Conserved oviposition preferences in alpine leaf beetle populations despite host shifts and isolation

Abstract

Abstract 1. Choosing the plant on which to lay their eggs is the last act of care that most female herbivorous insects bestow upon their offspring. These decisions play a pivotal role in insect–plant interactions, placing host preference under strong selection and contributing to the diversity of phytophagous insects as one of the first traits to adapt to new hosts.2. This study presents a test of whether extreme isolation and exposure to different host plants can produce intra‐specific divergence in oviposition preference in alpine insects. Geographic variation should impose selection to fine‐tune host plant ranking and specificity to the plants normally encountered, to avoid wasting time during the very limited reproductive season experienced at high altitudes.3. Beetles from five populations of Oreina elongata differing in host availability were offered three natural hosts: Cirsium spinosissimum, Adenostyles alliariae, and Adenostyles glabra. A novel application of a continuation ratio model (logistic regression) was made to sequential no‐choice experiments, combined with quasi‐likelihood analysis of multiple‐choice experiments.4. The results show little geographic variation in host plant choice: all populations strongly preferred Cirsium in multiple‐choice trials, and in no‐choice experiments laid around 47% of their remaining eggs during each stage, almost regardless of the host present.5. Enemy‐free space seems to explain the preference for Cirsium, but isolation and exposure to different plants has clearly not caused local adaptation in host plant ranking or specificity. Reasons for this conservatism despite divergence in other characteristics are discussed.