Spread of Rapid `Ohi`a Death in Puna May By Hindering `Amakihi Population
In 2001, Environment Hawai`i reported on research by U.S. Geological Survey entomologist Dennis LaPointe and colleague Carter Atkinson suggesting that climate change-induced temperature increases will likely shrink the habitat ranges of native forest birds, as warming allows disease-carrying mosquitoes to reach those high elevations that once served as a refuge from deadly avian malaria.
However, researchers with the agency had also found something promising: `amakihi (Chlorodrepanis virens) in lowland forests in Puna, on the east side of Hawai`i island, were actually thriving where malaria was widespread.
“Things were looking good for `amakihi. As we continued to look at that project, looked at genetics, … we found that this lowland population was genetically distinct and somehow evolved a tolerance to malaria. So this was a good thing for Hawaiian `amakihi. It really stressed the significance of these lowland `ohi`a forests. Unfortunately, good news in the conservation community doesn’t last very long,” LaPointe said at the annual Hawai`i Conservation Conference held in Waikiki July 24-26.
Enter rapid `ohi`a death, or ROD, a disease caused by Ceratocystis fungi that infest wounded `ohi`a trees (Metrosideros polymorpha) and eventually starve them of water. Surveys in 2010 estimated that the disease spanned some 2,500 acres on Hawai`i island. By 2014, about 15,000 acres had been heavily affected. This year, one of the two fungi species responsible for the disease on Hawai`i was found in `ohi`a on Kaua`i.
To see whether this disease, which has been decimating one of the keystone species of native Hawaiian forests, is having a cascade effect, LaPointe and other scientists revisited in May and June of 2016 transects in Puna last surveyed in 2004 and took a census of the bird community there. In addition to the original 268 stations from the study, they added 45 more transects in the lower Puna subdivision of Leilani Estates, which was hit hard and early by ROD, he said.
They also estimated the percent mortality in `ohi`a compared to observations made by the U.S. Forest Service in 2014 and used National Oceanic and Atmospheric Administration data and aerial photographs to assess changes in land cover.
The first obvious change was a slight uptick in bird species diversity. In addition to `amakihi, they found introduced Japanese bush warblers, Japanese white eyes, yellow-fronted canaries, and saffron finches. What also became obvious was that the `amakihi population had decreased more than any other bird species, he said.
While the relative abundance of all species had decreased across the board, the estimated densities of `amakihi and Japanese white eyes had dropped the most, with the former decreasing by 70 percent and the latter by 33 percent, he said.
But was the decrease in bird (especially `amakihi) densities due to ROD-induced `ohi`a loss? LaPointe said there were definitely fewer birds where the `ohi`a stands were 90 to 100 percent dead. Because `amakihi, in particular, rely heavily on seasonal nectar, ROD could be a driving factor in their population decline, he said. (`Amakihi is a species of Hawaiian honeycreeper.)
At most stations, `amakihi detections decreased, and at some, such as those in Nanawale, detections plummeted, he said, noting that at Leilani Estates, about a mile away, and adjacent roadways, more than half the `ohi`a were dead.
Since the 2016 survey, volcanic flows from Kilauea have completely changed the landscape of lower Puna. “It’s kind of as if the volcanic eruption is making ROD a moot point,” he said, adding that `amakihi may be moving out of the area into the northern tracts of Leilani Estates, where, “hopefully for now it serves as a refuge.”
Rust Causes `Ohi`a Dieback
In O`ahu’s Ko`olau Range
At the conference, several speakers discussed the ongoing efforts to monitor and prevent the spread of rapid `ohi`a death, which has had a quantifiable and devastating effect on `amakihi. But Ceratocystis huliohia and Ceratocystis lukuohia aren’t the only infective threats to the trees.
Will Weaver, natural resource manager for the Ko`olau Mountain Watershed Partnership on O`ahu, discussed how Puccinia psidii, a rust first detected at an O`ahu plant nursery in 2005, caused a massive dieback of `ohi`a in 2016. Coincidentally, that’s the same year the state Board of Agriculture — at long last — authorized the Department of Agriculture to hold public hearings on rules to curtail domestic imports of the rust’s potential plant hosts.
Weaver said his organization observed an alarming amount of defoliated `ohi`a of the glaberrima variety at Poamoho and Kipapa in the latter part of the year. Both areas are on the leeward side of the Ko`olau range. The tremuloides variety was also affected, he added. (There are eight varieties of the species.) Samples of the plants, however, tested negative for ROD.
His team then conducted an aerial survey in February 2017, which revealed that from Waimanu to Helemano, tens of thousands of `ohi`a were defoliated, all around the 700-meter elevation gradient.
Months later, plant pathologist Janice Uchida from the University of Hawai`i’s College of Tropical Agriculture and Human Resources confirmed that the dieback was due to `ohi`a rust, although genetic testing to determine if it’s a new strain has not been completed. A secondary fungus, Neofusicoccum parvum, was also occurring on the island at the same time, but it was not thought to be the primary cause of the dieback, Weaver said.
While the rust has been around for more than a decade, little impact has been seen on `ohi`a up until now, he said. To determine what factors contributed to the outbreak, he evaluated 2016 rainfall data for Poamoho and noticed a “very interesting trend.” During the month of August, the area received almost 15 inches of rain over and above the average rainfall, he said.
“Potentially there was the perfect kind of scene for the rust to go crazy,” he said.
With climate change possibly leading to more summer storms, there may be future rust outbreaks, Weaver suggested.
He noted that about half of the affected trees that weren’t fully defoliated are showing some regrowth, which is encouraging. Many of the other trees, however, simply turned black, which is troubling since `ohi`a is a common host for endangered native tree snails, he said.
Lungworm Incidence is High
In Rats Tested from East Hawai`i
This year, rat lungworm disease has infected four people in Hawai`i, including a toddler on O`ahu, a visiting adolescent from New York, and two adult men, one from Maui, the other from West Hawai`i. Last year, there were 18 confirmed cases.
The disease, caused when larvae of the nematode Angiostrongylus cantonensis are eaten and eventually make their way to the brain, results in a type of meningitis that can involve severe and chronic pain, difficulty moving, nausea, fever, and even death. The parasites are expelled by infected rats, picked up from the soil by slugs and snails, which then contaminate whatever they come across, including leafy greens meant for human consumption.
But how widespread is the parasite? In June, University of Hawai`i scientists revealed that several species of slugs and snails from Kaua`i, Maui, O`ahu, and Hawai`i tested positive for rat lungworm.
With regard to infection rates among rats, which are the definitive hosts of the parasite, a team of University of Hawai`i at Hilo researchers and a USDA biologist published the article, “High prevalence of Angiostrongylus cantonensis (rat lungworm) on eastern Hawai‘i Island: A closer look at life cycle traits and patterns of infection in wild rats (Rattus spp.),” in the journal PLoS ONE last December. At the conservation conference, co-author Chris Niebuhr, the biologist with the USDA’s Animal and Plant Health Inspection Service, discussed the findings and reported something new: the nematodes have now been detected in coqui frogs.
The researchers took a closer look at the life cycle traits and patterns of infection of wild rats in East Hawai`i, considered to be the epicenter of the disease. From the 545 wild rats collected from in and around Hilo, as well as Leilani Estates, they detected 3,148 adult worms.
They found that 73 percent of the rats had adult worms in their lungs and 94 percent had some stage of the parasite in their tissues.
Some types of rats had greater infection rates than others. Specifically, more than 90 percent of Polynesian rats (Rattus exulans) had adult worms, compared to only 50 percent of black rats (Rattus rattus) collected from the same locations. But, the black rats had more of the parasite’s larvae in them.
“We suspect it might be a difference in susceptibilities. We want to look closer,” Niebuhr said. Body mass was also a factor in infection levels. The larger a rat is, the older it is, and the bigger/older black rats had fewer adult worms, while the smaller black rats had the same high levels as those of Polynesian rats, he said.
“Over time, Rattus rattus seems to be acquiring some sort of immunity,” which could have management implications, he said.
Even so, “[t]he exceptionally high prevalence of A. cantonensis infection in Rattus spp. in east Hawai‘i Island is cause for concern and indicates the potential for human infection with this emerging zoonosis is greater than previously thought,” the article concludes.
What is not well understood is whether infection rates fluctuate seasonally and what role parasite hosts other than snails, slugs, and rats play, Niebuhr said, adding that those issues will be tackled in future studies.
He said that recent testing has already shown that Asian semi-slugs collected in East Hawai`i are 96 percent positive for the parasite. The semi-slugs have been shown to carry especially high concentrations of the parasite and have been blamed for the increase in disease cases in recent years.
“These are very high numbers,” Niebuhr said of the semi-slug infection rates. “We’re looking to [test] three more times to see if the high levels are maintained. … Since there’s super high numbers, if you’re going to investigate potential alternative hosts, this is the place: mongooses, myna birds, coqui frogs …,” he continued.
He noted that 53 percent of golden tree frogs in New Caledonia were found to carry infective larvae. In Hawai`i, coqui frog concentrations are as high as 55,000 per hectare in East Hawai`i. They also eat semi-slugs and are a prey item of rats, he said.
He then announced that 11 coqui have been analyzed recently and all have tested positive for the rat lungworm parasite in multiple tissue types, including the stomach, brain, liver, heart, and lungs. Even so, he said it’s still unknown what role reservoir hosts such as coqui are playing in disease transmission. “I do have some ideas. … You feed an uninfected rat a coqui, check its stool. I certainly think this is something to pursue further,” he said.
(For more background, read, “Rat Lungworm Disease: Has Arrival of a New-to-Hawai`i Mollusk Made It Worse?” from our December 2011 issue.)
— Teresa Dawson
For Further Reading
`Amakihi and Rapid `Ohi`a Death
- “As Temperatures Rise, So Do Mosquitoes — And That’s Bad News for Hawai`i’s Birds,” January 2011;
- “Lowland ‘Amakihi Gain Ground In Battle Against Mosquito-Borne Disease,” August 2013;
- “`Ohi`a Disease on Big Island Poses Threat to Native Forests Statewide,” and “Quarantine Rule for `Ohi`a Fungus Not Likely to Happen Anytime Soon,” June 2015.
- “NEW & NOTEWORTHY: Vacancies; `Ohi`a Rust Rule; and Hope for the Wiliwili,” October 2007;
- “`Ohi`a Rust Threat Is as Large as Ever, But Ban on Imports Is Allowed to Lapse,” September 2008;
- “Board of Agriculture Approves Draft Rule To Block Import of `Ohi`a Rust Carriers,” April 2015.