NOTE: this pest is not known to spread in or on firewood. It is included in the Gallery of Pests for general information purposes only.
The wiliwili tree (Erythrina sandwicensis) is endemic to the Hawaiian Islands. It grows primarily in tropical dryland forest below 600-meter (1,968 feet) elevation and on scattered volcanic substrates characterized by high viscosity—loose, rough blocks of basaltic lava (Hawaii Department of Agriculture, 2025). Largest populations are found in the North Kona District on the Big Island and leeward slopes of Haleakalā in Auwahi Ahupua‘a on Maui (Hawaii Department of Agriculture, 2025).
Wiliwili trees are large, reaching up to 45 feet tall, and slow-growing. Among their specialized adaptations to arid conditions is being deciduous and photosynthesizing by the thin outer bark and stems. Their often gnarled appearance led to the name, wiliwili, which means “to wind, twist, writhe, crank, turn, screw” (Hawaii Department of Agriculture, 2025).
Like other dryland plants on the islands (Hawaii’s dryland forests host 25% of all the islands’ endangered plants; it is estimated that only 5-10% of these dryland forest ecosystem remains. [ Hawaii Department of Agriculture, 2025]), the wiliwili faces numerous threats, including fire – fueled in part by invading fountain grass, as well as feral ungulates (LaRosa, pers. comm., 2005). The latest threat is the Erythrina gall wasp (Quadrastichus erythrinae).
The wasp had been named only in 2004, from dying coral trees in Singapore and the Mauritius and Reunion islands (Hurley, 2005). Within a few years, the wasp had been found in numerous locations from the Indian Ocean and Asia across the Pacific to American Samoa, Guam and Hawai`i (Reimer 2007; CABI website) and Puerto Rico (Hawaii Department of Agriculture, 2025). The wasp was detected in Miami, Florida in autumn 2006 (Anon. 2006), in Brazil in 2014 (Culik 2014), and Mexico in 2017 (Palacios-Torres, 2017).
The Erythrina gall wasp was discovered on Oahu in April 2005 and shortly afterwards found on all the major Hawaiian islands (Reimer, 2007).
As of 2005, 24 of the 116 species in the genus Erythrina had been documented as hosts (Fulton, pers. comm., 2005). In the United States, concern focuses primarily on the wasp’s damage to the native wiliwili, a tree of great societal, cultural (Hurley, 2005) and ecological importance. The state’s largest intact stands of native wiliwili are on Maui, where the wiliwili is the keystone species of the low dryland forest (Madeiros, pers. comm., 2005). Widespread infestations were observed by September 2005 (Monson, 2005). By 2010, it was observed that wasp damage to the wiliwili was most severe at low elevations. Mature trees were dying, the biggest trees first (Madeiros pers. comm. April 2010). All seedlings had died and saplings were in steep decline at the reserve protecting the best remaining wiliwili ecosystem — Pu`u-o-kali (“hill of waiting”) (Madeiros pers. comm. April 2010).
Concerned Hawaiians quickly initiated a suite of efforts aimed at protecting the endemic wiliwili tree. Scientists and private citizens began a program to collect seeds for preservation (Ellis, pers. comm. 2005; Reimer, 2007). They also tried to protect trees in urban settings through injection or soil drench with systemic insecticides. This approach was unsuccessful (Hawaii Department of Agriculture, 2025).
Meanwhile, scientists at the University of Hawai`i-Manoa (Wright, Rubinoff, and Bokonon-Ganta 2007) and Hawai`i Department of Agriculture (Loope pers. comm. January 2006) immediately began searching for biocontrol agents in South Africa, Tanzania, & Kenya in 2005-2006 (Hawaii Department of Agriculture 2025). A promising candidate – a previously undescribed wasp, Eurytoma erythrinae – was collected in East Africa in 2006. After testing for host specificity (Reimer 2007), the Eurytoma wasp was approved for release in 2008 (Ohira, 2008).
Eight thousand Eurytoma erythrinae were released at various sites on O‘ahu, Maui, Kaua‘i, Hawai`i, and Moloka‘i. Within a few months wiliwili trees began showing signs of recovery, with healthy, non-galled new leaves and vigorous overall growth. By the second year after release of E. erythrinae, more than 60% of young shoots were found to be free of damage by EGW; proportions of trees with minimal galling damage remained low in most locations. Depending on location, recent weather, and time of year, parasitism rates by E. erythrinae on EGW larvae ranged from 20% to 100%. Wiliwili flowering and fruiting resumed, and number of flowers produced has increased each year post-release (Hawaii Department of Agriculture, 2025).
Data from ten years of monitoring (2008 – 2018) (Kaufman et al. 2020) show that:
- By 2011, mortality rates attributed to stress from gall wasp infestation had fallen to 21% for wiliwili trees located in natural areas.
- Gall damage to foliage continued to decline, particularly after 2016.
- Flowering and seed set improved; seed set had increased from less than 3% of trees in 2008 to almost 30% with mature seed in 2011.
- However, little recruitment of seedlings has occurred due to the impacts of multiple other invasive species, especially the seed weevil (Specularius impressithorax), which can cause seed crop loss exceeding 75%.
The problem is that the larvae of the original biocontrol agent, E. erythrinae feed in larger EGW galls that harbor many individual EGW. These large galls are on the stems and petioles, so wasp feeding affects primarily leaves of mature trees. Smaller galls tend to form on young Erythrina seedlings and the reproductive parts (flowers and fruits). These are typically avoided by E. erythrinae, so reproduction is severely reduced (Hawaii Department of Agriculture, 2025).
A second candidate biocontrol agent, Aprostocetus nitens, had been collected during the search in Africa in 2005-2006. This species was scientifically described in 2009. Studies of A. nitens’ life history conducted by the Hawaii Department of Agriculture revealed that it can reproduce parthenogenically – that is, eggs do not need to be fertilized to produce female offspring. Newly emerged adult females contain one or two mature eggs; they continue to produce mature eggs, laying an average of 139 eggs throughout their lifespan (up to 120 days). In addition, a short life cycle – from egg to mature adult in about 20 days – mean populations can build up quickly. Host specificity testing determined that A. nitens did not feed on seven other gall-forming insects, including four species native to Hawai`i. Finally, A. nitens is very unlikely to compete with the already-established biocontrol agent Eurytoma erythrinae since they utilize different host gall locations on the plant (Hawaii Department of Agriculture, 2025). A. nitens can live on individual EGW in the smaller galls found on flowers, seed pods, and young seedlings. Therefore, it is expected to protect reproductive structures and increase recruitment of wiliwili.
Preparation of a petition to USDA APHIS to allow release of this second biocontrol agent began in 2019. A Draft Environmental Assessment was submitted to Hawaii’s Office of Environmental Quality Control in December of that year. Public comments supported release (Hawaii Department of Agriculture, 2025). However, the document went through several revisions before final submission to USDA APHIS, probably in late 2024. APHIS’ review then was delayed. APHIS sought public comment on the proposal in summer 2026. The schedule calls for these to be completed by October 2026 (M. Wright, pers. comm. 2026).
If release of Aprostocetus nitens is approved, the agencies will prioritize initial releases on Oʻahu, Maui, and Hawai`i (Hawaii Department of Agriculture).
Recovery of the wiliwili tree continues to be hampered by the other threats, including inability of seedlings to survive in areas invaded by alien grasses and prone to fire. Even mature trees that have been attacked by the gall wasp are more vulnerable to fire.
The gall wasp also caused considerable damage to urban plantings of exotic coral. In the early stages of the invasion, the Department of Parks and Recreation of the City and County of Honolulu removed about 1,000 dead erythrina trees at great expense (Reimer, 2007).
Little concern has been seen in the U.S. mainland, although both native and exotic species in the Erythrina genus are found in mainland North American and—especially in tropical America. There are two native Erythrina species in the southern United States: Erythrina flabelliformis is found in the Sonoran desert and E. herbacea ranges from Texas to South Florida and North Carolina. There are 72 Erythrina species native to the Americas (Kaufman et al. 2020). As noted above, the erythrina gall was detected in Brazil in 2014 and Mexico in 2017 (Palacios-Torres et al. 2017).
USDA Forest Service scientists and managers have developed a conservation priority-setting framework for forest tree species at risk from pest and pathogens and other threats. The Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation) uses FIA data and expert opinion to group tree species under threat by non-native pests into vulnerability classes and specify appropriate management and conservation strategies. The Project’s initial focus was on tree species native to the North American continent. Project scientists are still assessing threats to trees on the Hawaiian and Caribbean islands (Potter pers. comm.)
Sources
Anonymous. 2006. Florida Department of Agriculture and Consumer Services. Erythrina Gall Wasp, Quadrastichus erythrinae Kim, in Florida. 25 October 2006
CABI website: https://www.cabi.org/isc/datasheet/46220
Culik, M.P., D. dos Santos Martins, J. Aires Ventura and V. Antonio Costa. 2014. The invasive gall wasp Quadrastichus erythrinae (Hymenoptera: Eulophidae) in South America: is classical biological control needed? https://www.tandfonline.com/doi/abs/10.1080/09583157.2014.900735
Ellis, David. 2005. Plant physiologist, U.S. Department of Agriculture, Agricultural Research Service. Personal communication. David.ellis@ars.usda.gov.
Fulton, Will. 2005. Horticulturalist, National Tropical Botanical Gardens – https://ntbg.org. Personal communication. wfulton@ntbg.org.
Hurley, T. 2005. Tiny wasp may kill off native trees. Honolulu Advertiser Newspaper. Thurley@honoluluadvertiser.com.
Kaufman, L.V., J. Yalemar, M.G. Wright. 2020. Classical biological control of the erythrina gall wasp, Quadrastichus erythrinae, in Hawaii: Conserving an endangered habitat. Biological Control Vol 142, March 2020
LaRosa, Anne Marie. 2005. Forest Health Coordinator, Institute of Pacific Islands Forestry, U.S. Department of Agriculture Forest Service. Personal communication. alarosa@fs.fed.us.
La Salle, John. 2005. Head of the National Australian Insect Collection, CSIRO Entomology. Personal communication. John.lasalle@csiro.au.
Loope, Lloyd. 2005 and 2006. Research Scientist, U.S. Geological Survey. Personal communication. Lloyd_loope@usgs.gov.
Medeiros, Arthur C. 2005 and 2010. Research Biologist, U.S. Geological Survey. Personal communication. acm@aloha.net.
Monson, V. 2005. Efforts to control erythrina gall wasps fail. The Maui News. vmonson@mauinews.com.
Ohira, R. 2008. Predator wasps released to save wiliwili trees. Honolulu Advertiser Newspaper. November 25, 2008.
Palacios-Torres, R.E., J. Malpica-Pita, A.G. Bustamante-Ortiz, J. Valdez-Carrasco, A. Santos-Chávez, R. Vega-Muñoz and H. Vibrans-Lindemann. 2017. The Invasive Gall Wasp Quadrastichus erythrinae Kim in Mexico. Southwestern Entomologist. https://www.bioone.org/doi/abs/10.3958/059.042.0405
Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S., Prioritizing the conservation needs of US tree spp: Evaluating vulnerability to forest insect and disease threats, Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/
Reimer, N.J. 2007. Hawaii Department of Agriculture. Field Release of Eurytoma sp. (Hymenoptera: Eurytomidae), for Biological Control of the Erythrina Gall Wasp, Quadrastichus erythrinae Kim (Hymenoptera: Eulophidae), in Hawaii. Draft Environmental Assessment.
Wright, M.G., D.Z. Rubinoff, A.H. Bokonon-Ganta. 2007. Proposal: Initial Exploration (in Africa) and Genetic Work to Accelerate Efforts at Biological Control of the Erythrina Gall Wasp Quadrastichus erythrinae Kim. For consideration for funding by Maui County.