Tiny Boring Beetle Could Bring Down Rare Monarch of Hawai‘i’s Dry Forests

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In 1912, when botanist Joseph Rock discov ered the mehamehame tree in South Kona, he estimated its population as just four trees – one female, three male. In the years that followed, the towering mehamehame (Flueggea neowawraea), which can grow more than 100 feet tall, was found on all the major islands. Rock’s estimate of their population was multiplied a hundred-fold.

But by the end of the century, the mehamehame was in trouble. Its habitat had been degraded by cattle and other grazing animals. Drought and fire had taken a toll. And a tiny invasive insect, the black twig borer, was making inroads into native forests, with the mehamehame among the most vul nerable species.

The mehamehame was placed on the fed eral endangered species list in 1994, when the U.S. Fish and Wildlife Service estimated the total number of trees globally at no more than 167, distributed among 28 separate popula tions on the four largest islands. By then, every tree was infested by the borer.

More than a decade later, the global popu lation of mehamehame has further declined – to between 132 and 139 individuals, according to Christa Russell, plant conservation program coordinator with the Fish and Wildlife Service in Honolulu.

On the Big Island, near the spot where Rock first identified the tree, three or four living trees are all that remain in the wild. According to Lyman Perry, the state De partment of Land and Natural Resources’ botanist for the island of Hawai‘i, viable seed has been collected from only one in recent years. When Perry visited the area with Rick Warshauer, a researcher with the U.S. Geological Survey’s Biological Re sources Division in Volcano, they found only a handful of the trees Warshauer had seen years earlier still living. Where the rest had been were now only the dead skeletons of these once enormous trees, described by the late naturalist Wayne Gagné as the monarch of the dry forest.

Variations on a Theme
The same story of decline has played out with depressing frequency across the islands for many other tree species, especially those found in the drier native forests. The beautiful hard wood uhiuhi (Caesalpinia kavaiensis) – its population decimated by fire, pigs, cattle, and all manner of other insult – may well be pushed into extinction by the black twig borer. The global population of mahoe (Alectryon macrococcus), its reproduction already depressed by seed-eating rats, consists of just a few dozen trees that are also battling the borer for survival.

The number of other species at risk from the borer runs into the dozens, and other invasive insects threaten dozens more. For the wiliwili tree (Erythrina sandwicensis), once used by native Hawaiians for surfboards and canoe outriggers, the threat is a bruchid beetle from Africa, suspected to have been brought into the state as an accidental hitchhiker on jewelry made from seeds of a closely related species. Wiliwili is not on the official list of threatened or endangered species, but in South Maui’s dry forest, it is about the only native tree that isn’t. Unless something stops the beetle infestation there, the landscape of South Maui could lose this last important remnant of native dry forest.

A scale insect threatens the famed hala groves of East Maui. Like wiliwili, hala are not endangered, but the infestation could bring about dramatic changes in a landscape attrac tive to tourists and beloved by locals.

For the critically endangered Cyanea superba and other rare lobelias, introduced slugs may be preventing reproduction in the wild and impairing the survival of outplanted individuals. “Slugs may turn out to be a limiting factor for lobelias everywhere,” says Vickie Caraway, state botanist for the Depart­ment of Land and Natural Resources. “You can’t go anywhere without seeing slugs.”

While most concerns have focused on the visible threats (ungulates and rats in the forest, invasive weeds, growth of fuel for fires), less attention and money have been dedicated to the smaller, more insidious pests. “Inverte brates and diseases are harder to figure out,” says Russell. “The effects may not be notice­able until long after the pest has reached a point where it has had a devastating impact.”

The pests that botanists and entomologists have identified is by no means a complete list of all invertebrate and disease pests that may be harming native Hawaiian flora. “There are lots of other critters out there that we don’t know of,” says Caraway, the state botanist. “We have so many invasives that come in every year, it’s practically a certainty that we’re always going to be facing something new.”

A Tiny Terror
The blacktwig borer, Xylosandrus compactus, is not one of those new pests, however. Its presence in the islands dates back to 1931, when it was found on elderberry imported from Singapore. In November 1961, the borer was discovered again on pink tecoma trees in Kailua, O‘ahu. Within a decade, the beetle was found on all four major islands and was known to infest at least 65 different species of plants (including 17 native trees and shrubs).

The borer belongs to a group of insects called ambrosia beetles, with the name deriv ing from the manner in which they feed their larvae. The females bore channels, or galleries, into twigs, cultivate a fungus – the “ambrosia” that gives them their name – and then deposit eggs in the channels. The developing larvae feast on the fungal ambrosia. When they are adults, the females breed with males in the same brood and then leave the infested plant to start the cycle anew.

Hawai‘i has 21 endemic species of ambrosia beetles, which seem to prefer wood in dead and dying trees. According to Arnold Hara, an entomologist with the University of Hawai‘i’s Center for Tropical Agriculture and Human Resources, the black twig borer goes for living plants that are under stress. “They won’t attack vigorous trees,” he says. “When sap flow is strong, it’s a natural defense to the beetle.” With so many of Hawai‘i’s dry forest trees surviving in harsh conditions, stress is a way of life, making them especially vulnerable to the borer.

In 1970, C.J. Davis, a botanist with the state Department of Agriculture, sounded the first alarm in a short paper prophetically titled, “Black Twig Borer Threatens Native Trees.” “Periodic observations on the various islands indicate that [the beetle] is steadily penetrating forest regions, increasing its host range and causing considerable damage to common and rare tree species,” Davis wrote.

He described a trip to Palikea Gulch in the Wai‘anae Mountains made by a party of bota­nists, entomologists and graduate students. They found the borer on 14 different host plants, including nine natives.

In 1979, Arnold Hara and John Beardsley, Jr., published a more comprehensive article on the borer, drawn largely from research Hara did in connection with his master’s degree. By then, the number of known hosts had grown to more than 100 species of plants in 44 families.

Among infested plants listed by Hara and Beardsley were some 18 species, representing 14 families, endemic to Hawai‘i. Other hosts included commercially important crops from Annonaceae (including such fruits as cheri moya and soursop) to Zingerberaceae (gingers). The beetle also affects coffee plants, and is sometimes called the coffee twig borer.

Controlling Questions
To control the black twig borer, the state Department of Agriculture introduced several parasitoid wasps in the 1960s and 1970s. None became established. In recent years, bio-con-trol programs have become more difficult, with increased awareness of the possibility of harming native species by introducing para sites of closely related pests. Still, in 1995, James Nakatani, then the chairperson of the state Board of Agriculture, wrote in his com ments on a recovery plan for Big Island plants that “biological control should be considered as a long-term control method.

Nakatani said his Plant Pest Control Branch had no funds for research on bio-control agents, but would be willing to work on the project if funds were provided. No funds became available, and for more than a decade the state has not pursued a bio-control agent for the borer.

Pesticides have been approved for use on infested plants, but the effectiveness of most of these depends on direct contact with the target animal. In the case of the black twig borer, which spends most of its life inside tunnels whose opening has been described as a pin hole, spraying infested plants has uncertain results. Systemic pesticides are also available for use on landscape plants. They work by being taken up through the vascular system of the affected plant and are able to hit the beetles where they live. One of the chief drawbacks to systemic pesticides is that they are to be used only on ornamental and landscape plantings, not in forests. In any event, the systemic pesticides are labor intensive and expensive, mitigating against their eventual use in pro tecting newly planted forest trees.

An even greater question arises when ad dressing chemical control of the black twig borer. While the approved pesticides may kill the beetles, it is not clear that the beetles and their tunnels are the only things that damage the infested trees. The ambrosia fungus, Fusarium solani, is suspected of causing can kers, root rot, and rapid wilt syndrome that can be lethal, even if the beetle infestation is elimi nated.

Kapua Kawelo, who works with the U.S. Army on forest restoration projects in the Wai‘anae Mountains of O‘ahu, has seen first hand the ravaging effects of the black tree borer on mehamehame trees. Borers on young trees that had been treated with a systemic pesticide were killed, but the trees died anyway, she said. In another experiment, greenhouse workers tried to mimic the physical damage of the insects to see if the boring alone – without the fungus – was sufficient to kill the tree. Those trees survived, she said.

In January, Kawelo and her team planted 15 mehamehame trees, grown from seed at the state’s rare plant nursery on O‘ahu, at the bottom of a gulch, in an attempt to see how the trees fared in an environment where the soils were rich and water was relatively more abun dant. At last check, she said, the trees were doing well, even though they had not been dosed with any pesticide since leaving the greenhouse. (The greenhouse treatment is essential, says Kawelo: the seedlings “are attacked constantly.”)

Yet another approach to controlling un wanted insects involves the use of chemical attractants and repellents. While this has not yet been tried in Hawai‘i, the technique is common practice elsewhere.

Last month, the Hawai‘i Invasive Species Council received a grant application seeking $21,500 in state funds to help pay for research on controlling the black twig borer through the use of pheromones. If the project is approved, the state money will be matched by equivalent support from the Forest Service of the U.S. Department of Agriculture.

“Most attempts to trap out beetles for pest management have not resulted in reductions in damage,” the proposal states, “but there are a few successful cases.” Most of those involve what is described as a “push-pull” strategy, in which a repellent pheromone is placed in the center of the area where beetle control is desired, while attractants are placed in traps at the perimeter.

“Consideration of non-target impacts is an important aspect that we will include in our approach in this study,” the proposal states, referring to the 21 native ambrosia beetle species.

Nancy Gillette, an entomologist with the Forest Service in Berkeley and one of the participants in the planned research, told Environment Hawai‘i that the behavioral chemicals don’t kill the beetles, a fact that may make their use in Hawai‘i more accept able in light of the desire not to harm natives. “The mode of action of verbenone, which is what we would ultimately hope to use, isn’t really totally understood, but it doesn’t kill the beetles. In fact, there is argument about what to call it because we don’t know whether it actually repels the beetles or just stops them from flying to ward the attractive source.”

One published report “suggests that [verbenone] interrupts the flight (slows down the flight muscles) toward the source of attraction (a host or mate),” Gillette wrote in an email.

Gillette expressed some surprise that the technique had not been tried in Hawai‘i, “since the approach has worked well with a lot of other scolytids.” (The black twig borer belongs to family of beetles known as Scolytidae.)

Healthy Forests, Healthy Trees
Regardless of the approach taken to con trolling the black twig borer – pesticides, attractants, pruning and destruction of in fested branches – all are labor intensive and time consuming. For individual trees, these approaches may be helpful. But for restor ing entire forests to health, they are of limited use.

“Are we gardening or outplanting?” asks Vickie Caraway, the state botanist. “We have to weed, maintain, fence, and water until nursery plants get established in the wild.”

And for some plants, outplanting in a wild setting simply may never be an option. “If some plants are to survive,” says Caraway, “they may have to be landscape plants.” It is not an appealing prospect for Caraway. When endangered native plants are sold as ornamentals, she notes, some people will always be tempted to plant them out on their own, without regard to whether the plants belong in a given area.

Art Medeiros, a biologist with the U.S. Geological Survey’s Biological Resources Division on Maui, sees more of a continuum than a divide between gardening and resto ration. Medeiros has been working to restore dry forest areas of South Maui for decades, hoping to reverse, tree by tree, the devasta tion wrought by cattle, drought, weeds, seed predation, and other insults.

“There’s no example of a dry forest where trees are not under stress,” Medeiros told Environment Hawai‘i. Some trees, such as mahoe, may now be extremely vulnerable to the black twig borer and other pests, he acknowledged, but if the stresses on those trees can be reduced, the trees will have a better chance of survival.

Just as the factors leading to devastation built upon themselves – grazing removed understories, prevented regeneration of na tives, allowed weeds to invade, and fostered a drier environment – so, too, can the elements of forest restoration work together to build vigor, Medeiros says.

“It’s the very same tenet we use when we protect ecosystems from invasive weeds,” he said. By encouraging undergrowth, outplanting key species and making sure they are well watered at the outset, “they become more weed-proof as we increase their integrity. They have more vigor, and greater resistance to weeds.”

— Patricia Tummons

Volume 15, Number 11 May 2005

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