178 DISCUSSION Despite severe EGW pressure at the treatment site, native wili- wili trees treated with trunk-injected imidacloprid maintained their pretreatment canopy conditions over the 13 months of this study; untreated trees or those treated with emamectin benzoate experienced significant canopy loss. Previous studies have dem- onstrated the toxicity of systemic imidacloprid to EGW (Xu et al. 2006; Xu et al. 2008); however, their goals were to evaluate many treatment, formulation and site combinations in a short period of time. The present study focused on evaluating imidacloprid and emamectin benzoate applied with one delivery system to native Hawai`ian wiliwili in natural, dryland conditions. The site, cli- matic and tree conditions were not ideal for insecticide injection and subsequent distribution in the plant, and should provide a conservative estimate of the recovery possible following injec- tion. Xu et al. (2006) report that an imidacloprid concentration of about 8 ppm in Erythrina leaves reduces EGW adult emer- gence by 80%. The authors use this value as the threshold for an effective concentration level in the study. Observed concentra- tions of imidaclopid in leaves at 35 d (7.7 ppm) and 13 months (23.3 ppm) correspond well with the 80% reduction in wasp emergence and exceed by 2-12 times the concentrations previ- ously reported for dryland forest E. sandwicensis injected with a different system (Sidewinder and Imicide®; Xu et al. 2006). Application method and formulation play important roles in the movement of systemics into target plant tissues. Various imi- dacloprid formulations and application techniques were evaluated by Xu et al. (2008) on the tall columnar form of E. variegata near Hilo, Hawai´i (3050-4575 mm rainfall per year) (120-180 in). Treatments included Merit 2® applied as a soil drench, Merit 200 SL® (Bayer CropScience) and IMA-jet stem-injection using the Arborjet Tree I.V., Pointer® formulation injected with the Wedg- le system (ArborSystems, LLC.), and Imicide dispensed through Mauget capsules (JJ Mauget Co, Arcadia, CA, U.S.). Mean con- centrations of imidacloprid in leaves from four replicate trees ex- ceeded 8 ppm by 21 days post-injection, and remained above this level in trees treated with either IMA-jet or Merit 200 SL until the final sample period at 12 months (Xu et al. 2008). Trees treated with IMA-jet peaked at about 350 ppm at 15 weeks post-treat- ment. Although the Merit 200 SL (0.17 g/g AI) treatment applied more active ingredient, lower residues were detected compared to the IMA-jet treatment. The Imicide and Pointer injections, along with the Merit 2 soil drenches, never produced concentrations different than zero during the 12-month study period. In another experiment, six E. sandwicensis were injected using Imicide and Sidewinder (as above) in a natural environment superficially sim- ilar to the site at Pu´u-o-kali (Waikoloa, Hawai´i island; 305 mm rainfall per year (12 in/yr) versus 375–500 mm per year (15-20 in/ yr) at Pu´u-o-kali). Mean tissue concentrations were determined three months post-injection and were too low to effectively limit EGW emergence (< 2 ppm) (Xu et al. 2006; Xu et al. 2008). Soil type, rainfall and rooting habit may all be important for uptake and movement of imidacloprid and this is probably magnified by soil applications, which are exposed to an increased number of ex- trinsic variables. Systemic insecticide drenches appear to be more effective when used to treat containerized seedlings and saplings due to the confined root system as compared to trees in landscape settings (Xu et al. 2008). As per label instructions, Imicide and Pointer treatments were applied at much lower AI rates compared Doccola et al.: Systematically Applied Insecticides for Treatment of Erythrina Gall Wasp to either Merit 200 SL or IMA-Jet, which likely affected imida- cloprid distribution to target tissues and concentration durability. Emamectin benzoate has not been previously evaluated against EGW, but abamectin, which is chemically similar, has been as- sessed against galling wasps (including EGW) in a limited number of studies. In Kentucky, Eliason and Potter (2000) stem-injected Abacide® (1g/100g abamectin; JJ Mauget Co.), at its label rate of (0.004 g/ml/cm DBH) (0.01 g/ml/in DBH) against horned oak gall wasp, Callirhytis cornigera (Osten Sacken) (Hymenoptera: Cyn- ipidae), in ornamental Pin oak, Quercus palustris Münchh. Their goal was to kill larvae within established galls and indeed they ob- served high levels of larval mortality in galls; however, the treat- ment had no effect on initiation of new stem galls. When the same rate and method were used to inject Abacide into E. sandwicensis and E. variegata in Hawai´i, little activity was observed against EGW (Xu et al. 2008). Causes of the observed variation in effec- tiveness among studies are uncertain, but could relate to toxicity differences among insect species, different formulation diffusion and distribution patterns among tree species, differences in tree physiology or different effects of heterogeneous environments. The low emamectin benzoate residue concentrations and the lack of treatment durability in this study were surprising. Em- amectin benzoate formulations have provided excellent residual activity against other insect pests when applied using similar dosages, e.g., eastern tent caterpillar, Malocosoma americanum (F.) (Potter et al. 2005), and southern pine engravers, Ips spp. (Grosman & Upton 2006). Takai et al. (2003) injected a differ- ent emamectin benzoate formulation (40 g/L; Shot Wan Liquid Formulation, formerly Novartis, now Syngenta, Japan; applied at 10 g AI/m3 of tree) into pines for nematode control and recovered residues of 0.08-2.09 and 0.15-2.14 ppm at 3 and 15 months re- spectively. After 35 d the study authors recovered residue levels of emamectin benzoate similar to those observed at three months by Takai et al. (2003); however, after 13 months this study’s lev- els were not detectable while theirs were similar to their three- month results. Whether this study’s drop in emamectin benzoate concentrations at 13 months was related to leaf fall and refolia- tion, limited hydraulic conductance of active ingredient into the leaf tissue at application, or something else is not known. At the time of treatment, this study’s trees had already experienced canopy injury and loss from EGW infestation. It seems plausible that this could alter uptake, movement and storage relationships more significantly with a large, water insoluble molecule like em- amectin benzoate. Moisture availability may also have played a role in the translocation of emamectin benzoate in the dry, upland environment over time. As mentioned, the site at Pu´u-o-kali is hot and dry with only seasonal rainfall. Relative to emamectin benzoate, movement of imidacloprid may be less dependent upon hydraulic conductance due to its smaller molecular weight (about 25%) and increased water solubility (about 21 times) (EX- TOXNET, Extension Toxicology Network, Pesticide Informa- tion Profiles, IMIDACLOPRID, and Roberts and Hutson 1998). The success of imidacloprid applications in this study and oth- ers suggests that this active ingredient is effective for maintaining Erythrina canopy and tree health for > 1 year under severe pres- sure from EGW. In particular, IMA-jet injections with Arborjet Tree I.V. provided concentrations of imidacloprid sufficient to kill 80% of EGW in galls by 35 d. Concentrations at 13 months, following a leaf senescence and refoliation event, were even ©2009 International Society of Arboriculture
July 2009
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