Sticky plant traps insects to enhance indirect defense

Background/Question/Methods

Plant-provided foods for predatory arthropods such as extrafloral nectar and protein bodies provide indirect plant defense  by attracting natural enemies of herbivores, enhancing top-down control. Recently ecologists have also recognized the importance of carrion as a food source for predators; since sticky plants are widespread and often entrap and kill small insects, we hypothesized that this insect carrion could play an important role in determining predator abundances, and, thus, herbivore control, on sticky plants. In a natural field experiment we manipulated the abundance of entrapped insect carrion on an annual sticky plant, tarweed (Madia elegans), then tracked predator abundance, herbivore damage and plant fitness. We also measured the natural relationships between density of viscid glandular trichomes, entrapped carrion and predator abundances on tarweed.

Results/Conclusions

Experimental carrion augmentation increased the abundance of a suite of predators, decreased herbivore damage and increased plant fitness. We also found a positive correlation between viscid trichome density and naturally entrapped carrion, as well as  naturally entrapped carrion and predator abundance on control plants. We suggest that entrapped invertebrate carrion may function broadly as a plant-provided food for predators on sticky plants. The benefits gained by increasing the abundance of predators adapted to sticky plants (e.g. certain heteropterans and spiders) may offset the well-established costs associated with trichomes excluding generalist predators (e.g. lacewings, ladybeetles and parasitoids). While glandular and hooked trichomes probably evolved as direct defenses against herbivory, we conclude that one of their current functions is to catch food for predators as a means of enhancing indirect defense.