Long-term cold storage of a commercial biological control agent for aphids, the predatory midge Aphidoletes aphidimyza (Rondani), was investigated as a way to reduce rearing costs and to preserve genetically important lines. Prolonged cold storage of last instars in cocoons at temperatures of 1-11 degree C in total darkness was sufficient as an environmental cue to induce diapause, even though larvae had been reared under conditions that would not induce diapause. Mortality and emergence patterns of midges diapausing in response to cold storage did not differ from those of larvae reared under diapause-inducing conditions (8-h day at 21 degree C, 16-h night at 15 or 18 degree C) before cold storage. For commercial application, cold storage regimes with lowest mortality (< 10%) and highest percentage of emergence in the first 4 d of the adult emergence period were: 2 wk at 10-11 degree C, up to 4 wk at 5 degree C, and up to 2 mo at 1 degree C after acclimation for 10 d at 5 degree C. Larvae stored at 5 degree C survived 8 mo with <9%, mortality, and fecundity of females was not significantly different than in unstored controls. These effects are useful for preserving A. aphidimyza lines for future research. However, the adult emergence period was too protracted for commercial application. Methods for synchronizing emergence (including cold chill at 1 degree C and 2-4-d heat shock at 33 degree C) were tested, but only desiccation for a week, followed by remoistening, gave promising results.