Biological Control of Gall Wasp at the Waikoloa Dry Forest Recovery Project

D. T. Faucette1,2, R. C. Bautista3, J.A. Yalemar3 and B. G. Brand1 1Waikoloa Dry Forest Recovery Project (WDFRP), Hawaii, USA 2Colorado State University, Hawaii, USA 3Hawaii Department of Agriculture, Honolulu, USA

Abstract

Like other dryland plants in Hawaii, the tree (Erythrina sandwicensis) faces numerous threats, including: a non-native seed weevil Specularius impressithorax; fire – fueled by invasive fountain grass; and feral ungulates. In April 2005, the erythrina gall wasp (EGW), erythrinae was discovered infesting the Indian coral tree (tiger’s claw), Erythrina variegate, on the island of Oahu in Manoa Valley. By June 2005, it was infesting the native wiliwili trees within the Waikoloa Dry Forest Recovery Project on the Big Island of Hawaii. Gall wasp infestation causes gall formation and the swelling of tissue on the leaves, petioles, stems and practically all green parts of the plant, including flowers and seed pods. Heavily galled leaves and stems result in a severe reduction of the growth and vigour of the trees. Photosynthesis is greatly reduced and the tree eventually dies.

The native wiliwili tree is a tree of great societal, cultural and ecological importance. The state’s largest intact stands of native wiliwili are most likely to be found in the Waikoloa area. Wiliwili and the uhiuhi tree (Caesalpinia kavaiense) are the only native trees remaining in the area. All other native species have succumbed to fire, ungulate browsing and human consumptive uses. In an effort to protect the wiliwili from the EGW, scientists began injecting and soil drenching with systemic insecticides (at great expense). This practice has not proven successful. The only hope for the species now is the establishment of a biological control agent.

Proceedings of the FORTROP II: Tropical Forestry Change in a Changing World, 17-20 November 2008, Kasetsart University, Bangkok, Thailand 144 FORTROP II: Tropical Forestry Change in a Changing World

The Hawaii Department of Agriculture (HDOA), is set to release Eurytoma sp. (: ), a parasitoid that tunnels through the gall chambers to feed on the immature stages of the EGW, into the Waikoloa Dry Forest Recovery Project area in the summer of 2008. Project staff along with HDOA will monitor the release and report on its efficacy. Some findings are expected by the start of the rainy season in November 2008.

Keywords: Waikoloa Hawaii, Erythrina sandwicensis, Erythrina gall wasp, Hawaii Department of Agriculture (HDOA)

Introduction

Tropical dry forests are perhaps the most endangered ecosystem in the world. The tropical dry forests in Hawaii may in fact be the most threatened of all. Less than 10% of the original forests remain and what little is left is fragmented and degraded by non-native ungulate browsing, fire, invasive species and even theft (Cabin et al., 2002). All of these factors have played and still do play a significant role in the demise of the remnant, native, dry lowland, wiliwili forest patch located half a mile south of Waikoloa Village on the island of Hawaii.

Like other dryland forest plants in Hawaii, the wiliwili tree (Erythrina sandwicensis) faces numerous threats, including: a non native seed weevil Specularius impressithorax; fire – fueled by invasive fountain grass; and feral ungulates. In April 2005, the erythrina gall wasp (EGW), Quadrastichus erythrinae was discovered infesting the Indian coral tree (tiger’s claw), Erythrina variegate, on the island of Oahu in Manoa Valley (Heu et al., 2006). By June 2005, it was infesting the native wiliwili trees within the Waikoloa Dry Forest Recovery Project (WDFRP) on the Big Island of Hawaii. Gall wasp infestation causes gall formation and swelling of tissue on the leaves, petioles, stems and practically all green parts of the plant, including flowers and seed pods. Heavily galled leaves and stems result in a severe reduction of the growth and vigour of the trees. Photosynthesis is greatly reduced and the tree eventually dies (Draft, 2007). No new seeds have been collected from the trees for the last two years. Volume 5: Dry Forest Ecology and Conservation 145

The wiliwili tree is summer deciduous, meaning that it drops its leaves during the hot, dry summer when extended drought conditions prevail. This habit may be extending the life of the trees, as fewer galls are able to form during the summer months when very little growth takes place on the trees. If the wiliwili tree is to be saved from extinction, a successful biological control program needs to be established soon.

Materials and Methods

Study Site

Waikoloa Village is a rapidly growing suburban community (population approximately 4000) located in the district of South Kohala on the island of Hawaii. The Waikoloa Village Association (WVA), which represents the community through an elected board of directors, manages the village golf course and approximately 4050 ha of land that surrounds the village which are deed-restricted as open space. The WVA board of directors supports this project and has agreed to a long-term license agreement with the Waikoloa Village Outdoor Circle. The Waikoloa Village chapter of The Outdoor Circle (WVOC) engages in beautification and maintenance of areas within and around Waikoloa Village for the benefit of residents and visitors. The Waikoloa Dry Forest Recovery Project (WDFRP), has been granted a 15-year lease in order to restore a native dry lowland wiliwili forest (Erythrina sandwicense) habitat over 100 ha of open space south of the community of Waikoloa Village. The remnant forest patch lies half a mile south of Waikoloa Village at approximately 300 m elevation on the northwest flank of the island of Hawaii.

The forest stand occurs on a relatively young Mauna Kea a’a flow that originated from Puu Hinai (elev. 439 m) and sweeps downslope toward Puako. Estimated to be between 4.4 and 7.1 thousand years old (Wolfe and Morris, 1996), the flow has a very rough and blocky texture that has precluded access to cattle. Like other young and rough flows that still support remnant patches of dry forest, this flow has remained less colonized by exotic grasses resulting in protection for its inhabitants from wildfire.

Rainfall within the area has been estimated anywhere from 1800 to 2300 mm/yr. The majority of this precipitation falls during the rainy season between October and March. The understory within the project site and 146 FORTROP II: Tropical Forestry Change in a Changing World much of the entire region is dominated by fountain grass (Pennisetum setaceum), a perennial bunch grass and buffelgrass (Censhrus ciliaris). Fountain grass first appeared on the Hawaiian Islands in 1914, when it was introduced as an ornamental (Jacobi and Warshauer, 1992).

There are approximately 200 live wiliwili trees in the vicinity of the project site. This number is matched almost to the tree by the dead snags that dot the landscape. These trees most likely died due to old age, but it leaves one to wonder about the structure and function of the forest 200 years ago before the introduction of fountain grass and ungulates. When the fact that there are no wiliwili in the seedling, sapling or pole stages is considered, the prognosis for this forest is not good. If the remainder of the trees were to succumb to the EGW, Hawaii would have lost something special as the native wiliwili is a tree of great societal, cultural and ecological importance. Faced with the extinction of the federally listed uhiuhi tree, which has fewer than 30 trees left in the wild and the sudden and immediate threat to the wiliwili tree, the WDFRP was formed. This group is made up of concerned residents of Waikoloa Village, whose goal is to protect, preserve and perpetuate the wiliwili forest which lies just a half mile from their doorsteps.

The Erythrina Gall Wasp (EGW)

The EGW is a devastating invasive species of African origin. It was only recently described as a new species in 2004 (Kim et al., 2004) based on samples collected in Mauritius, Reunion and Singapore. By 2005, it was detected in trees in the Waikoloa area. It has quickly spread across the world and can be found in such places as Hong Kong, mainland China and Thailand, as well as other nations around the Pacific (Heu et al., 2006). The movement of plant materials and global air travel are two of the possible reasons for the spread of the EGW around the world in such a short period of time (Draft, 2007).

There is little life history recorded for the EGW in the scientific literature. With very little information to guide them, the Hawaii Department of Agriculture (HDOA) set out on exploratory missions to Africa in order to find potential biocontrol agents. The initial focus centred on: 1) Tanzania, as it had at several native species in the genus Erythrina and it was in close proximity to the initial outbreaks of the pest in Mauritius and Reunion; and 2) South Africa and a species in the genus Quadrastichus. Volume 5: Dry Forest Ecology and Conservation 147

In early 2006, a predominant parasitoid (Family Eurytomidae) was collected in Tanzania. It was the dominant parasitoid reared from the galled leaves of a native Erythrina. Research back in Hawaii indicated that the female eurytomid preferred the Erythrina gall wasp over the non-target gall formers (NTGF). Given a choice, the female parasitoids visited and parasitized the gall forming EGW infesting the Erythrina plants 10 times more than they did the plants infested by NTGF.

The HDOA is set to release the biocontrol into the WDFRP as soon as the United States Department of Agriculture’s and Plant Health Inspection Service (APHIS) reviews the data compiled by the state. Given the history of biocontrol introductions into Hawaii that have gone wrong, authorizing agencies are far more careful before issuing permits. WDFRP staffs have been working with the HDOA in order to save the wiliwili tree. The project site will be one of the initial release points for the biocontrol within the State.

Results and Discussion

As the release date for the bioccontrol has been pushed back pending approval from the Federal Government, no field data has been collected on the efficacy of the parasitoid. It is hoped that there will be some results by the end of the rainy season in March 2009. The beginning of the rainy season seems to be the ideal time to release the parasitoid as the trees will be actively growing and adding biomass. Behavioural observations indicated that female EGW preferred to oviposit in young shoots, while avoiding expanding or mature leaves (Heu et al., 2006).

Conclusions

For the past 200 years the wiliwili tree has been able to withstand the onslaught of invasive species and an increase in fire frequencies. Up until three years ago, its demise was slow but steady. With the accidental introduction of the EGW, its decline has been greatly accelerated to the point where it may be extirpated from the islands. If the wiliwili tree and ultimately the dry forest surrounding Waikoloa are to be saved, then action to control the EGW is needed very quickly.

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References

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