The timing of critical events like mating, migration, and development has noticeably and recently shifted in many populations of diverse organisms. Here, we report a change in the breeding phenology of giant waterbugs, Belostoma flumineum Say (Heteroptera, Belostomatidae), in the northeastern United States. Waterbugs collected in 2005 and 2006 exhibited previously typical patterns of mating and reproduction: two annual reproductive peaks in which overwintered adults mated in the spring and young adults from a new generation mated in the fall. In 2012 and 2015, despite similar sampling effort, we detected no fall breeding activity in the study area. Reproductive behaviour under controlled laboratory conditions was also different between the earlier (2005 and 2006) and recent (2012 and 2015) years: waterbugs collected in recent years exhibited significant delays in reproduction (>30 days) under similar photoperiod and thermal conditions. We discuss potential causes of this dramatic change in reproductive behaviour, such as climate change, as well as possible negative impacts of the absence of fall reproduction on populations of B. flumineum in the study region.