Laboratory studies were carried out to determine the effect of high larval densities of Plodia interpunctella (Hubner) on the development and survival of the moth in two food media. Mortality was found to be higher when reared on groundnuts than on artificial diet. Mortality was between 6.6 and 13.3% when larval densities were below five larvae on 3 g of both groundnuts and the artificial diet in 75 × 25 mm tubes. At higher larval densities (10 or more larvae on 3 g of medium) mortality ranged from 22.5 to 60% on the artificial diet, and on groundnuts it was between 30 and 65%. In both media, development period was prolonged and weights of moths at emergence was lower when larval densities were greater than 10 per tube.

When high densities were experienced between the first and the fourth instars individuals had protracted development, high mortality at larval stage and reduced weights of emerged adults and those subjected to high densities only at the final instars were less significantly affected. It seems that the fourth larval instar is the stage most sensitive to high densities.

Laboratory studies were carried out to determine factors which induce larval diapause in a Nigerian strain of Plodia interpunctella.

Larvae reared at constant temperatures of 20 or 30°C did not enter diapause. However, 13.3 and 37.7% respectively of larvae reared for 8 or 12 days (third or fourth instars) at 30°C and then transferred to 20°C entered diapause. Larvae transferred to 20°C immediately after hatch (first instar) and after 16 days (final instar) did not enter diapause. Developmental periods from egg-hatch to adult of diapausing individuals ranged from 136 to 151 days, while those of non-diapausing individuals reared at constant temperatures of 30 and 20°C ranged from 23 to 34 days and 60 to 72 days, respectively.

Different photoperiods did not induce diapause when reared at constant temperatures but exposure of larvae that experienced a drop in temperature from 30 to 20°C, at 4th instar stage to continuous light or to a long photoperiod (light 16 hr: dark 8 hr) caused a drop in the percentage larvae that diapaused.

Larvae reared at high larval density at 30°C did not enter diapause, and increased mortality was observed.