Plant predator
Plant predator

There’s a stealth war going on right in our own backyards: plants versus plant-eating insects versus insect-eating insects. Research by UC Irvine’s Kailen Mooney suggests the outcome – of particular interest to farmers – is a stalemate.

For a study published online March 26 in the journal Science, the assistant professor of ecology & evolutionary biology and three colleagues examined 16 species of milkweed, a flowering plant found throughout the Western Hemisphere.

They sought to determine the relationship among plant growth, the manner in which plants defend themselves against plant-eaters (such as with thorns and toxins), and the protection plants receive from predators that eat plant-hungry insects. The herbivores – bright yellow aphids in the study – damage plants; ladybugs, for example, can act as bodyguards, helping plants by eating aphids.

The researchers asked: Can plants have it all? Can they flourish and defend themselves against herbivores while at the same time attracting protection from ladybugs and other predators? The answer: no.

Milkweed species that grow quickly (a desirable trait) are – due to poor defenses – more vulnerable to insects that feed on them (an undesirable trait), making those plants more dependent upon predators for survival. Basically, milkweed falls into one of two camps: hard-to-eat, slow-growing plants that don’t need bodyguards or prone-to-attack, fast-growing plants reliant on outside protection.

Besides increasing general knowledge of how nature works, the finding could be important to farmers trying to develop herbivore-proof, quick-to-mature crops.

“We can breed plants to grow rapidly, but it appears that when we do, we’re weakening the plants’ immunity to herbivores, rendering them more needy of protection from potentially unreliable predators,” says Mooney.

And there may not be much we can do about it.

“Milkweed has been evolving for as many as 20 million years. Natural selection favors faster-growing plants and those that easily fight off insects,” Mooney says. “If nature hasn’t found a way to combine these traits, perhaps it’s something that cannot be done.”

Cornell University scientists Rayko Halitschke, Andre Kessler and Anurag Agrawal also worked on the study, funded in part by the National Science Foundation.