Tim Decker hoped some lichen he found in the California hills last summer would contain an exotic strain of yeast that would give a unique flavor to his home-brewed beer.

Mr. Decker put the lichen in a jar with a sugary solution to see if the liquid would ferment, a sign that yeast, which consumes sugars and releases alcohol, could be present. Weeks later, a small wormlike grub appeared, likely crawling out of a bit of wood attached to the lichen.

“I...