<p>The developmental traits and cocoon-site selection behavior of last-instar larvae of the moth <i>Illiberis tenuis</i> (Butler) (Lepidoptera: Zygaenidae), a vineyard pest in East Asia, were investigated. Green larvae were observed feeding voraciously during June and July, with individuals turning pinkish at the last instar. Prior to pupation, larvae ceased feeding, purged their gut contents, and lost over 60% of their maximum body weight. In laboratory observations, cocoons were consistently constructed on hard surfaces, such as the undersides of dead leaves, beneath stones, against neighboring cocoons, or along the container walls. The cocoons exhibited an elliptical, mound-like shape. Laboratory experiments revealed strong preferences for moist soil and fallen leaves, whereas field observations showed that dark-pink larvae typically descended along grapevine trunks to the base, rather than dropping directly to the ground. These findings offer new insights into the adaptive significance of cocoon construction, pre-wandering color change, and settlement behavior in this species, and suggest practical methods for capturing mature larvae using ground-deployed materials and trunk-base traps.</p>