whereas some species only feed on one or two species of fungi. In woodlands near the Ecosulis head office is the scarce fungus weevil, the larvae of which lives in King Alfred’s cakes fungus – first found by our Managing Director Cain on the back of my shirt during a lunchtime walk.
In Rot Holes
Rot holes in trees are fascinating. Both dry and wet rot holes provide a habitat for a wide range of invertebrate species. Woodlice are numerous in these places, brown tree ants and hornets make their nests in them and many species of beetle lay their eggs in them, including lesser stag beetles, noble chafer beetles and rhinoceros beetles. Wet rot holes provide suitable habitat for gnats and the larvae of some hoverflies and crane-flies. Rot holes can also provide suitable habitat for nesting birds and roosting bats.
Inside Dead Branches and Trunks
The larvae of many species of beetle live inside dead branches and tree trunks. Longhorn beetles are among the most conspicuous of the adults, and often have interesting patterns and huge antennae. “D”-shaped exit holes in branches are almost certainly caused by jewel beetles. Holes of this shape in hawthorn trees are always from the hawthorn jewel beetle, which was formerly thought to be scarce until this method of recording them was used. The adults are very elusive, however, and despite having recorded them on many sites across the UK, I have still yet to see one!
The Old Ones are the Best…
Large veteran trees usually harbour many more invertebrate species than younger trees as they contain a wider variety of features. Such trees are of immense value to nature conservation and the food chain, and should be preserved wherever possible. But trees of all sizes are also important to invertebrates, and any trees can usually yield at least a few species
Ecosulis' ecological consultancy team undertakes invertebrate surveys associated with trees and all other habitats across the UK.