Arboriculture & Urban Forestry 38(2): March 2012 Arboriculture & Urban Forestry 2012. 38(2): 41–49 41 Treatment Strategies Using Imidacloprid in Hemlock Woolly Adelgid (Adelges tsugae Annand) Infested Eastern Hemlock (Tsuga canadensis Carrière) Trees Joseph J. Doccola, William Hascher, John Joseph Aiken, and Peter M. Wild Abstract. Due to the widespread establishment of hemlock woolly adelgid (Adelges tsugae Annand) (HWA) across the range of eastern hemlock (Tsuga canadensis Carriere), woodland trees may be infested for many years before treatment is made. Symptoms of prolonged infestation in- clude extensive dieback and thinned canopies. Imidacloprid, a systemic neonicotinoid insecticide, is a useful and effective tool to manage HWA. In this study, mature, large diameter trees in poor condition were treated with imidacloprid. Trees were treated once by trunk and/or soil injec- tion in Asheville, North Carolina, U.S. Following application, changes in tree growth, HWA density and imidacloprid residues were measured for three years. Trees treated with imidacloprid recovered, whereas the untreated trees continued to struggle. Trees injected with imidacloprid ac- cumulated compound in the canopy, facilitating refoliation and the imidacloprid persisted for three years. This extended activity of trunk-in- jected imidacloprid was attributed in part to slow upward movement through the restrictive tracheid vascular system and to perennial needle retention. The imidacloprid soil injection was slower to act systemically, but has potential for longer-term activity. Researchers suggest the combi- nation of tree and soil injection for immediate and long-term (4+ years) activity as an effective and economic strategy to protect high-value trees. Key Words. Eastern Hemlock; Hemlock Woolly Adelgid; Imidacloprid, Soil Injection; Tree Injection. Eastern hemlock (Tsuga canadensis Carrière) covers a wide range of the United States, from New England, New York, Pennsylvania, and the Mid-Atlantic states, expanding west- ward from central New Jersey to the Appalachian Mountains, and continuing south into northern Georgia and Alabama. In the Midwest, hemlock occurs predominantly in eastern Ohio, northern Michigan, and Wisconsin (USFS). First iden- tified in the early 1950s in Virginia, U.S., hemlock woolly adelgid (Adelges tsugae Annand) (Hemiptera: Adelgidae) (HWA) is an invasive insect that infests hemlock trees. It is considered a pest of Tsuga canadensis Carrière and T. caro- liniana Engelmann. The biotype of HWA found in eastern North America genetically matches populations native to the island of Honshu, Japan (Havill et al. 2006). HWA occurs in India, Japan, and China, on four Asiatic species, includ- ing T. dumosa Eicher, T. forrestii Downii, and T. chinensis Pritz (Montgomery et al. 2000). Symptoms of HWA infes- tation include canopy thinning, twig dieback, tree decline, and ultimately death. HWA has spread and now infests ap- proximately half of the eastern native range of hemlock. Low temperatures may play a role in HWA mortality, there- by limiting its northward spread. Although the cold tempera- ture sensitivity of HWA varies seasonally, substantial mortality occurs at temperatures of -30°C to -25°C (Costa et al. 2004). Such low temperature extremes may occur infrequently, or not at all, in the southern extent of the hemlock range. At least one study suggests that hemlock decline is more rapid in southern Appalachia compared to northern areas (Nuckolls et al. 2009). persicae, the peach tree aphid) in one week in Q. palustris tree in- jection compared to eight weeks for soil application, whereas in HWA has piercing–sucking mouthparts and feeds within the xylem parenchyma cells (McClure 1987) on twigs at the needle base. The xylem parenchyma is living symplast that is rich in carbohydrates and other essential nutrients; as adelgids feed on these food resources, less is available for foliar growth and other essential functions (Shigo 1989). As an infestation pro- gresses, twig lengths decrease, followed by loss of tip growth. McClure (1991) suggested an inverse relationship between HWA population and hemlock growth. This growth loss oc- curs with increasing HWA pressure and may take three to four years; the impact may be compounded by drought or poor tree vigor. Ward (1991) suggested a threshold of 25–30 HWA/100 needles as negatively impacting twig growth. Doccola et al. (2007) suggest a threshold of 2–4 HWA/cm twig growth. McClure (1992) investigated the efficacy of organophosphate insecticide injections and implants for the control of HWA. He reported significant HWA mortality four weeks and five months after treatment. The efficacy of soil injected imidacloprid against HWA has been reported (Steward and Horner 1994; Steward et al. 1998). The translocation of imidacloprid applied to the soil and by trunk injection have been the subject of some investigation (Tattar et al. 1998; Dilling et al. 2010). Tattar et al. (1998) studied three species (Quercus palustris, Tsuga Canadensis, and Pinus strobus), one of each pair receiving soil-applied imidacloprid, the other receiving trunk-injected imidacloprid. Imidacloprid reached lethal concentrations (established at 0.15 ppm, the LC95 for Myzus ©2012 International Society of Arboriculture
March 2012
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