Survival of soil-dwelling larvae of the polyphagous whitefringed beetle, Graphognathus leucoloma (Boheman), increased by ≈2-3-fold in annual pasture irrigated, but otherwise undisturbed, in a summer-dry environment. Rate of larval development was unaffected by the unseasonal moisture. The next generation of larvae appeared in late summer, ≈2 mo earlier than usual. The primary cause was oogenesis induced by unseasonable high-quality food plants resulting from the irrigation; moisture probably enhanced egg survival and induced hatching. Fewer larvae in an irrigated potato crop than in both wet and dry pasture indicated that deleterious effects of cultivation outweighed the survival-enhancing effects of irrigation. However, whitefringed beetle abundance, and hence its pest potential, are likely to be increased by irrigation in infrequently cultivated perennials such as alfalfa.