Arboriculture & Urban Forestry 35(4): July 2009 Arboriculture & Urban Forestry 2009. 35(4): 173–181 173 Systemically Applied Insecticides for Treatment of Erythrina Gall Wasp, Quadrastichus erythrinae Kim (Hymenoptera: Eulophidae) Joseph J. Doccola, Sheri L. Smith, Brian L. Strom, Arthur C. Medeiros, and Erica von Allmen Abstract. The erythrina gall wasp (EGW), believed native to Africa, is a recently described species and now serious invasive pest of Erythrina (coral trees) in tropical and subtropical locales. Erythrina are favored ornamental and landscape trees, as well as na- tive members of threatened ecosystems. The EGW is a tiny, highly mobile, highly invasive wasp that deforms (galls) host trees causing severe defoliation and tree death. The first detection of EGW in the United States was in O´ahu, Hawai´i in April 2005. It quickly spread through the Hawai`ian island chain (U.S.) killing ornamental and native Erythrina in as little as two years. At risk are endemic populations of Erythrina as well as ornamental landscape species in the same genus, the latter of which have already been killed and removed from O´ahu at a cost of more than USD $1 million. Because EGW are so small and spread so quickly, host injury is usually detected before adult wasps are observed, making prophylactic treatments less likely than therapeutic ones. This study evaluates two stem-injected insecticides, imidacloprid (IMA-jet®) and emamectin benzoate, delivered through Arborjet Tree I.V.® equipment, for their ability to affect E. sandwicensis (wiliwili) canopy demise under severe EGW exposure. IMA-jet, applied at a rate of 0.16 g AI/cm basal diameter (0.4 g AI/in. dia.), was the only effective treatment for maintaining canopy condition of wiliwili trees. Emamectin benzoate, applied at a rate of ~0.1 g AI/cm basal diameter (~0.25 g AI/in. dia.), was not effective in this application, although it was intermediate in effect between IMA-jet and untreated trees. The relatively high concentrations of imidacloprid in leaves, and its durability for at least 13 months in native wiliwili growing on a natural, dryland site, suggest that treatment applications against EGW can impact canopy recovery even under suboptimal site and tree conditions. Key Words: Coral Trees; Emamectin Benzoate; Erythrina sandwicensis; Imidacloprid; Tree Injection; Wiliwili. The erythrina gall wasp (EGW), Quadrastichus erythrinae Kim (Hymenoptera: Eulophidae), believed native to Africa, was first described in 2004 from specimens collected in Taiwan (Kim et al. 2004). The gall wasp is currently a serious pest of ornamental and native Erythrina (coral trees) in multiple distant locales and is predicted to spread to five continents, including the entire tropical and subtropical world (Li et al. 2006). In Hawai´i (U.S.) and else- where, Erythrina have been planted extensively at airports, parks, private residences, and farms. They are also commonly used for windbreaks and soil and water conservation, and the Hawai`ian endemic species, Erythrina sandwicensis O. Deg. (Fabaceae) is a keystone species of low-elevation dryland forests as well as an important Hawai`ian cultural and ethnobotanical resource. Erythrina sandwicensis, known locally as wiliwili, can grow 9–13 m (30–45 ft) tall, with a broad, spreading habit (Wagner et al. 1990). Leaves are alternate, compound, 13–30 cm (5.1–12 in) long, with a long slender leafstalk. The three leaflets are short- stalked with the most distal being larger than the other two. Leaflets are 4–10 cm (1.6–3.9 in) long and 6–15 cm (2.3–5.9 in) wide (Wagner et al. 1990). Wiliwili grows in dry forests on leeward slopes of all the main islands to an elevation of 600 m (2000 ft) (Powell and Nakao 1992). Rhizobial and endomycor- rhizal root associates fix nitrogen and solubolize phosphorous, important adaptations for survival in its native habitat (Powell and Nakao 1992). Outer bark is thin and stems are photosyntheti- cally active. Wiliwili is dry-season deciduous; on the island of Maui, Hawai´i, leaves are most frequently present from Novem- ber through April depending on rainfall (Powell & Nakao 1992). Erythrina species are the only known hosts for EGW. The genus includes about 115 species native to tropical and sub- tropical regions (Neill 1993); at least 59 appear to be hosts for EGW (Messing et al. 2009; W. Fulton, National Tropical Botani- cal Garden, pers. comm.). In botanical gardens in Hawai´i, the most severe injury has been observed on species of non African origin, supporting the putative African origin of EGW (Mess- ing et al. 2009). Erythrina’s showy flowers make them popular ornamentals that are cultivated in landscapes and horticultural gardens in locations such as southern California, Florida and Hawai´i (Figure 1A, Figure 1B). The largest collection of Eryth- rina species on the continental United States is found at the San Diego Zoo, where 53 species are currently growing (San Diego Zoo 2008). The degree of worldwide susceptibility to EGW is uncertain because Erythrina species differ in their host-quality for the wasp and many species have not yet been exposed. In- dian coral tree, Erythrina variegata (L.) Merr., native to India and Malaysia, is used as an ornamental and street tree in warmer re- gions worldwide and is a favorite host of EGW. Two forms of E. variegata, a tall, columnar, windbreak form and a more spread- ing, ornamental form, were nearby to initial EGW detection sites in Hawai´i and likely facilitated its spread into native forests. The EGW was first detected in the United States on O´ahu, Hawai´i in April 2005 (Heu et. al. 2006) and spread to the other Hawai`ian ©2009 International Society of Arboriculture
July 2009
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