Facultative diapause is a life history trait that allows insects to undergo continuous development when conditions are favorable or to enter diapause when they are not. Insect voltinism can have an impact on the success of a weed-biological control agent because additional generations can increase agent population growth and reduce late-season recovery in the target weed.The most common factors that cue diapause are photoperiod and temperature; however, the role of nutrition is increasingly being recognized. We conducted a laboratory experiment to examine the effects of photoperiod and foliage age on diapause induction, pupal weight, and pupal development time in Hypena opulenta (Christoph) (Lepidoptera: Erebidae), a biological control agent for invasive swallow-worts in North America. A factorial experimental design was employed whereby H. opulenta was reared at long (16:8 h light:dark) and short (12:12 h) photoperiods on young and old swallow-wort foliage (Vincetoxicum rossicum Kleopow) Barbar. (Apocynaceae). Photoperiod was the only factor that affected diapause induction in H. opulenta. While foliage age did not affect diapause induction, it did affect pupal weight and pupal development time, with older foliage resulting in lower pupal weight and extended pupal phase. In field conditions, these impacts could affect pupal mortality through reduced winter preparedness and increased exposure to predators.These results support H. opulenta voltinism models based on photoperiods and indicate that the tendency of captive-reared H. opulenta to enter diapause after 2 or 3 generations, even in the absence of short photoperiods, is not a result of changing foliage age.