326 Raupp et al.: Reducing Populations of Elongate Hemlock Scale Figure 1. Elongate hemlock scale is one of the most destruc- tive armored scales attacking hemlocks and other conifers in the United States. foliage kill crawlers and provide control (McClure 1977a; Stim- mel 2000; McClure 2002).This approach is most useful in land- scape settings where trees can be completely covered and are not obstructed or near water. McClure (1977a) found that incomplete coverage of the canopy resulted in rapid resurgence of EHS resulting from elimination of mobile natural enemies or en- hanced host quality resulting from relaxation of intraspecific competition. Unfortunately, foliar applications to large trees in urban settings are often complicated or impossible as a result of proximity of buildings, presence of vehicular and pedestrian traf- fic, and restrictions related to pesticide movement and exposure to nontarget organisms. Similarly, in remote locations in natural forest stands such as at the headwaters of small mountain streams, it may be impossible to access infested trees by vehicles with hydraulic applicators. In these situations, systemic insecti- cides may provide the only means of administering insecticides. The purpose of this study was to examine the efficacy of two approaches for controlling EHS: 1) foliar applications of the insect growth regulator pyriproxyfen (Distance; Valent Corpo- ration, Walnut Creek, CA) and horticultural spray oil (Horticul- tural Oil; Lesco, Cleveland, OH); and 2) soil and trunk injections of the systemic insecticides imidacloprid (Merit; Bayer Environ- mental Science, Research Triangle Park, NC; Imicide; J.J. Mauget Corporation, Arcadia, CA), dinotefuron (Valent Corpo- ration), and acephate (Acecaps 97; Creative Sales, Incorporated, Freemont, NE). The objectives of this study were threefold. First, studies were conducted to determine if single applications of insecticides listed previously provided significant control of EHS. Second, investigations were made to determine differences in scale popu- lations to applications of insecticides over the course of a single growing season. Finally, scale populations were examined on treated and untreated trees 1 and 2 years after the initial appli- cation of insecticides to determine if the abundance of EHS was reduced on trees treated with insecticides for more than 1 year. MATERIALS AND METHODS Several hundred specimens of T. canadensis populate the grounds of the United States National Arboretum in Washington, ©2008 International Society of Arboriculture DC. Before the study, we identified 115 hemlocks infested with EHS. Sixty-three hemlocks were selected for use in this study. Trees were selected on the basis of proximity to each other, similarity in size, history of pesticide treatments, and accessibil- ity for pesticide applications and sampling. Trees were excluded if insecticide applications posed risks such as drift to adjacent properties, residues on vehicles, or exposure of visitors to the arboretum. All hemlocks were mature trees, parts of managed landscape plantings, and ranged in size from 11 cm (4.4 in) to 94 cm (37.6 in) diameter at breast height (dbh). They had not been treated with any insecticides for at least 5 years before the ex- periment and received no insecticide applications other than those in this study. Trees were randomly assigned to one of seven experimental regimes. Nine trees were left untreated to serve as controls. Nine trees received a foliar application of pyriproxyfen (Distance) at a rate of 0.24 L per 378.5 L (8 oz per 100 gal). Nine trees received a foliar application of horticultural spray oil (Lesco Horticultural Spray Oil) at a rate of 2 gal per 100 gal (2 L per 100 L). Foliar sprays were applied to runoff with an FMC/John Bean sprayer (FMC, Houston, TX). Nine trees received a basal soil drench of imidacloprid (Merit 75 WP) at a rate of 1.96 g per 2.54 cm dbh (0.07 oz per in dbh) applied in 1.9 L per 2.5 cm per in dbh (0.5 gal per in dbh). When present, mulch was removed before application of Merit and replaced after the application. Nine trees received injections of imidaclo- prid formulated as Imicide delivered by Mauget injection using 3 mL (0.1 fl oz) capsules. The number of capsules used per tree was determined by measuring the dbh in inches, dividing this number by two, and injecting the tree with the resultant number of capsules. Injection sites were evenly spaced around the cir- cumference of the tree. Holes were drilled at a slight downward angle at the root flair area (approximately 6 to 8 in [15 to 20 cm]) aboveground level using a clean 11/64 in (0.4 cm) drill bit. Holes were drilled to a depth of 3/8 to 1/2 in (0.60 to 1.3 cm). Nine trees received applications of acephate formulated as Acecap 97 implants. The number of implants was determined by multiply- ing the dbh of the tree in centimeters by 3.14 and dividing the product by 10.16. Holes for implants were made at 4 in (10.2 cm) intervals starting approximately 12 in (30.5 cm) inches above- ground level and spiraling up and around the tree using a clean 3/8 in (0.95 cm) drill bit. Holes were drilled at a right angle to trunk to a depth of 1.25 in (3.2 cm). Nine trees received appli- cations of 10% dinotefuran with Arborjet injection systems. Trees were treated with 10% dinotefuran formulation at 4 mL per 2.54 cm (0.14 fl oz per in) dbh. Two of the smallest diameter trees were treated using the air hydraulic injector at 5 mL (0.17 fl oz) per injection site. These trees received three injections evenly spaced around their circumference. Holes were drilled at a right angle to the trunk to a depth of 0.63 in (1.6 cm). Re- maining trees were treated with the Arborjet Tree I.V. system. These trees received one injection site per in (2.5 cm) dbh. Holes were drilled at a right angle to the trunk to a depth of 0.63 in (1.6 cm). An average volume of 70.5 mL (2.38 fl oz) of dinotefuran was applied per tree. Davidson and McComb (1958) were the first to report “that active crawlers and all stages developmental stages of both males and females are present throughout the year” in Maryland. Davidson and McComb (1958) noted and McClure (1978) later confirmed that peak periods of crawler activity occurred in late spring and early summer (May to July) and in late summer and early fall (August to November). Oil is a contact insecticide and
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