Country diary: a fearsome insect assassin has moved in to our log pile

Phil Gates

Late June 1964, and I had recently been discharged from hospital, after coming close to losing my sight. My parents bought me an inspired gift, a camera, to keep me occupied during convalescence and I spent many happy, sometimes frustrating, hours learning to photograph insects in the garden. That is when I first encountered Rhyssa persuasoria, Britain’s largest ichneumon wasp.

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It had landed on the fence: a black, three-inch-long, hyperactive insect with red legs and twitching antennae, whose slender ovipositor – harmless, but resembling a sting – accounted for half its length and for its common name: sabre wasp. But that sword-like tail appendage is really a scabbard, sheathing an egg-laying drill that is almost as fine as horse hair.

I raised the camera, click: out of focus, just a blur when I developed the film. Disappointment. That was the last I saw of this extraordinary parasitoid species until yesterday, 56 years later.

It was fluttering against the window, with amber tinted wings that seemed too small to support such an ungainly body. I trapped it under an upturned glass and this time had a better camera, within easy reach.

Its first priority was to groom those long antennae. They are the portal to a sensory world that no human can experience, so sensitive that they can precisely locate wood-boring insect larvae hidden deep inside a tree trunk. It was once thought that vibrations caused by chewing jaws gave away prey location, but scent seems to be its victim’s undoing. When the ichneumon arches its body and begins exploratory drilling it is guided by the odour of the grub’s frass inside its tunnel. The laying of a single egg seals the host’s fate, to be slowly eaten alive when the parasitoid’s larva hatches.

Rhyssa persuasoria is typically a species of forests. During the 35 years we’ve lived here I have had to cut down cypresses, a walnut, a hornbeam and a whitebeam that grew too tall for our modest garden, but always stacked the timber under the hedges as a habitat for wood-boring insects. Build log piles and they might come, I hoped. Evidently, they have, luring in this mesmerising assassin, which until yesterday was just a childhood memory.