Host specificity and activity synchronization drive frog-biting midge incidence on torrent frogs (Hylodidae) in southeastern Brazil

Filipe C Serrano , Juan Diaz-Ricaurte, Laura Alencar, Marilia Gaiarsa, Isabela Máximo, Luís Felipe Toledo, Marcio Martins

Frog-bitting midges (Corethrellidae) are widespread micropredators that feed on the blood of frogs. Furthermore, frog-biting midges carry pathogens such as Batrachochytrium dendrobatidis (Bd), an important cause of worldwide amphibian declines. Female midges usually target calling male frogs by using acoustic cues. However, how midges target frogs that use conspicuous visual cues, especially diurnal species, is still poorly understood. We explored micropredation by corethrellid midges on two syntopic diurnal frogs, Hylodes asper and H. phyllodes, in Brazil’s Atlantic Forest. We tested host selection (species and sex) as well as their spatial and temporal patterns. Midges were recorded exclusively on H. asper, indicating host preference, potentially linked to acoustic parameterss preferred by midges. Males were more frequently targeted, particularly those calling near the main current of the streams. Yet, midge abundance per individual did not differ between sexes, with unusual high abundance in females. Midges concentrated on the nostrils and gular region, likely due to higher vascularization. Midge activity was strictly diurnal, despite frogs being available at night while resting exposed on vegetation. Micropredation levels were strongly influenced by precipitation, especially under high maximum temperatures. These patterns suggest that host traits, as well as environmental conditions, shape micropredation dynamics for frogs that use visual cues. Considering that midges can carry pathogens, their micropredation could influence infection risk at the population level. This may be particularly critical for H. asper, a species with rapid annual turnover and documented susceptibility to Bd, which highlights the importance of integrating behavior and environmental factors into disease risk assessments.