Assessment Notes

Justification

Butyriboletus frostii can be found throughout eastern North America, areas of southwest United States, Mexico and into Central America. It is considered rare and can be fairly common in eastern North America and Mexico. Its status in southwest United States and Central America is more uncertain. The slow decline in hardwood forests in some areas of its range may correspond with similar declines in B. frostii. However, overall it is not thought to approach the thresholds for consideration as threatened and so assessed as Least Concern.

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Taxonomic notes

Currently placed within the genus Butyriboletus (Wu et al., 2016). Was moved from the genus, Exudoporus (Vizzini, 2014), and before that from Boletus. There is a subspecies in Florida, Exudoporus frostii floridanus, sufficiently distinct as to be considered separately (Singer, 1947). It remains unclear whether the southern range of B. frostii is the subspecies floridanus, Nuhn et al. (2013) place the records from Costa Rica into this subspecies.

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Geographic range

Butyriboletus frostii can be found throughout eastern North America, areas of southwest United States, Mexico and into Central America (Kuo 2020). Aside from Costa Rica, its distribution elsewhere in Central America is unknown.

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Population and Trends

More research is needed on the population size and trends. It is not considered rare and can be fairly common in eastern North America and Mexico. Its status in southwest USA and Central America is more uncertain. Slow declines of hardwood forests in some areas of its range may correspond with similar declines in this species.

Population Trend: Decreasing

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Habitat and Ecology

It forms mycorrhizal associations with many hardwood trees, particularly oaks (Morris et al. 2009), but also pine (Vozzo and Hackskaylo 1961) and Madrone in Mexico (Arora 1986). It has been reported as common in moderately dense oak forests (Singer 1947) and on sandy soils (Smith and Weber 1980). It forms sporocoarps between July and October, producing mushrooms either singly or in groups.

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Threats

The species is impacted by the reduction of substrate available to it, namely oak (Bessette et al. 2017). The oak population of eastern North America is threatened by the impacts of climate change, invasive species, growing deer populations, and sociopolitical land disputes that disrupt adequate land management processes (Dey 2014).

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Conservation Actions

Preservation of old growth oak forests and regeneration of oak populations will help bolster Butyriboletus frostii by providing sufficient habitat for subpopulations to grow. More specific research on any direct threats to B. frostii would be a useful in addition to the current body of research, as much is already known about indirect threats to B. frostii via reduction of its main substrate: oak. Additional surveys are also needed to better understand the B. frostii population size and trends, especially in the more isolated areas of its range (southwest of the United States and Central America).

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Use and Trade

Butyriboletus frostii is an edible mushroom harvested by foragers. It is described as having a fruity taste (Alabama Mushroom Society 2018).

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Source and Citation

Mueller, G.M. 2025. Butyriboletus frostii. The IUCN Red List of Threatened Species 2025: e.T273228947A273243796. Accessed on 22 November 2025.

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• Country occurrence