Why are there so many herbivorous beetles?
Save J. B. S. Haldane’s parody that the creator must have had “an inordinate fondness for beetles,” biologists believe that plant-feeding beetles have been very successful in multiplying into innumerable forms and species because they were able to track and follow the diversity of plant forms. This association has successfully made plants and herbivore insects together the largest group on earth, occupying more than half of the total biodiversity described today. However, plants are hardly passive when under insect attack, but have evolved an arsenal of defenses to reduce the fitness of herbivorous creatures desperately looking for fresh green leaves, stems, fruits or roots of the plant. Thus, we have been studying the evolution of spines, trichomes, waxes, sticky latex, and toxic chemical molecules that deter or directly kill hungry pests. A good example of such anti-herbivore defenses in plants is found in the very diverse genus Asclepias. Also commonly known as milkweeds, all species in the genus produce latex upon disruption of their aboveground tissues. Moreover, this group of medicinal plants (hence the connection to the Greek god of medicine, Asclepios) is known to produce toxic molecules (cardenolides) capable of altering cardiac rhythm in animals.
Professor Anurag Agrawal at Cornell University, who has dedicated the past ten years to studying milkweeds and their associated herbivore fauna – along with post-doctoral researcher Sergio Rasmann, now at the University of Lausanne, who has always been amazed by the mysterious wonders of life beneath our feet – decided to address the question of what promotes or constrains the ability of beetles to feed on multiple plants species. To answer this enigma, they forced the root-feeding larvae of the common red milkweed beetle (Tetraopes tetraophthalmus) to feed on its true host plant (the common milkweed, Asclepias syriaca) and on 17 other milkweed species. What they found, although seemingly obvious, conforms to major theories that address why we have so many beetles. Milkweed species that are more distantly related to the true host, the common milkweed, were poor hosts for beetles, whereas species that are closely related to the common milkweed were good hosts. This is not to say, however, that evolutionary history by itself explains everything. In fact after dissecting various ecological and physiological parameters of each milkweed species studied, and using statistical analysis that can remove the effect of evolutionary history, the researchers found that survival of the beetle larvae is mainly affected by the production of toxic cardenolides in the roots. Interestingly, the evolution of cardenolides seems to be affected by the habitat in which the plant lives.
Highlighting potential mechanisms of what promotes or constrains interactions between organisms is crucial if we want to be able to understand and explain the incredible diversity of life on earth. Examples hidden in the soil, like this one – where interactions between plants and animals are constrained by evolutionary history, the defenses of the plant, and the habitat in which the organisms live – are needed to increase the efforts for reducing the losses in biodiversity we are currently seeing. Hopefully, we will never have to say to our grandchildren: “Once upon a milkweed…”
Images: shown is the intimate relationship between the plant, the common milkweed Asclepias syriaca, and its host, the red milkweed beetle (Tetraopes tetraophthalmus). The beetle spends its entire life on the plant. Both male and female adults feed on the leaves and flowers of the plant (left), whereas the larvae feed on the roots and rhizomes of the plant (right). The milkweeds can reduce the feeding by secreting white sticky latex in the leaves (see the droplet forming after a first bite in the middle vein of the leaf), as well as by producing toxic chemical compounds (cardenolides) in all organs of the plants, such as the roots. (photographs: left by Anurag Agrawal © and right by Sergio Rasmann ©).
