Country diary: An autumnal netherworld of galls, blisters and lesions | Nic Wilson

Mid-autumn is surely the peak of conspicuous abundance in the countryside. Hazelnuts, crab apples and sloes ripen in the hedgerows, while musings on mists and mellow fruitfulness flow effortlessly from writers’ pens. And though it’s true that the scrubland beside the alder carr is glistening with the glossy lustre of rosehips this morning, the blackberries still plump and sweet after days of rain, it’s the underside of life that I’ve come to explore in all its excrescences and stains – a netherworld of galls, blisters and lesions.

Many of the trees on the Dump (the local name for this irrepressible patch of scrub) host fungal or invertebrate intruders. Hairy pimples (home to minuscule gall mites) protrude on stalks from the veins of field maple foliage, and upturned hazel leaves are peppered with a paprika-dusting of rust pustules. Squatting under a tall oak, a sycamore flashes black blotches haloed in yellow, pockmarks of the tar spot fungus. But the oaks themselves have the most populous underworld. Almost every leaf has an illicit dwelling – or a whole settlement – flourishing on its underside, most with incongruously stylish names such as silk button, smooth spangle, cherry and oyster gall.

Silk button galls are by far the most plentiful. They’ve erupted in clusters, thousands of mini-Cheerios produced by the trees in response to tiny parasitic gall wasps laying their eggs in the leaf tissue. Though only intended as homes for one, some of these gingery hoops are likely to contain uninvited housemates known as “inquilines”, laid in the galls by another species of wasp. Even more mind-blowing, several chalcid wasps prey on the gall wasps. These hyperparasites (parasites that live in or on other parasites) insert their long ovipositors through the gall wall to lay their eggs. Once their young hatch, they devour the host larvae inside like some demonic lodger.

These fungi, mites and wasps are unfairly maligned in some quarters. But though they fester, consume and disfigure, they rarely do any lasting damage and they’re living miracles in their own right. Autumn is as much about these underappreciated organisms and the role they play in healthy ecosystems as the hips, nuts and berries in the foragers’ hedgerows.

• Country diary is on Twitter/X at @gdncountrydiary

• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 20% discount