When I think of parasites, I typically think of creatures such as tapeworms, fleas, ticks, and leeches—nasty invertebrates that drain the life out of us humans. Then, I might recall that some higher animals can also be parasites, such as the deep sea anglerfish. You’ve likely seen pictures of anglerfish, with their huge, pointy teeth in a large head, followed by a tapering body. When these fish mate, the male permanently attaches itself to the female and lives off her bloodstream for the rest of his life, his sperm his only contribution. (See National Geographic’s video of a particularly beautiful anglerfish species.)
Eventually, I may remember that plants can be parasites too. Mistletoe is the common name for a number of species in the order Santalales. They all parasitize trees, from oaks to Ponderosas. You may have seen Pinedrops, leafless spikes topped with red flowers, while hiking in the forest. Or perhaps you went to see the Corpse Flower when it bloomed recently at the Denver Botanic Gardens.
Dodder, is a parasitic plant that looks like a mess of yellow-to-orange string strewn across the leaves of small forbs. I remember seeing Salt Marsh Dodder smothering the pickleweed in the salt flats around San Francisco Bay. This photo was taken at Bitter Lake NWR, in New Mexico. Dodder is an annual, but produces zillions of long-lived seeds to ensure successive generations. The seeds germinate like any angiosperm, growing both stems and roots. But once the stems encounter a host plant, the roots die, and the dodder becomes completely dependent on its host—to the host’s detriment.
Witchweed (Striga sp.) is yet another genus of parasitic plant. The 26 species are native to tropical Asia, Africa, and/or Australia; one (S. asiatica) has been introduced to the Americas. Three Striga species cause significant crop damage, including S. asiatica.
Like dodder, witchweed is an annual, with seemingly innocuous pink or red flowers. Those flowers, however, produce incredible numbers of seeds—between 90,000 and 500,000 per plant! The seeds are dispersed by wind, wildlife, and especially us humans, who unknowingly carry them on clothing and machinery, and can remain viable in the soil for a decade or more.
Witchweed seeds need to germinate in the presence of a host, or the new plant dies. Even before the sprouting seeds break the surface of the soil, it has attached itself to the roots of its victim. There, it diverts water and nutrients from the host plant, weakening and ultimately killing it. Meanwhile, the witchweed rapidly reaches maturity, blooming and setting seed, and starting the process all over again.
Since witchweed parasitizes a number of annual plants, or crops harvested annually, such as corn, rice, sorghum, and sugarcane, I wondered how the seeds “know” when to germinate. Sprouting while the field is fallow would be fatal. It turns out that the embryo inside that seed can “smell” its prey!
The host plants’ roots give off various chemicals that normally serve to inhibit the growth of competing plants. However, witchweed has receptors that can sense the presence of these chemicals, and instead of acting as herbicides, their presence triggers germination. Sneaky.
How do parasitic plants access their host’s nutrients? They have fangs!
Officially called haustoria, these are modified stems, slender structures that pierce the cells of the host plant much like the pointed straw that comes attached to a juice box. (The plants can’t jab with it, however, it has to grow into place.) Some plants, such as dodder, have haustoria on their roots, while in parasitic fungi, the haustoria is a branch or tube growing from their hyphae.
So it turns out that some plants are vampires, and not as harmless or passive as they’d like us to believe. The good news is that they’re only dangerous to other plants. While there are plenty of microbes, fungi, worms, and other creatures that we need to beware of, I wasn’t able to turn up any plants that parasitize humans.
Witchweed photo: By Marco Schmidt  [CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], from Wikimedia Commons
Haustoria photo: Haustoria of Cassytha pubescens on a host, Mt Field National Park, Tasmania, Australia. By Chrissicc [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)%5D, from Wikimedia Commons