Erythrina Gall Wasp [Quadrastichus erythrinae Kim]

The erythrina gall wasp (EGW) was first discovered in Hawaii in Honolulu in April 2005 and rapidly dispersed throughout the Island of Oahu. Within six months, it had spread statewide and devastated nearly all of the most susceptible species of Erythrina trees. The alarming rate at which it spread had researchers both in the government and private sectors racing for ways to combat the invader with only limited success using imidacloprid formulations.

The Erythrina Gall Wasp Biocontrol Project was initiated in August 2005 with the planning of an exploration by the branch’s Exploratory Entomologist in East Africa in December 2005. Preparation to receive shipments of natural enemies required information on the gall wasp biology and its development on Erythrina plants. No life history data was found in scientific literature because this wasp was a newly described species. Thus, studies were initiated in the HDOA Insect Propagation Facility by the EGW project leader and assistant. Erythrina variegata was chosen as the host plant to conduct these studies because of its susceptibility to the gall wasp, availability of seeds, and ease of propagation.  Seeds were an efficient way to grow uniformly sized potted plants for laboratory work. Scarified seeds germinated in seven days and produced seedlings, six to eight inches tall, in three to four weeks.  Plant pests, mainly mites, thrips, and a fungus disease (powdery mildew), have presented problems during plant propagation. Plants are constantly monitored and routinely culled to eliminate pest infestations. EGW biology studies were also undertaken to determine the relationship with its host plant. Laboratory testing and observations were done to determine what plant parts were attacked and how long the different wasp stages required for development. 

The search for EGW natural enemies evolved into a cooperative effort between the University of Hawaii (UH-CTAHR-PEPS) and the Hawaii Department of Agriculture (HDOA). Africa was determined to be the best starting point for the search based on literature information, such as the number of endemic Erythrina species in those countries and the presence of Quadrastichus species in South Africa. Another decisive factor was the proximity of two islands off the coast of East Africa, Mauritius and Reunion, where initial outbreaks of the EGW were reported.

UH-CTAHR researchers selected Kenya, South Africa, and West Africa (Benin, Ghana, and Togo) for their exploration because of their professional contacts in those places.  HDOA biocontrol practitioners chose to concentrate their search in Tanzania because of its geographical location in East Africa and the highest number of endemic Erythrina species in all of Africa. However, the HDOA explorer began his trip in South Africa to check out the report of the presence of some species of Quadrastichus and to take advantage of better travel and shipping options. Nevertheless, the highest priority for the HDOA Biocontrol Program is to determine the native origin or range of the target pest species and to collect parasitoids found in association with the specific target species, or those very closely related to it. This strategy gives the best chance of finding biocontrol agents that will be highly specific to the target pest species.

Collaborators of the UH researchers in Africa sent one shipment each of galled Erythrina leaves from their respective countries, the first from South Africa during December 2005 and the second from Kenya during January 2006. The HDOA Exploratory Entomologist began his travel in late December 2005, collecting extensively in Tanzania and South Africa, and making a total of six shipments before returning to Honolulu in February 2006.

Three UH researchers traveled to South Africa and made four shipments during April 2006. Another UH researcher made collections from Benin, Ghana, and Togo during May and June 2006 and sent back three shipments.  As a result of these explorations, three potentially promising parasitoid species have been colonized in the HDOA Insect Quarantine Facility (IQF) in Honolulu.  Specimens have been sent to specialists for species determination. This may take quite a while because, like the EGW when it first invaded Taiwan, Singapore, Mauritius, and Reunion in 2004, these parasitic wasps will most likely be found to be unknown species so they will have to be described by specialists and given a species name. One species belongs to the family Eurytomidae and the other two to the family Eulophidae. Each parasitoid will have to undergo host specificity testing in the IQF to determine that it is suitable for release in Hawaii as a biocontrol agent to suppress EGW infestations.

The data collected from the studies must provide assurance that the release will have minimal risk of impact on non-target organisms and native ecosystems in the Hawaiian Islands.