Characterising Spectral Sensitivity and the Role of Sunlight Intensity in Japanese Beetle ( <i>Popillia japonica</i> ) Surveillance and Monitoring (Coleoptera: Scarabaeidae)

ABSTRACT The Japanese beetle (JB) Popillia japonica (Coleoptera: Scarabaeidae) has been a significant invasive pest for over a century in North America. Several studies have reported that plants under direct sunlight are preferred and trap colour affects the number of beetles captured, indicating that visual stimuli influence JB behaviour. Despite this, the influence of visual cues on trap efficiency remains poorly understood, and its visual system has not been characterised. To address this knowledge gap, we conducted a field trial using panels to manipulate sun exposure of traps to test the effect of sunlight intensity on JB trap performance. Our results indicate that JB flight activity is reduced under lower sunlight conditions and that visual cues influence trap performance. Fully shaded traps consistently captured fewer beetles when cloud cover was below 85%. To better understand JB colour perception, we investigated its genome for visual opsin genes and analysed its photoreceptor sensitivity with an electroretinogram. We found evidence for a UV‐green dichromatic visual system, lacking the orange‐sensitive photoreceptor found in some Coleoptera. This may explain inconsistencies in trap colour preference reported in the literature and indicates that JB may not have been able to differentiate some of the colour treatments tested in published studies. To optimise JB trap capture, future studies should evaluate UV‐reflecting traps, control for spectral reflectance intensity, and investigate whether JB avoids low‐light conditions due to environmental risks or visual limitations.