Effects of a temperate heatwave on diel rhythms of insect activity: A comparison across habitats

The increasing frequency and intensity of heatwave events in temperate climates threaten to alter behavioural rhythms of ectothermic animals, such as insects. However, it is poorly understood how heatwaves affect daily activity patterns of insects, and whether shaded microclimates can moderate these responses. We investigated the impacts of a heatwave on the diel profile of insect activity, comparing effects across open, tree-covered, and hedged habitats. Using yellow pan traps, insect activity was monitored from 07:00 AM to 07:00 PM on ten non-consecutive days, including two during a heatwave. Insect counts exhibited a unimodal relationship with temperature. On heatwave days, open habitat exhibited a significant (81.9%) reduction in counts compared to two ‘non-heatwave’ days, one before and one after the heatwave. Smaller, non-significant reductions were observed in the tree-covered (32.3%) and hedged (17.8%) habitats. The diel rhythm of activity on non-heatwave days approximated a unimodal relationship, with model-estimated counts peaking around 03:00 PM; by contrast, heatwave days exhibited a bimodal profile, with predicted counts highest in the morning and evening. Such heatwave-induced deformations of activity patterns, modelled as interactions between heatwave and time of day, were significant across all three habitat types. The findings suggest that temperate heatwaves can markedly decrease insect activity levels, and that whilst shade-providing vegetational features may reduce this effect, diel patterns of activity are affected landscape-wide. As heatwaves become more frequent, the preservation of trees and hedges in temperate landscapes is likely crucial to support the resilience of insect activity and of wider ecosystem functioning.