THE TARGET: ERYTHRINA GALL WASP (QUADRASTICHUS ERYTHRINAE)

Damage on wiliwili caused by erythrina gall wasp (Quadrastichus erythrinae)

The erythrina gall wasp (Quadrastichus erythrinae) or “EGW” is a non-native, destructive pest to plants in the genus Erythrina, including Hawaiʻi’s native wiliwili trees (Erythrina sandwicensis). EGW insert their eggs into young leaf and stem tissue of plants, causing the formation of galls or abnormal growth in a plant. In high levels this results in a loss of growth and/or vigor, and sometimes defoliation and death of the host tree.
After extensive testing, a natural enemy of the EGW was released in Hawaiʻi starting in 2008 in order to control EGW populations. This natural enemy is Eurytoma erythrinae, a wasp native to Africa (Tanzania), and has significantly reduced the gall wasp infestations since its introduction. The release resulted in a significant reduction in EGW populations and recovery of wiliwili trees throughout the state, with no non-target effects on any other organism except the target invasive pest, EGW. Despite the success of the natural enemy in reducing EGW populations, ongoing monitoring of wiliwili trees has shown that flowering and seed production are still being negatively affected by the gall wasp, indicating further control of the pest can be beneficial to the recovery of wiliwili trees.
Distribution: Erythrina gall wasp (EGW) is known on every major island in Hawaiʻi. It was first described as a news species in 2004 from specimens from Singapore, Mauritius, and Reunion, and first found in Hawaii in 2005.

THE CANDIDATE: Aprostocetus nitens

An adult female Aprostocetus nitens. Photo courtesy of Hawaiʻi Department of Agriculture

Aprostocetus nitens is an ectoparasitoid proposed as a natural enemy of EGW, acting as a complementary control agent to the already released E. erythrinae to further reduce the EGW’s negative impacts on wiliwili trees.
Unlike E. erythrinae, A. nitens utilizes only one host individual to complete its development, and, therefore, can live on much smaller galls, such as those found in flowers, pods, and young seedlings. Having multiple, complimentary natural enemies of a pest provides multiple ways to control the pest species during different life stages or at different times, and can provide more robust control when possible.
Host testing results: Host specificity testing was completed at the Hawaiʻi Department of Agriculture Insect Quarantine Facility in 2009. Results indicate that A. nitens is host specific and has no preference for any of the seven non-target gall-forming species studied. Even if A. nitens were to parasitize these non-target species, it would be unable to produce any offspring because none of these species was shown to be suitable for supporting species’ development. For more information, see the draft environmental assessment.
Native range: Aprostocetus nitens is native to southern and eastern Africa. It was first described in 2009 soon after it was collected in Tanzania during an exploratory survey for natural enemies of EGW.

Draft Environmental Assessment — Published in The Environmental Notice for public comment on December 23, 2019
Final Environmental Assessment – Published in The Environmental Notice on February 8, 2023.
An application with the Hawaii Department of Agriculture for final permits allowing the importation of A. nitens into the state is in progress.
If the final permits are approved, A. nitens will be imported into the state, and initial releases will be implemented and monitored.