Integrating olfactory and visual cues for trap-based monitoring of the tropical nut borer, Hypothenemus obscurus (F.) (Coleoptera: Scolytinae), a key pest of macadamia in Hawaii

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Abstract
The tropical nut borer, Hypothenemus obscurus (F.) (Coleoptera: Curculionidae: Scolytinae), is a significant economic pest in Hawaii’s macadamia orchards, contributing to substantial pre- and postharvest losses. Existing management strategies relying on cultural controls and generalist predators have been proven inefficient. This study developed and evaluated a trap-based monitoring system integrating both olfactory and visual cues to improve pest monitoring and management for H. obscurus. Field trials conducted over 2 yr assessed various alcohol-based lure formulations and trap color preferences under commercial orchard conditions. Results demonstrated that lures containing higher proportions of methanol, particularly at a 100% concentration and 3:1 methanol-to-ethanol ratio, captured significantly more H. obscurus than ethanol-only lures. Trap placement at 1.5 m in the lower canopy yielded higher captures compared to ground-deployed traps. Color trials revealed that transparent bottle traps achieved the highest captures, followed by brown and red, while yellow and white traps were the least effective. In summary, methanol-baited, transparent bottle traps deployed in the lower canopy of trees proved to be most effective in capturing H. obscurus, highlighting the importance of integrating semiochemical and visual stimuli as well as trap placement to enhance trap effectiveness. The implications of our findings for the enhancement of early detection, timely interventions, and improvement of integrated pest management strategies for H. obscurus are discussed.

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