Human-modified landscapes are often composed of small and isolated natural habitat fragments immersed in agricultural and urban matrices. Within them, ecosystem services provided by wildlife, such as pest insect suppression, may decrease or even be lost leading to a substantial increase in agricultural production costs. Pest insect suppression by bats has been identified as an essential ecosystem service but remains poorly investigated. For example, we still lack a basic understanding of the proportion of pest insects that comprises the diet of many bat species. Here, we explored the diet composition of eight Brazilian bat species (78 individuals) in a human-modified landscape through the analysis of stable carbon (¹³C) and nitrogen (¹⁵N) isotopes. Bats were categorised into guilds: open-space aerial insectivores, narrow-space gleaning frugivores, or narrow-space gleaning nectarivores. We divided the insects collected into three groups depending on their δ¹³C values: forest, mixed, and pest insects. We found that open-space insectivorous bats had the highest proportion of insects in total in their diet — consuming primarily from the forest group (56%) and the pest group (34%). Interestingly, narrow-space gleaning frugivores also consumed pest insects (almost 20%). The narrow-space gleaning nectarivores had traces of insects in their diet, yet the actual proportion was inconclusive. Even though bats were from different guilds, with diets consisting mainly of plants and insects, the δ¹⁵N indicated that they fitted to similar trophic levels, as secondary consumers. Therefore, the trophic level of nectarivorous and frugivorous bats showed a more generalist diet than previously assumed. The proportion of forest insects in the diet of open-space aerial insectivores may indicate the importance of small forest patches as food resources for wildlife such as the ones included in human-modified landscapes. The bats' contribution to this ecosystem service could improve the economic conservation value of Neotropical bats in human-modified landscapes.