Precipitation and Interannual Variability Shape the Phenology and Abundance of the Endangered Butterfly Baronia brevicornis

Insects in tropical regions are subject to pronounced seasonal fluctuations in rainfall and temperature. A notable example is Baronia brevicornis, a panchronic butterfly species endemic to Mexico. Its reproductive season begins with the onset of the rainy period, which triggers the emergence of adults. Males exhibit four distinct color morphs, while females display three. To assess adult abundance, sex ratios, morph proportions, and wing size variation, we monitored a population of B. brevicornis during mating seasons (2015, 2017, 2018, 2019, 2021, 2023, and 2024). In addition, we experimentally investigated the effects of prolonged diapause and pupal survival. Our findings indicate a male-biased sex ratio. Morph frequencies and wing sizes in both sexes exhibited temporal variation. Adults emerged following one to two years of pupation when no water was provided during the first year. However, the proportion of individuals successfully emerging after two years was significantly lower than in those emerging after a single year. Our model suggests that accumulated precipitation, rather than temperature, plays a critical role in the probability of detecting adult butterflies. The optimal threshold (~ 70-150 mm) may be an adaptive strategy to cope with unpredictable rainfall, enabling individuals to postpone emergence when precipitation levels are insufficient to sustain host plants and nectar resources. Nevertheless, the potential fitness costs associated with prolonged pupation must be considered.