Biomass burning smoke pollution stimulates painted lady butterflies (Vanessa cardui L.) to increase flight speed

Smoke from biomass burning significantly degrades air quality due to high concentrations of particulate matter (PM_2.5) and trace gases. While the ecological and health impacts of smoke pollution are well documented, its effects on insect migration remain poorly understood. In this study, we conducted two experiments to investigate the flight performance of Vanessa cardui butterflies under varying smoke conditions and identify the mechanisms influencing their behaviour. Butterflies were tethered to flight mills (TFMs) for 6 h, during which flight speed, distance, and duration were recorded across clean-air conditions and three levels of PM_2.5 concentrations. Statistical analysis revealed that flight speed increases significantly as smoke concentration increases, although the increased range decreases. At a mean PM_2.5 concentration of 120 μg m⁻³, flight speed increased by 52 % compared to clean-air conditions. To determine whether particulate matter was driving this response, individuals were exposed to smoke with and without particulates. In smoke with particulates retained, butterflies exhibited nearly double the flight speed compared to filtered smoke, indicating that particulates play a key role in altering flight behaviour. Scanning electron microscopy revealed significant deposition of smoke particulates on the antennae and abdomen, suggesting a sensory or physical response triggering accelerated flight. We interpret these findings as evidence that Vanessa cardui accelerates flight in smoky environments as an escape response. This study highlights the remarkable sensitivity of butterflies to smoke pollution and provides novel insights into the ecological consequences of biomass burning, particularly its potential impacts on insect behaviour and migration dynamics.