Divergent antiherbivore syndromes in tarweed: Play it safe and hide or roll the dice and call for help
Specialist herbivores of plants can overcome even extreme direct hostplant defenses, to which plants have adapted means of tolerating their damage, luring predators to kill them, and/or hiding from them in time and space. Using divergent populations of a species of tarweed (Asteraceae: Madia elegans) and its arthropod community as a model system, we investigated a variety of functional herbivore resistance traits (i.e., apparency, tolerance, indirect defense) and plant reproductive output. We use these data to characterize the life-history syndromes of the populations and develop a broad framework to interpret their evolutionary ecology.
M. elegans, like many California tarweed species, is comprised of spring-flowering non-sticky populations and fall-flowering sticky populations. Spring-flowering plants avoid specialist herbivores (Lepidoptera: Noctuiidae: Heliothodes diminutiva) by flowering, setting seed and senescing before they emerge, but the plants are very small; most plants produce just a few fruits and there is very little variation in fruitset. Fall-flowering plants are larger and entrap insect carrion on their sticky hairs to lure scavenging predators for indirect defense, and they can tolerate more herbivore damage than spring-flowering plants. Fruitset for fall-flowering plants is quite variable but sometimes very high. We interpret these divergent life history strategies into a framework of a low-risk low-reward syndrome for spring-flowering plants versus a high-risk high-reward syndrome for fall-flowering plants, with herbivores being a key selective driver of the divergent syndromes.