Feeding Preference Differentiation in Sympatric Willow Leaf Beetles Driven by Host Leaf Trait Divergence

Insect herbivores often exhibit host preference and feeding differentiation underpinning fine-scale niche partitioning, yet their linkage to larval performance remains poorly understood. On the Qinghai–Tibet Plateau, two sympatric leaf beetles, Chrysomela vigintipunctata alticola (CV) and Phratora vitellinae (PV) (both Coleoptera: Chrysomelidae), co-occur on two willow species occupying distinct microhabitats: CV predominantly uses abundant Salix atopantha (SW), while PV specializes on rare Salix wangiana var. tibetica (LW) (both Malpighiales: Salicaceae). To disentangle intrinsic feeding response from natal-host induction, we combined dual-choice feeding bioassays with controlled natal-host rearing experiments using first-instar larvae originating from both hosts, and evaluated fitness consequences in terms of morphology and instar duration. PV larvae exhibited a consistent preference for LW, coupled with wider head capsules. In contrast, CV larvae showed no intrinsic preference; specifically, those from SW remained non-selective, while those from LW developed a delayed preference. CV larvae developed faster on nitrogen-rich SW, whereas LW contained higher salicylate esters. These results indicate that PV prioritizes growth and chemical defensive sequestration on LW, whereas CV prioritizes rapid development by utilizing the nutrient-rich SW. Our results reveal distinct resource-use patterns and life-history strategies that facilitate local co-occurrence of the two leaf beetles within the alpine meadow ecosystem.