Abstract Longhorn beetles (Cerambycidae) are a very diverse group whose species vary greatly in morphology and behaviour, particularly in diel activity. The type genus Cerambyx includes two large oak‐ling sympatric species ( Cerambyx cerdo and Cerambyx welensii ) that exhibit a crepuscular/nocturnal (dim‐light) lifestyle, but their actual diel activity remains largely unclear. We used phototraps in the wild to assess if diel activity depended on either species, sex or annual meteorology. Phototraps were highly effective, and the imaged activities were representative of beetles' life, including interactions with some vertebrate species. Both longhorns exhibited roughly similar crepuscular/nocturnal behaviour, with daytime activity being usually residual or erratic. Diel activity increased sharply just before dusk, peaked between dusk and early dark night (23:00–2:00 h), and then dropped progressively until disappearing at dawn. Diel activity was sexually dimorphic in C. welensii, as in some years females tended to be active earlier than males. Distinctly, in 2022, the warmest and driest year recorded in the study area, diel activity curves were flatter and wider, with C. cerdo activity increasing in the late afternoon (19:00–21:30 h) and C. welensii in the second half of the night (3:00–6:00 h). Furthermore, both species boosted daytime activity (diurnality) by more than three times, and diurnality also increased over the season that year. We finally hypothesise that, under the current climate change scenario, extreme droughts impact diel activity in these species, particularly boosting diurnality, in an attempt to locate water/food sources to counteract body dehydration.