The determination of larval phase coloration in the African armyworm, Spodoptera exempta and its consequences for thermoregulation and protection from UV light

Spodoptera exempta exhibits a density‐dependent phase polyphenism in which caterpillars reared in isolation (phase solitaria ) tend to be green/brown and cryptic while those reared in groups (phase gregaria ) are black and highly visible. Differences in coloration between solitaria and gregaria phase larvae become apparent in the third instar and are pronounced by the final instar. Larval rearing densities as low as two larvae per 250 ml container are sufficient to induce gregaria coloration in 61% of the insects and at higher densities this is the only coloration expressed. Larvae transferred from isolated to crowded conditions or vice versa tend to retain or adopt gregaria phase coloration which suggests that this is highly advantageous. Black coloration is also induced in single larvae reared with other species of caterpillar ( S. littoralis ). Isolated larvae reared at low temperature tend to become melanic while crowded larvae reared at high temperature tend to be less melanised although they do not adopt the solitaria phase coloration. These results suggest that phase coloration is determined by non species‐specific inter‐larval contact, although it can be modulated by temperature. Under laboratory conditions, gregaria phase larvae heat up faster than solitaria ; this is partially a consequence of their black coloration but their smaller size is a more important factor. The cuticles of the gregaria larvae absorb significantly more ultraviolet radiation than those of solitaria but this does not confer any protection when larvae are irradiated at 254 nm.