Leaf-cutting ants are a group of species of leaf-chewing ants belonging to two genera, Atta and Acromyrmex (Hymenoptera - Formicidae). These amazing ants practice fungiculture, and have been doing this for about 50 million years. The bits of leaves and flowers that they bring to the nest get chewed up, fertilized, placed in suitable “fungus-gardens” within the nest, and seeded with fungal material from previous gardens. Obviously, fungal gardens can be overrun by unwanted species, with disastrous results for the colony. To keep this from happening, the ants depend on selective antifungals made by actinomycetes (Actinobacteria).
Leaf-cutting ants live in an intensely studied tripartite mutualism with the fungus Leucoagaricus gongylophorus, which provides food to the ants, and with two antibiotic-producing actinomycete bacteria: Pseudonocardia and Streptomyces.
The Pseudonocardia make an unusual antifungal called dentigerumycin, plus a relative of nystatin. On the other hand, a Streptomyces species associated with Acromyrmex octospinosus (pictured) was shown to produce the well-known antifungal candicidin. While the Pseudonocardia are vertically transmitted through the queen ants and may well have evolved with the ants (genome scanning has revealed that these Pseudonocardia have the genes needed to produce the nystatin-like antifungal), the candicidin production is widespread in environmental isolates of Streptomyces, so this could either be evidence of recruitment of useful actinomycetes from the environment…. smart ants.
Mushroom identification is tricky. (Oh before I go any further, did you notice these look kind of like dicks? It was pointed out to me on a couple mushroom identification sites, by what I can only assume were professional comedians.) Mushrooms change over time, starting as small knots of material that become spherical, then perhaps ovoid, before unveiling themselves and producing spores.
This group of mushrooms will absorb moisture, which will cause them to expand. The stalks will push the caps skyward, and the caps will spread wide, the smooth brown surface breaking into dozens of reddish brown scales on a white field. The underside of the cap, and the stem, are also white, though they will develop a reddish stain with age or bruising.
Helpful experts pointed me toward Leucoagaricus americanus*, for this fungus. The habitat is fairly distinctive: wood chips and tree removal sites. This was in a mulched area in a parking lot.