The lesser grain borer, Rhyzopertha dominica, is a major pest of stored grain and is considered to have originated from India, but has subsequently spread throughout the world where grain is produced and stored. Adults lay eggs on the outside of kernels of grain and larvae after hatching from the egg tunnel into kernels and completing development to adulthood inside. Little is known about diapause in this species, so the influence of day length on development and adult reproduction was investigated. Surprisingly, R. dominica was sensitive to day-length, and their developmental duration was lengthened and their reproduction reduced when exposed to short-day photoperiods even under optimal temperatures. Our findings suggest that the number of generations of field populations could potentially be overestimated using just cumulative temperatures. How R. dominica evolved and adapted to such diverse environments at different latitudes remains to be explored.