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. Like other dryland plants on the islands, the wiliwili faces numerous threats, including a non-native seed weevil, Specularius impressithorax (Hurley, 2005); fire – fueled in part by invading fountain grass; and feral ungulates (LaRosa, pers. comm., 2005). The latest threat is the Erythrina gall wasp (Quadrastichus erythrinae).
The wasp had been named only in 2004, by a scientist when examining specimens collected 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). 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 through injection or soil drench with systemic insecticides. The efficacy of the insecticide over the long term was not then known (Wright, pers. comm. 2005); While systemic insecticides such as imidacloprid can be effective in treating landscape trees, the approach is not considered sustainable for protection of native wilwili trees, owing to costs and difficulty in accessing tree populations to treat them repeatedly (Wright, pers.comm. 2018)
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) began the search for biocontrol agents. 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). Results were encouraging from the start (Madeiros pers. comm. April 2010).
Data from ten years of monitoring (2008 – 2018) show that
- By 2011, mortality rates attributed to stress from gall wasp infestation had fallen to 21% for wiliwili trees located in natural areas (Kaufman et al. (in press)).
- Gall damage to foliage continued to decline, particularly after 2016 (Kaufman et al. (in press)).
- 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 (Kaufman et al. (in press)).
- 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% (Kaufman et al. (in press)).
In 2019, scientists were preparing a petition for release of a second biocontrol agent, Aprostocetus sp., which is expected to further reduce gall wasp damage to flowers and seed pods on wilwili trees (Kaufman et al. (In press)).
Recovery of the 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). Scientists are trying to breed Erythrina trees resistant to the gall wasp for landscape use (Wright pers. comm. 2018).
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. (In press)). 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 & 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. In press. 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.