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Observations of plant-pollinator interactions are an important component of the study of the overall ecological setting of a plant species. Here, we report two species of sphingids as pollinators of Cereus aethiops, a wide-spread shrubby columnar cactus species from the Argentinian Monte vegetation. Cereus aethiops is remarkable for its desiccation tolerance during the dry winter season and rapid rehydration after the first rainfall during the summer season. Rehydration starts within 24 hours after the start of the first significant rain, and is complete within 7 to 10 days. Flower bud initiation is triggered by the first rainfall event, and the time from the first rain to the start of the flowering season shows little year-to-year variation. The flowers conform to the sphingophilous pollination syndrome, and the expected sphingid pollinators could be confirmed by direct observation. Diurnal plant-insect interactions in C. aethiops are insignificant and most likely do not contribute significantly to reproductive output.