Washington, July 23 : Indian-origin Cornell University researcher Anurag Agrawal says that milkweed plants seem to be shifting away from elaborate defences against caterpillars, and adopting a more energy-efficient approach.
He has observed that a monarch butterfly caterpillar, when about to devour a milkweed leaf, first disarms the plant's natural defence system by cutting its veins that deliver a toxic and sticky latex.
According to him, as the caterpillar has evolved specialised strategies to feed on the plant, milkweeds have started to put in more efforts into repairing themselves faster rather than resisting their predators.
"An important question with co-evolution is where does it end? One answer is when it becomes too costly. Some plants seem to have shifted away from resisting herbivory (plant eating) and have taken that same energy and used it to repair themselves," said Agrawal, Cornell associate professor of ecology and evolutionary biology, and lead author of a paper in the current issue of the Proceedings of the National Academy of Sciences.
The researcher says that his study sheds light on key theories of co-evolution, and shows that pressure by foraging insects makes plants diversify as they evolve new defensive strategies, and that such diversification follows trends in one direction or another.
During the study, he observed that some caterpillars cut a leaf's veins in a circle, and then eat it in the middle where the latex does not flow.
He also found that the monarch caterpillar had become immune to the heart poisons called cardenolides in the plant's tissues.
Using DNA sequence data to look at relationships between 38 species of milkweed, Agrawal and his colleagues found evolutionary declines in the plants' three most important resistance traits - hairs on their leaves, cardenolides and latex - and an escalation in their ability to re-grow.
Agrawal said that he was surprised to find that the plant became more tolerant rather than more diverse in its defences.
He speculated the reason could be because as its predators have become so specialized, the plant was better off choosing a new defensive tactic "to tolerate the herbivory damage instead of resisting it."
He, however, added that it was yet unknown whether such strategies had also evolved in animals trying to evade parasites.
He said that his study, funded by the National Science Foundation, might give plant scientists clues about profitable pest control strategies.