In teak plantations at Nilambur, Kerala,Hyblaea puera Cramer (Lepidoptera, Hyblaeidae) caused one or two waves of epidemic defoliations between late April and July, followed in some years by isolated, lighter defoliation between August and November. The insect was absent at other periods, although with a 3-week life cycle it can theoretically pass through several generations per year. The temporal and spatial distribution of infestation and certain behavioural characteristics of populations gave evidence of short-range migration of newly emerged moths. In a model proposed for population dynamics ofH. puera, no diapause occurs and a residual, non-migratory population exists in natural forests during the nonepidemic period. With the onset of general flushing of teak in February–March, the population starts building up and when a critical density is reached, migratory behaviour is triggered. Migration facilitates exploitation of new food sources and escape from larval parasites. Generally, after one or two epidemics, the population declines due to leaf maturity, natural enemies and density-dependent food depletion. This cycle of ups and downs, with attendant transition between migratory and non-migratory phases is repeated every year. If some steps of the proposed model are confirmed by further study, simple methods could be devised to manage this serious pest of teak.