State one step closer to using new wasp to save wiliwili trees
Insect would prey on the gall wasp that’s been damaging the native trees
In an effort to save wiliwili trees, the only species of its kind that naturally occurs in Hawaii, state officials are finalizing the details of a plan to fight off an invasive wasp that’s been threatening the native plants for decades.
Beginning with Oahu, Maui and Hawaii island, the state Department of Agriculture, with the support of the state Department of Land and Natural Resources and the University of Hawaii, proposed the statewide field release of another type of wasp, Aprostocetus nitens, a host-specific natural enemy of the erythrina gall wasp (Quadrastichus erythrinae) that’s been damaging wiliwili trees.
DOA expects to rear and release thousands of individuals of this wasp until the species is established. No particular timing of releases is planned.
Completing the final environmental assessment, which was posted in the state Office of Planning and Sustainable Development’s “Environmental Notice” earlier this month, is just one step in the process, said Darcy Oishi, acting administrator of the Plant Industry Division.
“Once we have permission from all regulatory authorities, we will have to remove the insect from containment and begin what we call mass rearing,” Oishi said Thursday night. “This will be a unique challenge for us because of the presence of the first natural enemy.”
Once everything is said and done, the department hopes to deploy A. nitens within six months of release from containment.
Maui Invasive Species Committee Acting Manager Teya Penniman said that they “very much” support this project.
“Wiliwili is a keystone species of dryland forests in the islands, which means it helps hold the entire ecosystem together,” Penniman said Thursday.
The invasive gall wasps were first discovered on Oahu in 2005. They spread quickly throughout the state and resulted in the death of thousands of Erythrina trees, including native wiliwili trees.
Wiliwili have been known to grow in lower elevation areas of West and Southwest Maui, portions of Upcountry and out to Kaupo.
MISC joined interagency efforts to map the spread of gall wasps to assess the feasibility of control, Penniman said, but when they were detected in trees beyond Makena, the collective opinion was that “direct control was simply not feasible.”
“The decline of the trees was so fast that the focus across Hawaii shifted to collecting seeds and launching an urgent search for an effective biocontrol agent,” she said.
Five comments or letters of support of the biocontrol plan were attached to the final EA published in the “Environmental Notice” earlier this month, including one from the late Fern Duvall of DLNR’s Division Forestry & Wildlife, who voiced support prior to his death in 2022.
“I am so impressed by the thorough vetting of the EGW predator agent for proposed release, and the noting of cultural importance of wiliwili in the document,” the longtime Maui biologist wrote in 2020.
Erythrina gall wasps are yellow 6- to 8-millimeter-long invasive pests. They create galls — swollen, tumor-like growths — in erythrina tree tissue that serve as cocoons for their larvae to mature. These tumors deform leaves and interfere with the plant’s ability to take in water and light, eventually killing it.
The state in 2008 initially tried using the Eurytoma wasp, a 2-millimeter-long insect that was collected from Tanzania, as a form of biocontrol to save wiliwili trees. They were successful at first, but these wasps only went after larger galls and not smaller ones formed on flowers, seed pods and seedlings, according to the report.
At the time, more than 8,000 individuals were released at sites on Oahu, Maui, Kauai, Hawaii island and Molokai. Within months, the Eurytoma wasp was established and wiliwili trees began to recover. After the second year, more than 60 percent of young shoots were damage free and by 2011, 90 percent of the targeted sample wiliwili trees had their canopy back.
“Despite the successes of E. erythrinae in controlling EGW, damage by EGW on reproductive parts of wiliwili and young seedlings persist,” the report said.
This is where the shiny black and metallic green wasp, Aprostocetus nitens, come in. Sizing in between 1.1 to 1.7 millimeters in length and originating from Africa, A. nitens don’t need to have a male for fertilization and offspring.
They prove to be more promising because “extensive experiments” show A. nitens and E. erythrinae have different feeding preferences, controlling gall wasps on different parts of the plant.
A. nitens can survive four days without food, living an average of 120 days.
“The release of A. nitens will complement E. erythrinae in suppressing infestations of EGW, and will improve the survival success of wiliwili, an ecologically and culturally important Hawaiian tree species,” the report said.
Studies indicate that both wasps intended for biocontrol do not have negative impacts on cultural, historical or natural resources, or with one another, the report said.
The state DOA would release mature adults of A. nitens on infested trees, focusing on severe infestations first.
State officials and environmental experts said more must be done to save wiliwili as a culturally significant floral species and the primary habitat as well as dryland forest for traditional cultural practices.
Nearly all parts of the plant are used by practitioners, Penniman said, and some trees are estimated to be hundreds of years old.
“For these reasons protecting all remaining populations of wiliwili is imperative as this will help to ensure that its environmental and cultural significance is not diminished,” the report said.
The cost of the broader project to do biocontrol work on the erythrina gall wasp is estimated around $700,000, Oishi said, with the bulk of the funding used for foreign exploration to catalog, collect, rear and evaluate potential biological control agents.
Secondary costs include maintaining the insects, facility costs and conducting field releases and assessments.