Rapid Invasion of Wasp Threatens
Hawai`i’s Endemic Wiliwili Trees
The Institute of Pacific Islands Forestry is housed in a brand new building in Hilo. Plants used in its landscaping are mostly native to Hawai`i, which is only appropriate, given that one of the institute’s chief missions is to figure out ways to protect indigenous forests against alien invasive species.
It is no small irony, then, that the native wiliwili trees gracing the institute’s campus are falling victim to a tiny gall wasp, a relatively recent arrival to the islands that is destroying the natives and other ornamental coral trees belonging to the genus Erythrina.
The wasp, Quadrastichus erythrinae, is probably as old as the hills, but was named by scientists only in 2004, when it was found on Singapore and Mauritius and Reunion, two islands in the Indian Ocean near Madagascar. (It is now known that Taiwan had the wasp as early as 2003). By April 2005, it was discovered on the Manoa (O`ahu) campus of the University of Hawai`i. Within weeks, it had been reported on all the major islands.
To date, efforts to save the wiliwili trees have been frustrated. Pruning trees of infested areas has done no good. Tests are ongoing in the Pu`u Wa`awa`a area of the Big Island to see if a pesticide injected into the trees can deter the wasp, but Robert Hauff, forest health coordinator for the Department of Land and Resources is not terribly hopeful.
For one thing, chemical treatment of individual trees is not a practical solution for protecting landscapes where wiliwili is dominant. Second, says Hauff, the protection offered by the pesticide is likely to vanish as soon as the treated trees lose their leaves, which usually occurs every winter. Trees with mature leaves are less likely to be infested by the gall wasp, he told Environment Hawai`i. But as soon as the mature leaves fall, the tree becomes vulnerable, he said.
Lloyd Loope, research scientist with the U.S. Geological Survey at Haleakala National Park on Maui, told a Senate panel in August that “today, as a result of the arrival of the Erythrina gall asp, the possibility of survival of wiliwili … into next year is even in doubt.”
Ken Teramoto, head of the state Department of Agriculture’s Plant Pest Control Branch, told Environment Hawai`i that he has “never seen a pest insect that disperses so readily and does its damage in so sort a period as this gall wasp does.”
The only positive he can identify is its narrow range of targets: “Fortunately, it only attacks Erythrina.”
Searching for Control
No one is sure how the wasp arrived in the islands. “I’ve heard speculation that it came in on plants from Taiwan,” Hauff said. But the wasp or leaves infested with the wasp could have also fallen into cargo containers from Singapore, Hong Kong, Taiwan, or mainland China as well, all areas where Erythrina trees are plagued with the gall wasp.
Teramoto said he suspects Taiwan might be the source for other infestations. “Because we now know that Taiwan had it in 2003, and Taiwan has made some efforts to increase their agricultural production with ornamentals, I have a feeling that Taiwan was probably one of the first – if not the first – to see this creature. They must have brought in infested plants from somewhere,” most probably Africa.
And that is where Teramoto is focusing his search for a biological control agent – another insect that preys on the gall wasp. Last month, Mohsen Ramadan, the DOA’s exploratory entomologist, was scheduled to depart on a two-month trip to Tanzania, in hopes of finding a likely control agent.
Why Tanzania? According to Teramoto, South Africa has lots of researchers, and if the pest were present there, “they’d have already found it and it would not have been an undescribed species” in 2004. Further north, in Kenya, there is the Commonwealth Agriculture Bureau, whose scientists also probably would have described the wasp were it present in that country. “Another reason we selected Tanzania is that Mohsen felt that in his literature studies, he found out that Tanzania has the most endemic Erythrina species of all the countries in Africa,” Teramoto added.
“We’re looking for a needle in a haystack,” Teramoto said. “But I don’t have any doubt that there’s a biocontrol agent out there that is able to do the job. The question is, how are we going to find it.”
Wiliwili Double Whammy
Even if a biocontrol agent is found, the native wiliwili is threatened by another serious pest: a seed-boring bruchid beetle, Specularius impressithorax, which lays its eggs on seeds. Larvae eventually eat their way out of the seed, leaving seeds with what has been described as a “shotgunned appearance.” A seed with just a few emergence holes may germinate, but as the holes increase, the seeds lose their viability. The bruchid beetle was first detected on O`ahu in 2002, Maui in 2002, and all the other major islands by2003. arrived in Hawai`i in 2002.
Although the bruchid beetle has been present in the islands longer than the gall wasp, Teramoto says the search for a biocontrol agent has not begun yet. “The beetle is not as quick-acting as the gall wasp, so we’re trying to understand the gall wasp first,” he said.
To protect native wiliwili from these pests, the Center for Conservation, Research, and Training at the University of Hawai`i’s H.L. Lyon Arboretum has begun banking seeds. Lauren Weisenberger, who processes the wiliwili seeds, says that the big effort to collect them began around the end of August-beginning of September, “pretty soon after the news spread about the gall wasp.”
Although the arboretum wants to avoid receiving seeds bearing beetle larvae, Weinberger says, she still finds seeds with borers in them. “Sometimes a seed will look absolutely perfect, but if you look close, you can see a tiny hole. It’s kind of tricky.”
Weinberger added that although the gall wasp affects mainly leaves, “we did get one collection that had galls on the fruit pod itself. A bunch of wasps came out of that.”
By mid-December, Weinberger said the arboretum had received “hundreds of thousands of seeds,” with more to come over the next couple of months, as wiliwili trees on Kaua`i are expected to set seed.
At roughly the same time the wiliwili gall wasp was identified, researchers at the University of Hawai`i’s Agricultural Diagnostic Service Center, a division of its College of Tropical Agriculture and Human Resources, received from a Waimanalo nursery owner a sample of an `ohi`a plant infected with an unknown rust. Before this, no rust had been known to affect `ohi`a (Metrosideros polymorpha). A month later, a rose apple tree (Syzygium jambos) on O`ahu’s Maunawili Trail was seen to be heavily infested with a similar rust. By July, the rust was seen on the endangered Eugenia koolauensis, or nioi, and a guava tree.
In November, Sahobin Zhong, a plant pathologist at CTAHR, confirmed a preliminary identification of the rust as Puccinia psidii Winter. By that time, it had been found on all major Hawaiian islands.
In his testimony last August to the Senate Committee on Energy and Natural Resources, Lloyd Loope said the rust probably came to the state in a shipment of plants from Florida or a foreign country in the neotrpics. The rust “poses a potentially formidable threat” to Hawai`i’s native forests, he said. “This is of course alarming since `ohi`a comprises over 80 percent of Hawai`i’s still-intact forest.”
According to the state Department of Agriculture, the rust has a wide range of host plants, including eucalyptus, paperbark trees, guava, rose apple, allspice, jaboticaba, and other trees in the Myrtaceae family. No fungicide has been approved for use on infected trees, but the DOA recommends “good sanitation practices” to keep trees healthy. This includes removal and destruction of infected plant parts and keeping foliage dry, when possible. Even though the disease is widespread, the DOA also is asking everyone to refrain from transporting any of the host plants from one island to another.
— Patricia Tummons
Volume 16, Number 7 January 2006