After the cherry tree was cut down, I stacked some of the wood into a couple of piles so that it could rot down and continue to provide a habitat for the various wood-boring insects, and those who use the holes and tunnels once they are finished with by the original excavator. Looking through some of the pieces of wood, I saw fragments of metallic green insect body parts, and in looking for answers, for a few short and glorious days, I thought they might be the remains of the rare and endangered noble chafer beetle (Gnorimus nobilis). How exciting that would be.
What I thought might have been the remains of a noble chafer beetle
I got in touch with the RHS Advisory Service to ask for an opinion and it was the ever-helpful Andrew Halstead who replied, saying he thought the fragments were the remains, not of a rare species of beetle, but of common greenbottle flies, maybe Lucilia sericata. It was a bit of an ‘Oh…’ moment and slightly disappointing, but good to be put on the right track and it motivated me to look more closely at a creature I hadn’t previously given attention.
Going back to the pictures of those metallic green fragments and comparing them to pictures I found on various websites about greenbottle flies, I could see the similarities – dimples where hairs had been, the hard exoskeleton of the abdomen, sections of wing.
Another find was a larva in tunnel with a food supply. This bit of wood had broken open, so the larva probably won’t survive, but I did see the presence of pupae, which should survive and will leave the rest of the pile well alone, so the species with in it can develop without further interference from me.
A larva in a tunnel provisioned with flies
Next question: how did the greenbottle flies get into the tunnels inside the tree? If I hadn’t truly wanted them to be noble chafer beetles, and not flies, Andrew Halstead’s opinion should have jumped out at me. The flies, he said, “would have been captured by a species of solitary wasp and paralysed by the wasp's sting. Some species of solitary wasp make their nest sites by tunnelling in rotten wood. Once the tunnel has been made, the female wasp provisions the nest with a store of insects. Eggs will be laid and in due course the wasp's larvae will feed on the paralysed flies. There are many species of solitary wasps and various species will use other insects, such as caterpillars, weevils, leafhoppers, small flies or aphids as the food store for their larvae”. That raised another question: which solitary wasp could it be?
One of the tunnels in the rotting wood
Another wood pile nearby, this one in the courtyard, had already become home to solitary wasps of the genus Ectemnius. I’d seen them going in and out of their holes in the early autumn of 2010. Comparing the holes and tunnels in that wood pile with the new wood pile in the garden suggests that Ectemnius wasps could also be making use of the cherry tree. This is good news; solitary wasps are beneficial and useful to have around. They are what I call ‘cleaners’, taking pest insects such as aphids and chewing through rotting wood so it decomposes and returns to the soil. Once the snow has melted, I shall go and examine the holes again and see if I can spot one. Until then, this has been another pleasurable journey in finding things out.