Friend, foe – or merely lunch?

Milkweed is a perennial that is poisonous to some grazing livestock and a source of allergens to some humans. To most people, therefore, the plants are weeds – but to insects, they are a treasure trove of nutrients. In the woods, fields and untended gardens where they thrive, milkweed faces threats from bugs who love nothing more than to feed on their leaves or milky white sap all day.

A monarch caterpillar (Danaus plexippus) is seen dining on a bit of milkweed (Asclepias syriaca). Courtesy of Kailen Mooney, University of California, Irvine, and the National Science Foundation.

Researchers looking to develop fast-growing crops that are resistant to pest damage are interested in milkweed because varieties of the plant typically fall into two categories: those that grow quickly but are highly susceptible to pests, and those that grow more slowly, taking the time to develop less-digestible components. For protection, the latter types of milkweed plants depend upon predatory insects, such as ladybugs, to eat caterpillars, aphids and other attackers.

One caterpillar may not do too much damage, but a mass of them will. These are milkweed tiger moth (Euchaetes egle) caterpillars at the buffet. Courtesy of Kailen Mooney, University of California, Irvine, and the National Science Foundation.

Kailen Mooney of the University of California, Irvine, and his colleagues Anurag Agrawal, Rayko Halitschke and Andre Kessler, all of Cornell University in Ithaca, N.Y., are studying several milkweed species in an attempt to determine the intertwining relationships among plant growth, the defense mechanisms plants use against nibblers and the types of protection that plants receive from predator insects.

Shown are aphids (Aphis nerii) feeding on another milkweed sample. Aphids and other bugs skirt milkweed’s defense by using strawlike mouthparts to sip out the sap. Any plant toxins within are stored by the insects for their own defenses. Courtesy of Anurag Agrawal, Cornell University, and the National Science Foundation.

“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 needful of protection from potentially unreliable predators,” Mooney said.

Of course, fans of monarch butterflies will continue to cheer on the monarch caterpillar, which loves to feed on milkweed, to lay its eggs on milkweed and to build its cocoon on milkweed.

A larger group of aphids tackles another section of milkweed. Courtesy of Kailen Mooney, University of California, Irvine, and the National Science Foundation.

A predator insect (a member of the Pentatomidae family) is shown feeding on a monarch caterpillar. Some milkweeds depend on predatory insects to protect them from herbivore bugs, but such dependence is shaky at best. Courtesy of Anurag Agrawal, Cornell University, and the National Science Foundation.

Monarch caterpillars aren’t the only milkweed aficionados that are prone to predation. Here, an unlucky aphid has become a meal for a ladybug (Hippodamia variegata). Courtesy of Pavlos Skenteridis, Bio-insecta, and the National Science Foundation.