Chromatic Contrast Alone Does Not Predict Predation Risk in the Longhorn Beetle Rosalia alpina

Colouration can affect predation risk through camouflage, signalling, or other mechanisms that shape predator perception and decision-­ making. The Rosalia longicorn (Rosalia alpina), a flagship longhorn beetle of European beech forests, has a bluish-­ grey exoskeleton with conspicuous black spots, yet the function of this colour pattern remains unclear. We tested whether its blue body colouration is consistent with background matching to beech bark and whether black spotting reduces predation risk beyond that provided by the blue body colouration alone. We measured reflectance spectra of the blue body colouration, black spots and beech bark, modelled chromatic contrasts under avian vision, and deployed plasticine beetle models in three colour variants: black, blue and a natural pattern with blue and black spots. Visual modelling showed that both beetle colours were chromatically distinguishable from beech bark, but black spots had lower chromatic contrast than blue body colouration, suggesting that the black component may match the bark background more closely than the blue component. In the field experiment, attack occur- rence varied with colour, sampling session and their interaction. In September, blue and patterned models were less likely than black models to be attacked. Patterned models also received fewer marks than black models. By October, these colour differences had weakened or disappeared. These findings indicate that the blue body colouration and the natural colour pattern may reduce predation risk relative to an entirely black phenotype, particularly early in the season. However, we found no clear evidence that black spotting provides protection beyond that afforded by the blue body colouration alone. Alternative functions of the spots, including thermoregulation or signalling, remain plausible.