Arboriculture & Urban Forestry 35(4): July 2009 Methods for effective prophylaxis and rapid response are cru- cial for limiting EGW-caused Erythrina mortality. Short-term strategies center around systemic insecticides and longer-term strategies on traditional biological control (Heu et al. 2006, Xu et al. 2008). Insecticides applied by tree injection provide a num- ber of benefits which may include compact equipment for use in remote areas, little or no water required for application, less volume of active ingredient needed for treatment and potentially less environmental exposure compared to soil or foliar treat- ments, systemic movement of chemistry into the targeted tissues, relatively greater residual activity, and less frequent applications (compared to canopy sprays). Imidacloprid, applied as a sys- temic, caused significant mortality of EGW in Hawai´i (Xu et al. 2006). Xu et al. (2006) found that concentrations of imidacloprid in E. sandwicensis leaf tissue were correlated inversely with host injury and wasp emergence; as tissue concentrations reached 8 ppm, wasp emergence from galled tissues dropped by 80%. How- ever, this level of imidacloprid concentration was obtained only in trees located in an irrigated resort landscape. Concentrations of imidacloprid using either the Sidewinder® (Sidewinder Pty Ltd, Noosaville, Australia) or Wedgle® (ArborSystems, LLC, Omaha, NE, U.S.) injection systems on native wiliwili in dryland forests did not exceed 5 ppm after 4–5 months. Therefore, injection systems and/or formulations that produce higher tissue concentrations of imidacloprid are needed to improve protection options for trees in harsher environments. Emamectin benzoate (Syngenta Crop Protection Inc., Greensboro, NC, U.S.) has not been evaluated for efficacy in this system, but applications in other pest control situations suggest that it can provide longer residual activity than imidacloprid (Grosman et al. 2002; Doccola et al. unpublished). Imidacloprid is a widely used neonicotinoid and emamectin ben- zoate is a semi-synthesized avermectin compound with broad- spectrum activity against insects. This study evaluated imidaclo- prid and emamectin benzoate for their effectiveness in reducing host injury caused by EGW in a native wiliwili dryland forest. Both active ingredient (AI) formulations were injected with Tree I.V.® equipment (Arborjet, Inc., Woburn, MA, U.S.); imidaclo- prid as IMA-jet® (Arborjet, Inc.) and emamectin benzoate as a formulation that has evolved to TREE-äge® (Syngenta Crop Pro- tection, Inc.). IMA-jet is registered for use in the United States and TREE-äge has been submitted for registration to the United States Environmental Protection Agency. The study authors believe that environmental conditions at the site, along with the natural growth form of E. sandiwicensis and pre-existing canopy injury caused by EGW, combined to make results of this study conservative relative to applications implemented under more typical orna- mental conditions, especially those with supplemental watering. METHODS Sixty E. sandwicensis trees (20 per treatment) were selected for this experiment with consideration of their canopy health (average for the forest), diameter (avoiding outliers), location away from other experiments within the fenced exclosure, and nearness to equipment access at the site in Pu´u-o-kali, Maui, Hawai´i, USA (20°44’05.78”N 156°24’11.57”W). Pu´u-o-kali is on leeward southwestern Maui [210 m (689 ft) elevation], near Kihei, and experiences hot and dry weather with only seasonal rainfall (375–500 mm per year; 15-20 in/yr). Trees were esti- mated to be up to several hundred years old at the time of treat- ment and ranged from 23 to 74 cm ( = 41.7 cm) (9.6 to 29.6 in; = 16.4 in) in basal diameter (Table 1). Treatments were implemented in January 2007 (see below), about 1.5 years after initial detection of EGW on Maui. Thus, trees were infested and had suffered significant (but unquantified) canopy injury prior to treatment. Pre- and post-treatment canopy condition and injury were evaluated using three measures: canopy foliage density, the ratio of galled to total leaf area, and branch tip dieback. Cano- py foliage density was estimated for each tree using a spheri- cal densiometer (Model C, Robert E. Lemmon, Bartlesville, OK, U.S.; Lemmon 1956). Densiometer measurements were taken at the four cardinal directions with the user’s back to the tree bole. Total and galled leaf areas were estimated with a square centimeter grid from branch tips at the outer edge of the lower canopy. One branch tip was chosen at each of four randomly se- lected compass points. For densiometer and galled to total leaf area measurements, tree means were produced to provide a single value per tree for analysis. Branch tip dieback was estimated by visually examining the entire crown of each tree to categorize percent dieback as zero or in 25% increments (five categories). All measurements were recorded at time of treatment (0 months) and 13 months thereafter. Foliage was sampled for determina- tion of insecticide residues at 0 months, 35 days and 13 months. Study trees received one of three randomly assigned AI treatments: emamectin benzoate (Avermectin B1 , 4”-deoxy- 4”-(methylamino)-, (4”R)-, benzoate (salt), formulated at 4.21 g/100g concentration); imidacloprid {1-[(6-chloro-3-pyridinyl) methyl]-N-nitro-2-imidazolidinime formulated as IMA-jet at 5 g/100g concentration}; or none (untreated control trees). Insec- ticide application methods and rates were determined from ex- isting (IMA-jet) or submitted (TREE-äge) insecticide product labels. Dosage rates for the emamectin benzoate formulation were 1.6–2.4 ml/cm diameter (4–6 ml/dia. in) and 1.6, 3.2 or 4.8 ml/cm diameter (4, 8 or 12 ml/in diameter) for trees < 30 cm, 30–58 cm, and > 58 cm (< 12, 12–23, and > 23 in diameter) re- spectively, for IMA-jet. In applications against pests with chew- Table 1. Summary of mean treatment dosages, costs, uptake times and changes in galled to total leaf area and insecticide residues recovered from leaf tissues in Erythrina sandwicensis at Pu´u-o-kali, Maui, HI, U.S. Treatment Emamectin Imidacloprid Untreated DBHz (cm) 42.8a 41.2a 45.6a Dose (mL) 172a 131b N/A Cost (per tree) $45 $46 N/A Uptake Time (min) 16.7a 21.6a N/A z Means followed by the same letter in a column are not statistically different, n=19-20 trees per treatment. y Values are uncorrected for percent efficiency of chemical extraction. x Minimum detectable limit = 0.01 PPM. ©2009 International Society of Arboriculture % Galled/Total Leaf Area (cm2 (0 d) 3.39a 1.79a 1.98a PPMy ) (394 d) 13.56a 2.95b 11.07a (35 d) 0.63 7.67 0.00 (13 mo) NDx 23.34 0.00 175
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