Moisture and distribution of a keratophagous moth, Tinea occidentella

Keratophagous insects consume proteinaceous foods (animal hair, feathers, and other substances containing keratin) that are low or even lacking in water, which necessitates a source of moisture beyond the primary diet. Tinea occidentella Chambers 1880 (Lepidoptera, Tineidae), which feeds upon the keratin in fur and feathers in mammalian carnivore scat and in pellets of birds of prey (foods very low in water), affords an understanding of the ecology of keratophagous insects in nature. The common name “western clothes moth” is a misnomer as it does not eat clothes but only scat and pellets. In laboratory experiments, we found that larvae did not absorb water vapour directly from the atmosphere, and they died at 45% rh–55% relative humidity (rh). We found no evidence that larvae drank. Larvae grew normally, had high survival, pupation, and eclosion only when feeding at very high (>88%–99%) rh upon fur from pellets and scat; the fur absorbed water from the atmosphere. While scat and pellets are abundant throughout North America, T. occidentella is mostly restricted to the moist, mild climates of coastal central and southern California, where fog, dew, and morning high rh of 99% are common. Outside of this coastal envelope where growing season climate is warmer, drier, and becoming more so, the species is rare. An intriguing notion, requiring research beyond the present report, is that the warming and drying climate of southwestern North America will contract the range of this insect. The larvae of Tinea occidentella feed upon the keratinaceous, undigested fur and feathers in the pellets of raptors and the scat of carnivores, protein containing no water. The species is concentrated in coastal regions with very high relative humidity, develops upon food that has absorbed moisture from the atmosphere, and fails to develop at lower relative humidity. The warming and drying of southwestern North America could lead to a shrinking range of this insect.