Abstract The two sibling and syntopic odonate species Sympetrum striolatum and Sympetrum vulgatum are common and widespread in Central Europe. While S. striolatum has strong positive population trends, declines of S. vulgatum are observed. The aim of this study was to identify possible drivers of these diverging trends. We presumed that differences in egg development may lead to differences in survival until hatching. First, eggs laid in non-permanent or shrinking waterbodies may suffer of increasing drought periods. Second, differences in developement may cause increased size-mediated intraguild predation, a common cause of reduced survival in odonate larvae. Egg development time and hatching rates were recorded of eleven egg clutches of S. vulgatum and ten clutches of S. striolatum under simulated drought vs. water and direct vs. delayed development treatments. Hatching rates were reduced under drought conditions, and particularly so in S. vulgatum. We did not observe obligate winter diapause in any of the egg clutches. But, S. vulgatum varied widely in development between clutches, while the eggs of S. striolatum developed much faster and hatched highly synchronously. This would provide S. striolatum with a temporal advantage that may lead to a size-advantage over most S. vulgatum. We also found that S. vulgatum grew faster. Faster larval growth would only compensate for those S. vulgatum with fast egg development. The current population trends may be partly attributed to lowered survival of S. vulgatum under drought and by phenological and, thus, size benefits of S. striolatum. Implications for insect conservation Our results show that population dynamics of two closely related dragonfly species can be explained by climatically induced changes in their interactions. Understanding the causes and processes of behavioural changes resulting in differing population trends is fundamental for the protection of species.