Arboriculture & Urban Forestry 34(5): September 2008 Arboriculture & Urban Forestry 2008. 34(5):325–329. 325 Efficacy of Foliar Applications, Trunk Injections, and Soil Drenches in Reducing Populations of Elongate Hemlock Scale on Eastern Hemlock Michael Raupp, Robert Ahern, Brad Onken, Richard Reardon, Stacey Bealmear, Joseph Doccola, Paul Wolfe II, and Peter Becker Abstract. We examined the efficacy of two approaches for controlling elongate hemlock scale on eastern hemlocks in an arboretum. One approach relied on foliar applications of an insect growth regulator, pyriproxyfen, and horticultural spray oil when crawlers were abundant. The second approach evaluated soil drenches and trunk injections of the systemic insecticides imida- cloprid, dinotefuron, and acephate. Foliar applications of pyriproxyfen and horticultural oil provided superior levels of control of elongate hemlock scale compared with soil drenches, trunk injections, or implants of insecticides in the year that applications were made. After foliar sprays, population reductions were rapid and, in the case of pyriproxyfen, lasted into the second growing season. By the third year, significant differences in elongate hemlock scale populations were no longer found among trees treated with insecticides and those that were not. Imidacloprid applied as a soil drench had limited efficacy in reducing populations of elongate hemlock scale on one date in the first season. Acephate implants and trunk injections of dinotefuron did not reduce the abundance of elongate hemlock scale relative to untreated trees. Arborists can achieve high levels of control of elongate hemlock scale with foliar sprays of pyriproxyfen or horticultural oil applied when crawlers are abundant in spring. Key Words. Acephate; dinotefuran; Fiorinia externa; horticultural oil; imidacloprid; plant health care; pyriproxyfen; systemic insecticides. Several sucking insects in the order Hemiptera attack and kill hemlocks native to North America, including eastern hemlock, Tsuga canadensis, and Carolina hemlock, Tsuga caroliniana, in natural and managed landscapes. The hemlock woolly adelgid, Adelges tsugae (Adelgidae), is the most important pest of hem- locks. At least four species of armored scales (Diaspididae) at- tack T. canadensis in the eastern United States. Included in this list are three exotic species: elongate hemlock scale (EHS), Fior- inia externa Ferris; Nuculaspis tsugae (Marlatt); and cryptome- ria scale, Aspidiotus cryptomeriae Kuwana. The elongate hem- lock scale and N. tsugae are serious pests of hemlocks and sev- eral other conifers in the Northeast and Midatlantic regions (McClure and Fergione 1977; Stimmel 1986) (Figure 1). Cryp- tomeria scale has increased in importance in recent years in the Midatlantic region and is often observed at damaging levels in landscapes (Stimmel 1986; Gardosik 2001). A fourth native ar- mored scale, the hemlock scale, Abgrallaspis ithacae, is uncom- mon in the Northeast and Midatlantic region and rarely reaches damaging levels on either species of eastern hemlock (Stimmel 2000). EHS can decimate hemlocks (Garrett 1965; Wallner 1962; McClure and Fergione 1977; McClure 2002) causing dis- coloration of foliage, thinning and dieback of the canopy, and, ultimately, death of the tree. The death of trees in urban land- scapes results in significant economic loss. The loss of stands of hemlocks in forests threatens delicate upland watersheds and eliminates unique sources of germplasm when ancient hem- locks die. Davidson and McComb (1958), Garrett (1965), and Wallner (1962) were among the first to describe the biology, damage, and management of EHS in Maryland and New York, U.S., respec- tively. McClure and Fergione (1977) and McClure (1978) pro- vided detailed accounts of the distribution, abundance, and bi- ology of elongate hemlock scale and N. tsugae and their natural enemies in Connecticut. McClure (2002) provided the following summary of the biology of elongate hemlock scale: “The elon- gate hemlock scale completes two generations each year in the Southern and Mid-Atlantic States, but usually only one in the Northeast. Its life stages are broadly overlapping everywhere, so crawlers can be found throughout the spring and summer. Crawl- ers are the only stage capable of dispersing and establishing new infestations. Dispersal between trees is primarily by wind and birds. Females have three stages of development after the egg, while males have five. Within a day or two after hatching, crawl- ers of both sexes settle beneath the thin waxy cuticle on the lower surface of the youngest hemlock needles and begin to feed. The first-stage nymph for both sexes secretes a cover around itself as it grows. It then molts into a second feeding stage, continues to grow and add to its cover. The second-stage female then molts into the adult feeding stage. The second-stage male molts into a non-feeding prepupa and spins a cocoon, where it pupates before it emerges as an adult. The adult male mates with the female and dies soon thereafter without feeding. The adult female lays about 20 eggs within her cover. When crawlers hatch, they exit through a small opening at the posterior end of the cover. Elongate hemlock scale usually overwinters, either as an egg or as an inseminated adult female.” In Virginia and Maryland, U.S., EHS completes two genera- tions each year (Garrett 1965; Kosztarab 1996). Early attempts to control EHS focused on the use of contact insecticides such as dimethoate applied at the emergence of crawlers (Davidson and McComb 1958; Wallner 1962). Well-timed applications of con- tact insecticides such as horticultural oil or dimethoate applied to ©2008 International Society of Arboriculture
September 2008
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