Fly agaric (Amanita muscaria)

Well known for its red and white-spotted cap, this fungus has a mutually-beneficial relationship with birch trees in the forest.

Black slug (Arion ater) eating a fly agaric fungus (Amanita muscaria).
Fly agaric fungi (Amanita muscaria).
Fly agaric fungus (Amanita muscaria) just beginning to open out.
Cap detail of fly agaric fungus (Amanita muscaria).
Fly agaric fungi (Amanita muscaria) & birch trees

Global distribution

The fly agaric fungus is widely distributed in forests and woodlands of the temperate and boreal regions of the northern hemisphere, including Europe, northern Asia and North America. Its range extends from the northern half of Alaska and northern Scandinavia, Russia and Japan southwards to montane areas of Costa Rica and the Hindu Kush mountain range between central Afghanistan and northern Pakistan.

The fly agaric also now occurs in South Africa, New Zealand and parts of Australia, including Tasmania, where it has been inadvertently introduced, most likely as spores accompanying non-native pine trees (Pinus spp.) for plantation forestry. In Australia and New Zealand it is now considered a problem species as it forms mycorrhizal relationships with native southern beeches (Nothofagus spp.), and is thought to be displacing some indigenous fungal species.

Two main subspecies have been described for the fly agaric within its natural range, with Amanita muscaria ssp. muscaria occurring in Europe and Asia, and Amanita muscaria ssp. flavivolvata being distributed in North and Central America.

Distribution in Scotland

In Scotland, the fly agaric occurs throughout the mainland, the larger islands such as Skye, Mull and Rum, and has also been recorded in the Outer Hebrides. It mainly occurs in association with birch trees, both silver birch (Betula pendula) and downy birch (Betula pubescens), which are widespread throughout the country, although it is also thought to grow with other tree species.

The fly agaric has not been assessed for its conservation status at an international level, so it does not feature on the IUCN Red List of Threatened Species. As a widespread and common species it is unlikely to be at risk, and is not considered to be of conservation concern.

The fly agaric is a fungus in the Amanitaceae family and its main body consists of microscopic filaments known as hyphae that exist in the soil, and persist throughout the year. The filaments are like tiny threads, and the network formed by them is known as a mycelium.

The mushrooms that appear in late summer or early autumn, are the fruiting bodies, and with the fly agaric, these are the characteristic red-capped fungi with white spots that are commonly depicte in children’s storybooks etc. Fruiting is thought to be triggered by a combination of conditions, including a reduction in daylight hours, falling temperatures and increased rainfall, and fly agaric mushrooms are usually visible in late August or September. 

Fruiting begins with the fungal hyphae coming together to form an egg-shaped structure that breaks through the soil surface as it grows and expands. The young, red fruiting body is covered in a universal veil that is white and splits apart as the stem grows up and the cap opens out. As the fruiting body matures, the cap becomes flat, sometimes with a slight depression in the centre, and grows up to 20 cm. in diameter, making it as large as a dinner plate. 

On the underside of the cap, the closely packed gills are white and free, meaning that they are not connected to the stem. The cap is borne on a white stipe, or stem, which can be from 5–20 cm. in height and up to 2 cm. wide. The stem will often have a white, flaccid ring around it, just below the cap. At the base of the stem is a swollen or bulbous section known as the volva, and this is the remains of the universal veil, the membraneous structure that covered the young fruit body when it was emerging from the soil. The spots on the cap are also remnants of the veil, although they can disappear over time as the cap expands and opens out.

Large quantities of white spores are released from the gills, and these are oval in shape and microscopic in size. They are dispersed by the wind over large distances, but only a few will germinate and grow successfully to become new fungi. 

Fly agaric mushrooms can appear either singly or in small clusters, and in the latter they can be at different stages of their development, even though they are growing right next to each other. Individual fruiting bodies usually persist for 1 – 3 weeks, with the cap often turning a paler red or even yellow colour as it ages, before the whole mushroom very quickly rots and decomposes. 

The fly agaric is poisonous, but ingestion of it is very rarely fatal. Its common name is derived from the fact that it is in the fungal taxonomic order Agaricales and historically it was used for killing flies by placing dry powdered material from its mushrooms in milk.

In Scotland the fly agaric is usually seen fruiting in close proximity to birch trees, and this is because it has an ectomycorrhizal relationship with them. In this symbiotic, mutually-beneficial relationship, the hyphae of the fungus wrap around the root hairs of a tree. In the exchange that takes place at the plant-fungus interface, the fungus obtains sugars that the tree produces by photosynthesis, while the tree gains nutrients, such as nitrogen and phosphorus, which the fungus takes up from the soil. The fly agaric also has mycorrhizal relationships with some conifers, and, in other parts of its range, other broadleaved trees (eg. montane oaks in the mountains of Costa Rica). 

Several species of parasitic fungi attack the fly agaric, including one in North America known as ‘Amanita mould’ (Hypomyces hyalinus). The peppery bolete fungus (Chalciporus piperatus) is suspected of being parasitic on the mycorrhizas of the fly agaric fungus. 

Like many species of gill fungi, the fly agaric is used by fungus gnats – small two-winged flies that lay their eggs in the fungus, and the larvae then feed on the fungal fruiting body as they grow. Six species of common fungus gnats that utilise a range of gill fungi have been recorded in association with the fly agaric in the UK, including one of the most widespread gnats (Mycetophila fungorum).

Several families of beetles have strong associations with fungi, and in the case of the fly agaric these include a pleasing fungus beetle (Tritoma bipustulata), which feeds on the fruiting bodies, and rove beetles (Gyrophaena spp.), the larvae and adults of which are thought to predate on small fungus gnats and also possibly to feed on the fungal spores. 

As with many fungi, the fly agaric is fed upon by slugs, including the European black slug (Arion ater). The fruiting bodies are also consumed by small mammals, including the red squirrel (Sciurus vulgaris), and these appear to be unaffected by the substances in the fungus that are toxic for humans. Fly agaric fungi are eaten by reindeer (Rangifer tarandus) throughout their range from the Baltic Sea to the Bering Straits. 

There is a long history of human cultural use of the fly agaric, based on the active ingredients muscimol and ibotenic acid, which are hallucinogenic, with records from Scandinavia, parts of eastern Europe and particularly Siberia.

 

Alan Watson Featherstone

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