Zombie-Ant Fungus Identification Guide

What You Are Actually Looking For

This is not a mushroom in the ordinary sense and you will never find it by scanning the ground for a cap. What you are looking for is a dead carpenter ant with a dark wire growing out of its back, clamped to the underside of a leaf at about knee height. Once that search image is loaded, specimens start appearing in places you had walked past a hundred times.

The Setting

The classic habitat is humid lowland and montane broadleaf rainforest in the tropics and subtropics, with related members of the complex reaching warm-temperate forest in the south-eastern United States, eastern Asia and eastern Australia. The productive ground is shaded forest interior with still, saturated air: stream valleys, the base of slopes, dense sapling regeneration under a closed canopy.

Work the understorey between ankle and waist height and look at the undersides of leaves, not the tops. Walk slowly with the light coming through the leaf toward you, and an attached ant reads as a hard dark silhouette against the translucent blade. Specimens are strongly aggregated: you can cross a hundred metres of forest with nothing, then find several dozen ants within a few square metres. Those aggregations, often called graveyards, sit under the colonies whose foragers are affected, so once you find one, search the neighbouring plants carefully before moving on.

Step One: Read the Ant

The ant is normally a carpenter ant, Camponotus, and in parts of Asia a spiny Polyrhachis, typically 6 to 15 mm long. It is dead, dry, dull and slightly darkened, with the legs splayed or drawn under the body and the antennae often still in place.

The critical character is posture. The mandibles are locked onto a leaf vein, a leaf edge or a twig, and they do not release when you tug the body gently. On leaves the bite is usually on a major vein on the lower surface, and the body hangs beneath, aligned along or across the vein. That fixed bite, held long after death, is the first thing to confirm, because it separates a genuinely colonised ant from an ant that simply died in place.

Check also for a thin dull greyish bloom of hyphae emerging along the sutures of the cuticle, and note whether the abdomen looks shrunken and hollow.

Step Two: The Stroma

From the back of the head or, far more usually, the dorsal surface of the pronotum, the top plate of the first thoracic segment, a single unbranched stalk emerges. Two stalks occur occasionally; more is unusual.

The stalk is 0.5 to 2.5 cm long, sometimes reaching 4 to 5 cm in vigorous material, and only 0.2 to 0.6 mm thick. It is cylindrical to slightly flattened, dark brown to nearly black over most of its length, often paler brown or reddish toward the tip, and it ends in a sterile point. Handle it: it is tough and springy, bending and returning rather than snapping like a plant hair. It is typically straight or gently curved, directed away from the leaf surface so that the ant hangs beneath it.

Step Three: The One-Sided Perithecial Cushion

This is the character the species name refers to and the one that makes the identification. Somewhere along the upper half to two-thirds of the stalk, a distinct swelling or plate develops on one side only. It is an asymmetrical cushion roughly 1 to 4 mm long, darker than the rest of the stalk, matte, and finely pimpled.

Under a hand lens each pimple is the projecting neck, the ostiole, of a flask-shaped perithecium embedded obliquely in the cushion, and they sit in rough rows or irregular clusters. Because the cushion is confined to one flank, the stalk in silhouette looks lopsided, like a wire with a bud on its side. That asymmetry is diagnostic against relatives whose perithecia ring the stalk evenly or gather into a terminal head.

If you can take a razor section, the perithecia are ovoid to flask-shaped, on the order of 200 to 300 micrometres, each with a narrow neck reaching the surface and a cavity packed with long cylindrical asci.

Step Four: The Asexual Growth

Look at the joints. Short white to pale grey tufts and synnemata of the asexual state erupt from the leg joints, the antennal sockets and the membranes between abdominal segments. They are small, usually well under 2 mm, fine and pale, and they brush off easily.

These are a different structure from the dark stroma, and the two together, a pale fuzz at the articulations plus one dark wire from the thorax, are a strong combination. Do not mistake the pale tufts for a separate organism growing on the specimen.

Getting Spores

There is no conventional spore print here, because the spores form inside enclosed perithecia and are forcibly shot outward rather than dropped from an open surface.

To capture them, cut the leaf with the ant still attached and pin it inside a closed clear box with a glass slide or a square of black card positioned 1 to 2 cm below and slightly to the side that the cushion faces. Add a damp tissue at one end of the box to keep the air humid but do not wet the specimen itself. Leave it at room temperature for 12 to 36 hours and a fine whitish bloom appears on the slide, offset in the direction of discharge rather than directly beneath the stalk.

Under the microscope the ascospores are extremely long and narrow, thread-like and multiseptate. On a slide left damp for a few hours many of them germinate to produce short lateral stalks bearing small secondary spores, the capilliconidia, which are among the best confirmations you can get without sequencing.

Season and Conditions

In aseasonal rainforest specimens can be found year-round, with peaks in the wettest months. In seasonal and warm-temperate forest, fruiting concentrates in the humid season and ants are more often attached to twigs and bark than to leaves.

Stromata develop slowly over weeks, and old cadavers persist long after discharge is finished, so a good proportion of what you find is spent. The perithecial cushion decays and sloughs first, leaving a bare wire, so an old specimen may need to be scored as the complex without a species-level read.

Separating the Look-alikes on Structure Alone

• Beauveria bassiana coats the entire insect in a dense white powdery to woolly bloom, with no stalk and no perithecia, and it takes hosts across many insect orders.
• Akanthomyces and Gibellula species, mostly on spiders and moths, form pale, soft synnemata over the body rather than a single tough dark wire.
• Other Ophiocordyceps on ants, such as the myrmecophila group, produce perithecia that ring the stalk or gather in a terminal head, with no one-sided cushion.
• Ophiocordyceps nutans grows from true bugs on a long black wiry stalk, but its fertile part is a bright orange to red ellipsoid head at the tip, and the host is a shield bug rather than an ant.
• Cordyceps militaris and its relatives push a club-shaped orange stroma out of a buried moth pupa in soil or litter, not from an insect clamped above ground.
• Entomophthoralean fungi such as Entomophthora and Pandora leave flies glued to stem tips with the wings raised and white bands of hyphae bulging between the abdominal segments, and produce no stalk of any kind.
• Within the unilateralis complex itself, many regional species differ chiefly in the identity of the ant, the length of the stalk and the exact position of the cushion. Record the ant, the plant, the height above ground and the locality: morphology alone will get you confidently to the complex and not always past it.

Field Checklist

• Dead carpenter ant, mandibles locked on a leaf vein or twig, 20 to 30 cm above ground, on the leaf underside.
• Several to many specimens clustered in a small area.
• One unbranched dark wiry stalk from the dorsal pronotum or head.
• Stalk tough and springy, 0.5 to 2.5 cm, with a sterile pointed tip.
• An asymmetrical darker cushion on one flank of the upper stalk, finely pimpled with ostioles.
• Pale short tufts at leg joints and body sutures.
• Thread-like multiseptate ascospores, often germinating into capilliconidia.
• Humid shaded forest interior, wet season or year-round in rainforest.