Abstract Larval density, photoperiod and periodic changing of the food supply were investigated as possible causal factors in the retardation of development of the mature larvae of Gnatocerus cornutus (F.). Four larval densities, 1, 2, 4 and 8 larvae per gram of food, were exposed to photoperiods of 0, 12, 15 and 24 h per day. The effect of changing the food at 3-weekly intervals was investigated with 2 and 4 larvae per gram of food. The mean times for development from larval to adult stages, and survival rates revealed that increased larval density significantly increased development time, mortality and the incidence of cannibalism. Changes in photoperiod and food had little effect, showing that the effect of larval density is independent of lighting conditions and changing of the food medium.