Parasitic plants are not just a biological curiosity. Every year, parasitic plants damage farmers’ fields, particularly in Africa. Kristin Lewis, a junior fellow at the Rowland Institute at Harvard, is learning more about plants and their parasites.

For instance, in Africa, seeds of parasitic plants blow in from surrounding environments or are deposited in bird droppings. The plants that grow from those seeds attach to roots and stems, sucking vital nutrients, stunting the crop plants’ growth, and reducing yields.

To foil the parasites, African farmers have adopted the practice of planting fields twice, first with plants that are resistant to the parasites and then, after the parasites germinate and die, with their desired crop, such as sorghum.

“Most of them are very problematic plants,” Lewis said.

Lewis became interested in parasitic plants while studying plant defenses. Though they can’t fight back as an animal would, plants can generate a wide variety of substances that are toxic or distasteful to insects and other browsers.

In fact, some of our best-known commercial plants are popular exactly because of the unique qualities of the plant’s chemical defenses. Caffeine and nicotine, for example, as well as the hot spices contained in pepper plants, are all intended by the plant to discourage herbivores.

After studying plants’ reactions to insects eating their leaves, Lewis became interested in the interaction of a plant and its parasite.

She already knew that in some cases a parasitic plant shares the host plant’s defensive chemicals, as well as nutrients and carbohydrates. What she wants to find out is how much the two plants communicate.

If an insect attacks a parasitic plant, that insect is just a short distance from the host plant. Though normally the host wouldn’t benefit from helping its parasite, it’s possible the parasite could manipulate the host into shifting more defensive chemicals into the parasite to keep the leaf-eating insect at bay – over there. That would require some sort of communication from the parasite to the host saying, in essence: “Send help fast. It’s BITING me!”

Lewis said she’s still gathering data, but she remains fascinated by the active responses to the environment of plants, which on the surface seem so passive.