Plant spines deter herbivory by restricting caterpillar movement.

The spines of flowering plants are thought to function primarily in defence against mammalian herbivores; however, we previously reported that feeding by caterpillars on the leaves of horsenettle plants () induces increased development of internode spines on new growth. To determine whether and how spines impact caterpillar feeding, we conducted assays with three plant species that vary in spine numbers (, and ) and also manipulated spine numbers within each species. We found that caterpillars located experimentally isolated target leaves much more quickly on plants with experimentally removed spines compared with plants with intact spines. Moreover, it took caterpillars longer to defoliate species with relatively high spine numbers ( and particularly ) compared with , which has fewer spines These findings suggest that spines may play a significant role in defence against insect herbivores by restricting herbivore movement and increasing the time taken to access feeding sites, with possible consequences including longer developmental periods and increased vulnerability or apparency to predators.