Scarlet caterpillarclubCordyceps militaris

The scarlet caterpillarclub (Cordyceps militaris), also written scarlet caterpillar club, is a fungus that lives on moth pupae: in autumn a bright orange club pushes up through moss or grass from a pupa buried in the soil. It is the species grown for many cordyceps supplements, but it also grows wild across Britain and Europe, Ukraine included. It's small and easy to miss, so it's probably more common than records suggest.

Only several features together identify it. It's a single club 1–8 cm tall, with no cap, gills or pores. The upper part is slightly thicker, orange to scarlet orange, and finely pimply, like fine sandpaper. The stem is paler and smooth. If you carefully dig around the base you'll find a mummified pupa wrapped in white mycelium. Without the pupa it can't be told apart from orange club fungi such as Clavulinopsis, which grow in grass on their own.

Where does it grow in the UK and Europe? It's found in grassland, on woodland edges and in mossy mixed, broadleaf and pine woods, often on poor sandy or loamy soil. Of at least a dozen Cordyceps species recorded in Britain it is the commonest, yet it's still an infrequent find, partly because the clubs are usually shorter than the grass around them. It fruits from late summer through autumn, most often in mid autumn, and occasionally into early winter. If you find one, photograph it and leave it where it is.

How is Cordyceps militaris different from Cordyceps sinensis? They're different fungi, in different families. The caterpillar fungus of traditional Chinese medicine, now called Ophiocordyceps sinensis (yartsa gunbu), grows only high on the Tibetan Plateau and in the Himalaya, at 3,000–5,000 m, from the larva of a ghost moth rather than a pupa. It's scarce and can cost thousands of dollars a kilo, and it doesn't grow in Europe. Cordyceps militaris, by contrast, is easy to cultivate on rice, grain or silkworm pupae. So most products labelled cordyceps contain cultivated C. militaris or the fermented mycelium of related fungi, often sold as Cs-4, rather than wild sinensis. Their chemistry differs too: C. militaris contains cordycepin, while more recent work suggests O. sinensis doesn't make it.

What does the evidence say about the benefits? There is no convincing evidence that cordyceps treats any disease. Most of the data come from cell and animal studies. In people, C. militaris has been tested in a handful of small, short trials: healthy adults showed a modest rise in immune cell activity, and a mushroom blend containing it may have helped people tolerate high intensity exercise. A Cochrane review of Cordyceps sinensis for kidney disease found 22 trials, but their low quality meant no firm conclusions could be drawn. If you have a medical condition, a supplement is not a substitute for treatment.

What are the side effects and risks? Cordyceps may lower blood sugar and reduce platelet clumping, so it shouldn't be combined with insulin, diabetes medicines, warfarin or other blood thinners without talking to your doctor. One case of heavy bleeding after a tooth extraction has been reported in a person taking cordyceps daily. In Taiwan, two people developed lead poisoning from contaminated cordyceps powder, and wild Ophiocordyceps sinensis often contains high levels of arsenic. There are no safety data for pregnancy or breastfeeding. In the European Union, C. militaris is still classed as a novel food that hasn't yet been authorised for sale as food.

Is it the zombie fungus? No. The scarlet caterpillarclub parasitises moth pupae. The fungus that makes ants climb plant stems before it kills them, the inspiration for films and games, is a different species, Ophiocordyceps unilateralis from tropical forests. The real risks for people lie in supplements, covered above.

Is it edible? In China and Vietnam the cultivated clubs are cooked in soups, and China has approved them as food. Wild clubs are tiny, infrequent and of no culinary value in Europe, so we class it as inedible.

What else looks like it? Clavulinopsis species, small yellow to orange club fungi of lawns and unimproved grassland, are smooth, often twisted, have no pimples and no pupa underneath. Yellow stagshorn (Calocera viscosa) is yellow orange too, but branched like antlers, slippery and growing on conifer wood. Snaketongue truffleclub (Tolypocladium ophioglossoides) also rises from the ground, but its head is yellowish at first, then black, and its base leads to an underground false truffle. None of these is gathered for food, so the real risk with cordyceps lies in supplements, not in a mix-up in the woods.