The life history and host plant relationships of the mahogany webworm, Macalla thyrsisalis Walker (Lepidoptera: Pyralidae: Epipaschiinae), were investigated in Florida and the distribution of this species in the Americas was determined by examining museum specimens in the U.S. National Museum of Natural History. Macalla thyrsisalis is distributed throughout the natural range of American mahoganies, Swietenia spp., and some localities where mahoganies have been introduced. In Florida, natural infestations have been observed only on native West Indies mahogany, S. mahagoni Jacquin, but larvae feed on leaves of some close relatives of this species if placed on them. Honduras mahogany (S. macrophylla King) and Spanish-cedar (Cedrela odorata L.) are suspected to be hosts in Central and South America. In Florida, the adult moths fly in early spring. Larvae spin webs and feed on mahogany leaves during the ca. 5-week period of the spring flush, then leave the trees to pupate. Larval populations greatly diminish by early summer due to unknown factors. Parasitoids include Lespesia n. sp. (Diptera: Tachinidae), reared from 8.8% of 25 mahogany webworms, and two species of Braconidae: Habrobracon sp. near hebetor Say and Apanteles sp. Apparent relationships between mahogany webworm populations, spring leaf flush of mahoganies and photoperiodic and climatic factors are discussed.