In April 2005 a UH student in Manoa spotted strange swellings on the leaves of a coral tree (wiliwili haole), damage caused by a newly invasive wasp. Within months, the erythrina gall wasp (Quadrastichus erythrinae) had spread statewide. From attractive ornamentals to the tall wiliwili (‘Tropic Coral’) used in windbreaks to the native wiliwili of Hawai‘i’s dry forests, trees in the genus Erythrina succumbed to the attack. The female gall wasp lays her eggs in young leaves. As the wasp larvae mature, the leaves become too deformed to sustain the plant through photosynthesis. Severely infested trees lose their leaves and die.
CTAHR is battling the wasp on several fronts. Mark Wright, Daniel Rubinoff, Russell Messing, and former post-doctoral scholar Aime Bokonon-Ganta have collaborated with the Hawai‘i Department of Agriculture and researchers in Africa, the gall wasp’s continent of origin, to collect and identify erythrina gall wasps and their predators. Comparing the DNA of Hawai‘i’s invasive gall wasp to the DNA of gall wasps collected from locations throughout Africa helps researchers pinpoint where the Hawai‘i pest originated and thus where to look for its natural enemies. CTAHR collaborators in Kenya and HDOA entomologist Mohsen Ramadan, working in Tanzania, have collected several such enemies, wasps that lay eggs in Erythrina galls so that their offspring can eat the gall wasp’s larvae. HDOA is seeking state and federal approval to release one of these parasitoid wasps as a biocontrol agent.
CTAHR researchers have found a way to save individual trees by injecting the pesticide imidacloprid into their trunks. Arnold Hara developed and refined this injection treatment, while Qing Li established a new method to quantify how much of the pesticide reaches the leaves and how long it persists. For future ornamental plantings, Kenneth Leonhardt is propagating Erythrina varieties already in Hawai‘i that appear to resist the gall wasp. Despite these successes, an effective program of biocontrol remains the best hope for native wiliwili.