Study Links Miconia to Potential for High Erosion Rates in Hawai`i

posted in: March 2013 | 0

One of the state’s best tools against invasive species is biocontrol, according to state Department of Agiculture invasive species specialist Carol Okada. And in recent years, local resource management agencies here have received approvals to release carefully selected insects to save native wiliwili trees, stamp out stinging nettle caterpillars, slow the spread of water-hogging strawberry guava, and kill fireweed, which is toxic to livestock.

But so far, no biocontrol agent has been identified for Miconia calvescens, considered by many to be the worst invasive plant in Pacific Island wet forests. The plant, introduced to Hawai`i island in the 1960s, has since been found on Kaua`i, Maui, and O`ahu and is controlled almost exclusively by small crews on each island armed with machetes, pesticides, and GPS devices. Herbicide ballistic technology, which involves shooting balls of herbicide at the plants from a helicopter or from the ground, is a relatively new technique currently being explored. (Elsewhere in this issue, we report on a recently approved paintball pesticide that targets Australian tree ferns.)

Although miconia has become relatively widespread on Hawai`i island, dense stands like those in Tahiti have been confined mostly to ravines in Onomea, and manual control has been relatively successful in preventing its spread on the other islands. On O`ahu, for example, the O`ahu Invasive Species Committee found not a single miconia plant during a recent survey of previously infested areas.

Still, the article “Erosion Potenial Beneath Invasive Miconia Stands,” published online earlier this year by Wiley Online Library, suggests that should the plant spread, increased soil erosion would likely follow.

The article reports the results of research conducted by a team of scientists from the University of Hawai`i’s Geography Department, Japan’s Forest and Forestry Product Research Institute, and the National University of Singapore.

Using lasers to measure the size of raindrops in forested areas on the island of Hawai`i, they have found that areas dominated by miconia may be more susceptible to erosion than those without the invasive plant.

In 2007, the scientists, including UH climatologist Thomas Giambelluca, measured raindrops from three rainfall events at seven sites in east Hawai`i, one of which was infested with miconia. They then calculated the kinetic energy of those drops — or their splash potential — and determined whether or not they were likely to cause erosion.

They found that the effective kinetic energy was highest at the miconia site, which “supports the notion that miconia invasion could increase the erosivity of a site by affecting throughfall raindrop properties,” they state in their article.

Raindrop size at the miconia site (3-83 millimeters) was twice that of ambient rainfall (1-62 mm), they wrote. Although drop size in multi-storied `ohi`a forests was greater than that of miconia, the drops didn’t fall as far, and, therefore, didn’t hit the ground as hard.

“Compared with native `ohi`a stands, throughfall energy is greater in miconia stands because a higher proportion of large drops exceeds erosive thresholds,” they wrote.

Grounds beneath miconia stands are more vulnerable to erosion because the plant’s large leaves inhibit the growth of protective understory vegetation, they collect rainfall into larger drops, and they also decompose rapidly, leaving soils even more unprotected, they concluded.

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Coqui Control

Like miconia, the coqui frog (Eleutherodactylus coqui) has become well established on Hawai`i island, but is not yet ubiquitous on the other main Hawaiian islands. At a hearing held earlier this year before a handful of state senators, Neil Reimer of the DOA’s Plant Pest Control Branch underscored the importance of maintaining control efforts against this noisy hitchhiker.

O`ahu’s two designated coqui control experts caught 78 coqui in fiscal year 2012 over 91 site visits. They spent more than 500 hours looking for the frogs, Reimer reported.

So far, coqui have been detected consistently at only two sites on O`ahu, he said. Even so, he said, “without these two guys, O`ahu would be like Hilo,” where coqui calls are now ubiquitous at dusk, even downtown.

With a handful of established coqui populations on Maui, including a widespread infestation in Maliko Gulch, even the relatively isolated islands of Moloka`i and Lana`i are at risk. Coqui interceptors with the Moloka`i Invasive Species Committee found six coqui on Moloka`i last year and two on Lana`i, according to Teya Pennimann of the Maui Invasive Species Committee.

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Yellow Raspberry Biocontrol Hunt

One of the nastiest plants to invade Hawai`i forests is the Himalayan yellow raspberry (Rubus ellipticus). And not only is it a problem here, say experts in invasive species, it is one of the worst invasive species in the world. Its thick, thorny, impenetrable stands threaten lowland forests and displace native species, including the native Hawaiian raspberry. The Ola`a Forest tract of Hawai`i Volcanoes National Park is especially vulnerable to invasion by the plant.

The hunt to locate its natural enemies – and pinpoint a likely biocontrol agent – is the subject of an article in the January 2013 issue of Pacific Science. Over a period of six years, the article’s authors (five based in Chinese research institutions, one from the U.S. Department of Agriculture) collected potential natural enemies of the plant – both insects and pathogens – at more than 30 sites in southwestern China.

In Hawai`i, making sure that any biocontrol agent won’t take out the native raspberry is critical. Of the 60-plus species of pathogens that the researchers examined, five of them were found exclusively on the Himalayan raspberry and on no other species.

While tests on any of the possible biocontrol agents will take years, the researchers note that their work “adds additional candidates in the form of a rust and a leaf-spot fungus never before considered.” If they are used together with some of the insects identified, it “may improve the overall effectiveness of control programs.”

Tracy Johnson with the U.S. Forest Service’s Institute of Pacific Islands Forestry in Hilo noted that the work was supported with funds from the Hawai`i Invasive Species Committee and some Forest Service funds as well.

Because of the Hawaiian species of Rubus, Johnson said, “we need high specificity, and that’s a high bar” for any biocontrol agent. But, “we won’t know unless we try. Prospects of finding one are pretty decent,” he said, adding that a leaf beetle and a variety of rust looked promising.

The invasive-species research group CABI, with support from the Forest Service, is continuing the hunt for a biocontrol agent for the Himalayan raspberry, this time on the western side of the Himalayas, in India and Pakistan, Johnson said. “We’re piggybacking on their work” in the region, Johnson said, noting that CABI has special expertise in pathogens.

Teresa Dawson and Patricia Tummons

Volume 23, Number 9 — March 2013