Experts in Biocontrol Work to Match Hawai`i Weeds with Natural Enemies

posted in: October 2011 | 0

The challenges of dealing with invasive species can seem daunting, whether on a local or a global scale. But at a meeting last month in Waikoloa, the scientists, researchers, and natural-resource managers who are on the front lines in the war on weeds had to be cheered by news of breakthroughs in the science and art of identifying and employing biocontrol agents against some of the world’s most aggressive invasive plants.

The occasion was the 13th International Symposium on Biocontrol of Weeds, a gathering that every four years brings together the world’s leading researchers in the field. That it was held in Hawai`i this year was due, in large measure, to the doggedness of Tracy Johnson. As a research entomologist with the U.S. Forest Service on the Big Island, Johnson has dedicated much of his work over the last decade to identifying ways to knock back strawberry guava, also known as waiawi (Psidium cattleianum), which has already invaded much of Hawai`i’s native forests and threatens to destroy them if unchecked.

Johnson also organized the session on prospects for weed biocontrol in Pacific islands. Researchers from three continents discussed their efforts to identify pests of miconia, fireweed, and kahili ginger, among other plants.


Robert Barreto, of the Phytopathology Department of the Universidade Federal de Vicosa, in Brazil, had to look no further than his next-door neighbor’s yard to find a potential biocontrol agent for miconia (Miconia calvescens). What’s more, the tiny gall-forming nematode that appears to take this giant down a notch works on some of the other Melastome family members – clidemia and tibouchina, to name two – that have settled comfortably in Hawai`i and other Pacific islands.

Searching for a natural pest of these plants in their home range of Central and South America was complicated by the fact that, although they are too easy to find in Hawai`i, they are relatively obscure in Brazil. Barreto described miconia as “a nightmare of an invasive weed,” but, he added, in Brazil, “it is so uncommon it doesn’t have a common name. It occurs in the margins of rivers and in isolated patches, and is always hammered by insects and disease.”

Barreto entered into an agreement with the University of Hawai`i to look for biocontrol agents for miconia and other melastomes back in 1993. Initially, a fungus was found and shipped to Hawai`i, where it was introduced in 1997. It became established, but, as Barreto said, it didn’t have much impact. (On the other hand, when the fungus was released in Tahiti, it was much more effective on miconia there, allowing some native trees to return to the slopes. That the fungus was effective in Tahiti and not in Hawai`i illustrates the large consequences that subtle variances in the genetic makeup of different strains of a plant can have.)

The search continued, Barreto said, and while more than a dozen possible parasites or diseases of miconia were identified, the gall nematode was “the real interesting one.”

Once researchers knew what to look for, “we found it everywhere,” he said. Part of the problem in tracking it down, he noted, was that leaves that have been attacked “don’t look like miconia at all. It hammers the fruit, buds, and flowering parts.”

The microscopic nematode had not been scientifically identified until Barreto’s work. Now that he knows what to look for, Barreto said, he can find all the nematodes he needs for his studies on a Miconia ibaguensis plant near his house.

Tests of possible host plants on 102 species in 58 families have shown the nematode only infects Melastomaceae, Barreto said, and its potential as a biocontrol agent for miconia in Hawai`i is now thought to be “very high.”


Fireweed (Senecio madagascariensis) is a relative newcomer to Hawai`i, having arrived here only in the 1980s, probably in fodder imported from Australia. Since then, though, it has spread through the islands like – well, fire. More than 400,000 acres of rangeland in the state has been invaded by the weed, which is toxic to cattle and horses. Hundreds of miles of roadside are also covered with fireweed. In moist conditions, the tiny, daisy-like flowers paint the landscape yellow. In drought conditions, the fields of yellow turn crispy brown, with the dry plants providing tinder for any spark.

Andy Sheppard of Australia discussed the problems Australia has had with fireweed. “It’s the first invasive species you see” on arrival in Sydney, he said. “You spot it out of the airplane window at the Sydney airport.” It infests grazing areas in Australia, particularly in the east, from Victoria to Southern Queensland. While in the past it was found usually in coastal areas, it is now moving upland.

With costs of up to $9,000 per acre to control, and no control measure really being effective, the weed has become a political hot-button issue. It is the only weed named in manifesto of the ruling coalition parties, he noted, and finding an agent to control fireweed has become a high priority for anyone wishing to remain in office.

The strain of fireweed in Australia (and Hawai`i) actually comes from Natal, in southern Africa, and Australia’s search for a biocontrol agent led there. More than two dozen common insects and two fungi were found on fireweed, but none exerted any effective control over the plant.

Sheppard concluded on a glum note. “There’s a very low chance of finding a specific enough and effective enough agent,” he said, putting the long odds at 20 percent. Despite the “slim chance of success,” he said, there’s strong community support and political will.

Prospects for controlling fireweed in Hawai`i are not quite as grim, according to the DOA’s exploratory entomologist, Mohsen M. Ramadan.

“Unlike Australia,” Ramadan noted in an email to Environment Hawai`i, “we don’t have any native plant species in the whole tribe Senecioneae to care about. … The Madagascan arctiid moth, Secusio extensia, is our first fireweed biological control agent proposed for release, and it seems specific for members of Senecioneae based on host testing and my field observation during two seasons in Madagascar.”

Like miconia in Brazil, fireweed is not regarded as a weed in its native range. “In South Africa and Madagascar a handful of insects were attacking the plant from flower head to the roots, reducing its competitive ability and allowing kikuyu grass to grow,” Ramadan wrote. “Because of these insects and two rust pathogens, fireweed is not perceived as a weed in its native region, and it grows in batches of less than 100 plants per population.”

Even though Hawai`i has no Senecioneae plants, it does have 181 species of aster plants, he continued, with half of the species endemic to Hawai`i and many of them endangered. In experiments designed to see if the arctiid moth would use any related species as a host, or if it would use any species of environmental or agricultural significance as a host, results were encouraging. Of the species tested, the moth attacked only fireweed and two close relatives, Senecio vulgaris (common groundsel) and Delairea odorata (cape ivy), which is also a noxious weed in Hawaiian forests.

In an article published earlier this year in the Journal of Applied Entomology, Ramadan, K.T. Murai, also of the DOA, and Tracy Johnson write that they anticipate that impacts of release of the moth to plants outside the tribe Senecioneae will be minimal. Once released, they continue, the moth will “spread gradually at each release site, reaching damaging levels within a few years.”

The state Board of Agriculture has approved release of the moth. Now, Ramadan says, the hold-up is with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS).

APHIS, Ramadan says, “is concerned about sunflower in Hawai
`i, claiming that sunflower is a crop in Hawai`i, so why not use chemicals to kill fireweed instead of biocontrol.” On the island of Hawai`i alone, chemical control has been estimated to cost more than $11 million per year, according to the article in the Journal of Applied Entomology.

Ramadan says that the state responded with new tests that showed 11 percent of the moths will mature to adults on sunflowers, but only during forced-feeding, no-choice tests. Given a choice, the moths will reject sunflower as a host. “Also,” he says, “adults will not lay eggs on sunflower” and recent tests have shown that “sunflower will not sustain a continuous population of Secusio.”

Kahili Ginger

Invasive gingers have been the focus of Djamila Djeddour, a weed researcher with the London-based international organization CABI. All four Hedychium species she has been studying – kahili, white, yellow, and red ginger lily – are native to the Himalayas. Of those, kahili ginger is the most invasive, having been placed on the list of top invasive species by the International Union for Conservation of Nature.

The devastation caused by the massive rhizomes of kahili ginger are easily visible in areas of Hawai`i forest that have been invaded. But the species also is a huge problem elsewhere in the world – Australia, New Zealand, South Africa, and elsewhere. What makes it all the more difficult to control, Djeddour said, is the fact that it “invades pristine forests and does not rely on disturbance.”

In 2008, CABI began to look in the eastern Himalayas for possible biocontrol agents. There, her group found several promising biocontrol agents, including a fly, a “huge, very striking, colorful beetle,” a moth that “skeletonizes” the leaf, and a rust fungus. “There’s a large suite of natural enemies” of ginger, she said, including many new to science. “The inventory continues to be consolidated.”

A test plant list has been approved already for New Zealand, and a list for Hawai`i is “under review.” Testing of the potential agents is already occurring in the plants’ home range.

“Kahili ginger is a pantropical concern,” Djeddour said.


Unlike other presenters at the symposium, Flint Hughes, of the U.S. Forest Service’s Institute of Pacific Islands Forestry in Hilo, offered no analysis of prospective biocontrol agents or detailed genetic analysis of an invasive plant. Rather, he had “a humble proposal,” pleading with members of the assembled researchers to begin looking for a biocontrol agent for albizia, the invasive tree that has been Hughes’ special area of study.

“More and more I’ve been pulled into the societal and cultural impacts of this species as it invades across places in Hawai`i and across the Pacific,” Hughes said.

Albizia (Falcataria moluccana) is a huge tree with a large capacity for nitrogen fixation, he noted. “It is one of the fastest-growing trees in the world and is listed in the Guinness book of world records,” he said.

Describing it as a “nightmare tree,” Hughes recited a bit of its history in Hawai`i. It was introduced by famed botanist Joseph Rock, who, unaware of albizia’s potential, wrote that by the time his albizia trees had reached the age of nine, they were over a hundred feet tall – “a rapidity of growth almost unbelievable.”

“The only objection to the tree,” Rock wrote, “is its short-lived period, but as it is an abundant seeder, there should always be a good stand of this tree present.” Guffaws could be heard in the conference room as Hughes displayed Rock’s words on the large screen.

The canopy of a single albizia tree can cover half an acre and shade out native plants, especially `ohi`a, Hughes continued. It also facilitates invasion by non-native species, particularly waiawi.

Albizia has now become a concern of the state Department of Transportation, he noted. “As stands develop, they become more unstable and are of increasing danger to human health and welfare,” he said. In 2009, falling albizia branches caused two serious automobile accidents. On Kaua`i, the DOT spent $1 million per mile to remove 1,500 albizia trees along Kuhio Highway. Statewide, up to 100 miles of roads under the DOT’s jurisdiction are threatened by infestations of albizia.

“Finding a biocontrol agent for albizia is a high priority for the DOT,” he noted. Anyone looking for a project with a ready source of funding might well consider this green giant, Hughes said.

When his talk concluded, Hughes was the most popular man in the room.

Patricia Tummons

Volume 22, Number 4 — October 2011

Leave a Reply