A seed about the size of the head of a needle has grown into a big problem throughout much of the tropical world.
Relatively harmless in its native homelands, a plant called Clidemia hirta has been sprouting in non-native habitats for more than a century, expanding its range with worrisome zest while crowding out more desirable species. If not contained, the invasive plant’s damaging presence will only worsen with the passage of time.
Clemson University scientist Saara DeWalt has made it her mission to stop the invader in its tracks. For almost 20 years, she has been hunting down Clidemia hirta around the world in an attempt to uncover the secrets that will eventually lead to long-range biological controls.
Clidemia hirta is a densely branched, long-lived shrub that is native to Central and South America and the Caribbean Islands. It produces dark, purplish berries that are engorged with seeds and prized by birds and many other animals. In its native habitat, the plant tends to grow along roadsides and in areas where the soil has been disturbed, but it does not thrive in the more-sensitive understories of forests. However, starting more than a century ago, Clidemia hirta began an insidious migration across non-native areas, including parts of Asia, Africa, Australia and many islands in the Indian and Pacific oceans. In those places, the plant does proliferate in forests, competing with and often choking off more desirable species.
Clidemia hirta is a member of the plant family Melastomataceae, which mostly occur in humid tropical climates. Freezing temperatures kill the plant, so it is not a threat to South Carolina or any portion of the continental United States. But Hawaii, American Samoa and the Virgin Islands have been struggling with it for many decades.
“It got to Java and Fiji in the late 1880s, and then it reached the Hawaiian Islands in the early 1940s,” said DeWalt, an associate professor in the biological sciences department. “One idea was that it was a seed contaminant of coffee plants. The seeds are really small. And the plants do grow on the edge of coffee plantations in the native range,”
Funded by a $500,000 grant from the U.S. Department of Agriculture, DeWalt has travelled internationally to study the plant in its native and non-native regions. Also involved in the USDA project are fellow Clemson scientist Paula Agudelo and USDA Forest Service entomologist M. Tracy Johnson.
“I have seen it in native lands such as Costa Rica, Puerto Rico, Dominica and Brazil, but also in its introduced range in Hawaii, Malaysia and Australia,” said DeWalt, who first began to study Clidemia hirta in 1997 while working on her dissertation at Louisiana State. “And I have a network of scientists who has sent me material from other places in the world in its native range, such as Trinidad, Guyana, Suriname and French Guiana, and also where it is invasive, such as American Samoa, Palau, Borneo, Sumatra, Madagascar, Mauritius, La Réunion Island, the Seychelles and Tanzania.”
In Hawaii, where Clidemia hirta is invading the fragile understories of rainforests, DeWalt and her cohorts have made a fascinating discovery.
“Every individual in Hawaii is genetically identical, at least in terms of the molecular markers I’m looking at,” DeWalt said. “So it appears that the plants can make seeds without fertilization. So what’s happening is the maternal plant is making little clones of itself, and then birds eat the berries and disperse the seeds. Many other animals also love the berries. Because these plants have this asexual reproduction, one individual probably grew in Hawaii and became two and four, increasing exponentially.”
A formidable conglomeration of insects, fungal pathogens and nematodes (small worms) preys upon Clidemia hirta in its native regions. These natural enemies control its spread, especially in forests. But in areas where it has been introduced, the invasive plant seems to have fewer foes, enabling it to expand more robustly. DeWalt’s research has become focused on discovering the best — and safest — ways to introduce biological controls into the non-native areas that will knock back Clidemia hirta while not harming other species.
“In Hawaii, Clidemia hirta is everywhere, so how do you get everywhere and pull it out or spray it? It’s just not possible,” DeWalt said. “So if we were to introduce some kind of biological agent — be it an insect or a fungal pathogen or a nematode — where should we go in the native range to get the agent and which one of these will be the most effective at killing the plants?”
DeWalt and her cohorts have yet to discover the precise origination of the Hawaiian genotype, but they believe that southern Brazil is the leading suspect. This is important because the natural enemies in one area of the wide-ranging native habitat might differ as much as the plants themselves. In other words, nematodes from Costa Rica might not kill a plant in Hawaii as effectively as nematodes from southern Brazil.
“So we’re collecting one species of nematode from different parts of the plant’s native range and growing them on the Hawaiian plant genotype, as well as on another plant genotype that is invasive in Singapore,” said DeWalt, who has turned to nematodes as potentially the most effective biological control agent. “We have these two different invasive genotypes and then we’ve got nematodes from a bunch of different parts of the native range and we’re going to see which nematodes do the most damage on the plants from these various areas.”
Agudelo, a nematologist at Clemson, has focused her attention on the biology of the nematode Ditylenchus gallaeformans and the ideal conditions for infection of Clidemia hirta.
“The nematode causes galls and deformations on the plant, and we are evaluating which nematode populations are capable of causing the most disease,” said Agudelo, who is an associate professor in the College of Agriculture, Forestry and Life Sciences. “The more aggressive nematodes will be good candidates for biological control of the invasive plant.”
Much research remains to be done before the nematodes can be released en masse in the wild. But even as Clidemia hirta continues its relentless march, DeWalt and her cohorts remain hot on its trail.