Promising natural enemy could make tibouchina less invasive
KIA'I MOKU
Tucked away in the rain and fog of the West Maui mountains are uniquely Hawaiian treasures: high-elevation bogs carpeted by rare and restricted plants. The last few haha (Cyanea magnacalyx) existing in the wild anywhere in the world are found there. Dwarf ohia, two unique silverswords and many other plants are restricted to the high summit bogs and have adapted to the wet substrate.
Conservation crews monitor these bogs closely — they are in a section of West Maui that is fenced and protected from pigs, goats and deer. Nonetheless, pinhead-sized seeds of invasive cane tibouchina (Tibouchina herbacae) blow into the bog from the lower elevations and threaten the fragile ecosystems.
If crews did not remove tibouchina, it would take over the bog, crowding out the Cyanea and other endangered plants. It’s only in these ecologically sensitive bogs that crews remove tibouchina — there is simply too much of it on the surrounding slopes. As a result, it’s a continuous battle.
“Tibouchina is scattered throughout the 50,000 acre watershed of the West Maui,” says Chris Brosius, manager of the West Maui Mountain Watershed Partnership. “It’s found from sea level to the summit of Pu’u Kukui, but thrives in the wet windward slopes between 2,000 and 4,000 feet.”
Brosius’ crew helps to protect the bogs along with the rest of the watershed.
“West Maui is the steepest land area in the state, more dissected by cliffs than any other watershed in the Hawaiian Islands,” he says. Even if they had the capacity, “We couldn’t possibly control this species everywhere it grows.”
Tibouchina thrives where there is soil disturbance — whether from pigs or the landslides that helped shape the West Maui mountains. Tibouchina is often the first plant to arrive after a landslide.
Tibouchina belongs to the melastome family, a group of plants that are notoriously invasive in Hawaii — miconia and clidemia are both melastomes. An ugly second cousin to miconia, tibouchina is often leggy and scruffy looking. The leaves are 2 inches long, fuzzy and lack the purple underside that characterizes miconia; the purple flower is pretty, but small — unlike the closely related glory flower.
Biologists first noticed tibouchina in 1977 on Big Island and in 1982 near Waihee Ridge in West Maui. Within a few years, it had spread throughout Maui, Lanai and Hawaii island. By 2005 it was found on Oahu, likely introduced on dirty hiking boots. It has shown up on Molokai as well.
As a trailside weed, it can be problematic — hikers passing through a patch of tibouchina end up coughing as tiny hairs are knocked off and the plant can be a skin irritant.
Tibouchina is native to Brazil. It can be difficult to find there — occasionally appearing in wet areas and meadows. It only reaches 3 feet in height and dies back every year. But here in Hawaii, tibouchina can be 9-12 feet high. When the last year’s growth dies back, the plant resprouts from the roots, creating bamboo-like thickets and taking over habitat for native species.
Why would the plant behave so differently here? One explanation is the lack of predation. Tibouchina evolved in Brazil alongside insects that munch its leaves, seeds and roots. These insects are not present in Hawaii, so tibouchina grows and spreads unchecked. One of tibouchina’s natural predators is a small beetle — both the larvae and the adults devour tibouchina leaves.
The beetle, called Syphrae uberabensis, can only survive by eating tibouchina and other select melastomes, none of which are native to Hawaii. After 20 years of work collecting and evaluating the beetle’s impacts on other species in quarantine, U.S. Forest Service scientists are confident that the beetle poses no negative threat and are preparing to release it into the wild.
The syphrae beetle could be what takes tibouchina down a notch — reducing the threat it poses to the remaining habitat of Hawaii’s rarest native plants and animals.
“If we don’t intervene, this species will continue to encroach on high-value native areas, erode biodiversity and spread elsewhere in the state,” says Brosius.
Tibouchina is just one example of a widespread pest with dramatic impacts. You can help by cleaning your hiking and hunting gear to prevent spreading hitchhiking pests.
* Lissa Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. “Kia’i Moku,” “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.