Scarlet Caterpillarclub
Cordyceps militaris
Also called orange caterpillar fungus, trooping cordyceps, scarlet caterpillar club
A small bright orange club fungus that grows out of the buried pupa or larva of a moth. Digging carefully beneath the club reveals the orange-stained insect from which it arises.
Overview
Scarlet Caterpillarclub (Cordyceps militaris) is the type species of Cordyceps, a genus of ascomycetes that parasitise insects and other arthropods. It is the most frequently encountered member of the group in temperate woodland, and by far the easiest to spot, because its clubs are a vivid orange that stands out sharply against dark litter and green moss.
Above ground the fungus looks deceptively like a small coral or club fungus: a simple, unbranched, club-shaped or slightly flattened stroma 2-6 cm tall. The upper, swollen portion is finely roughened, and a lens shows it to be studded with the mouths of embedded flask-shaped perithecia.
The key to identification is what lies beneath. Excavating gently with a knife or fingers follows a paler stalk down to a mummified moth pupa or caterpillar, its interior replaced by dense orange-white fungal tissue. Without this connection the identification is incomplete.
How to identify it
Stroma
Club-shaped, 2-6 cm tall and 3-8 mm thick, unbranched, occasionally slightly flattened, lobed or forked at the tip. The fertile head is bright orange to orange-red and takes up the upper half to two-thirds; it is minutely pimpled from the protruding tips of the perithecia. Several clubs may rise from a single host.
Stalk
The lower part is smoother, paler orange to yellowish or nearly whitish, cylindrical and slightly tapering, and runs down into the soil or moss to meet the host. It is tough and pliant rather than brittle.
Host
A moth pupa or, less often, a larva, buried in soil or litter, sometimes in a decayed cocoon. The insect keeps its outline but is hollow and packed with firm white to pale orange fungal tissue, and its cuticle is often stained orange where the mycelium has grown through.
Flesh and spores
Internal tissue is firm, fibrous and white to pale orange, without colour change on cutting. Spores are extremely long and thread-like, breaking into short segments, and are forcibly discharged from the perithecia; the deposit is whitish.
Where and when it grows
Scarlet Caterpillarclub grows in damp deciduous and mixed woodland, especially in mossy ground, deep leaf litter, grassy woodland rides, banks and stream sides where moth pupae overwinter in the soil. It is also recorded from pasture margins, dunes and gardens where suitable hosts occur.
It is widespread across Europe, North America and temperate Asia. Fruiting runs from late summer into late autumn, peaking after rain, and clubs may appear singly, in twos and threes from one host, or scattered in loose troops where many pupae lie buried in the same patch of ground.
Ecological role
The species is an entomopathogen. Spores landing on a caterpillar or pupa germinate and penetrate the cuticle, after which the fungus grows through the body cavity as yeast-like cells and then as mycelium, gradually replacing the internal tissue with a firm mass called a sclerotium that preserves the insect's outer shape.
When conditions are right, usually after the host has overwintered underground, the fungus pushes one or more stromata up through soil and litter. Perithecia mature in the orange head and forcibly discharge thread-like ascospores into the air, which fragment into infective units and settle on vegetation and soil where new hosts may pick them up. The whole cycle depends on a steady supply of soil-dwelling lepidopteran hosts, so the fungus tends to recur in the same patches year after year.
Similar species
Simple orange coral and club fungi such as Clavulinopsis fusiformis and Clavariadelphus species can look similar from above, but they arise directly from soil or litter with no insect at the base, and their surfaces are smooth rather than pimpled with perithecial openings. Digging under the club is the decisive test.
Among the insect-associated species, Tolypocladium ophioglossoides has an olive-black to brown club with a yellow stalk and attaches by yellow cords to a buried Elaphomyces truffle rather than to an insect. Ophiocordyceps gracilis produces a much slimmer, pale ochre to reddish club from a buried caterpillar and lacks the strong orange colour, while Ophiocordyceps entomorrhiza has a slender brownish stroma with a distinctly separate darker fertile head. Cordyceps militaris is separated from all of these by its short, stout, uniformly bright orange, roughened head and an arthropod host.