The siren song of a sticky plant: Columbines provision mutualist arthropods by attracting and killing passerby insects.

Many plants provide predatory arthropods with food or shelter. Glandular trichomes entrap insects and may provision predators with insect carrion, though it has not been clear whether this putative benefit functions with natural amounts of carrion, whether plants actively attract insect "tourists," and how common this provisioning system is. We tested the hypothesis that a sticky columbine (Aquilegia eximia: Ranunculaceae) attracts passerby arthropods (a siren song leading them to their demise); that these entrapped arthropods increased predators on the plant; and that these predators reduced damage to the plant. Sticky traps baited with columbine peduncles entrapped more arthropod carrion than unbaited control traps. Predator abundance correlated positively with carrion abundance observationally, and experimental removal of carrion reduced predator numbers. Experimental removal of carrion also increased damage to reproductive structures, likely due to reductions in predator numbers. This indirect defense may be common; we compiled a list of insect-trapping sticky plants that includes over 110 genera in 49 families, suggesting a widespread convergence of this trait, even in non-carnivorous plants. The ubiquity of this trait combined with these experiments suggest that carrion entrapment should be viewed as a common and active process mediated by the plant for indirect defense.