Long-term monitoring of a population of greater horseshoe bat emphasises the importance of a pest beetle prey on demographic trends.

The global decline in insect biomass has far-reaching implications for terrestrial and freshwater food webs, impacting species reliant on insects as a crucial component of their diet. This issue extends to species traditionally considered agricultural pests, such as the common cockchafer . In the race to combat cockchafers through collection, insecticide use, and other control methods, the repercussions of their numerical fluctuations on predators, including species of high conservation importance like bats, have been largely overlooked. Drawing on 31-years of monitoring data for a greater horseshoe bat population in the Aosta Valley (Western Italian Alps), we investigated whether annual fluctuations in bat counts are influenced by cockchafer availability and weather conditions. Despite an overall positive trend in bat abundance, we observed pronounced annual fluctuations, mostly driven by cockchafer availability rather than variations in temperature and precipitation. Furthermore, we found a significant association between cockchafer availability and the median date of birth and birth rate of bats. Births occurred approximately 5 days earlier in cockchafer flight years, with earlier births also linked to warmer spring temperatures and higher numbers of warm days in April. Moreover, the ratio pups/older bats was 0.56 in cockchafer flight years, compared to 0.47 in other years. Our results underscore the importance of considering predator-prey dynamics when examining the long-term population trends of species of conservation concern. We recommend implementing restrictions on the use of chemicals and other potentially harmful practices that may diminish prey abundance or quality, including that of species considered as agricultural pests. In designing conservation strategies, a delicate balance should be struck between the current interests of farmers and the overarching goal of preserving biodiversity against potential future threats.