Arboriculture & Urban Forestry 39(1): January 2013 tions may be successfully managed in ornamental landscape plantings with commercial insecticides. Insecticides used for managing insect and mite pests of hemlocks include systemic insecticides (imidacloprid or dinotefuran), contact insecti- cides, insect growth regulators, and horticultural oil (Cowles et al. 2006; Raupp et al. 2008; Cowles 2009; Cowles and Cheah 2002a; Cowles and Cheah 2002b). Effective materials for targeting EHS include autumn or early spring application of dinotefuran as a bark spray or soil drench (Cowles 2010), interruption of development with pyriproxyfen, an insect growth regulator (Raupp et al. 2008), or application of horticultural oil (Raupp et al. 2008). Horticultural oil is especially important because it can effectively suppress hemlock woolly adelgid, armored scales, spruce spider mites, and hemlock rust mites, while having limited impact on beneficial predators and para- sites due to its non-residual properties (Davidson et al. 1990). The potential phytotoxic response of ornamental trees and shrubs to modern summer-type paraffinic horticultural oils is both unpredictable (from a phylogenetic perspective) and rela- tively uncommon (Baxendale and Johnson 1988; Davidson et al. 1990). In their severe test for phytotoxicity under drought con- ditions, Davidson et al. (1990) repeatedly sprayed 52 species or cultivars with 2% Sunspray 6E PLUS. Of these, there were 13 representative conifers, and changes in plant appearance were observed only based upon disturbance of glaucous waxes (such as Colorado blue spruce, Picea pungens) in two species, which may not be injurious to the plant, and highly variable injury to P. abies, which was attributed to possible excessive spray appli- cation. A slight yellowing on some leaves was noted for weep- ing hemlock (T. canadensis ‘Sargentii’), but not the species form of T. canadensis. In other studies of conifers Douglas-fir was found to be sensitive to horticultural oil, as demonstrated by reproducible chlorosis among treated trees (Varela et al. 1996), whereas true firs (Abies spp.) typically exhibit chlorosis of new growth if sprayed with oil during shoot elongation (Cowles, pers. obs.). The replicated plantings of T. canadensis and T. chi- nensis at Lasdon Arboretum provided an opportunity to inves- tigate and compare phytotoxic response to horticultural oil in a replicated trial using 15 individual specimens of each species. MATERIALS AND METHODS Research Site On September 30, 2003, a total of 18 specimens of T. chinen- sis were established in three different experimental plots, among six other species of hemlock (T. canadensis, T. caroliniana, T. heterophylla, T. mertensiana, T. sieboldii, T. diversifolia) in randomized plots at Lasdon Arboretum (Weston and Harper 2009). Plots were approximately 12 m × 14 m, ranging from full- partial sun (Front Gate and Magnolia Garden Plots) to full shade (Hemlock Hedge Plot) established within fenced perimeters to protect trees from deer herbivory. Soils where the hemlock plant- ings were established were clay-based and had previously had significant amounts of organic matter incorporated into them through annual mulching with 5–10 cm depth of wood chips, since their time of establishment. High levels of earthworm activ- ity make maintaining an adequate mulch cover difficult. Over- all conditions during the summer prior to oil application were warmer and drier than normal, as indicated by averages from 7 Figure 1. Monthly average temperature maximum and minimum data, and precipitation data for Yorktown, New York, U.S., for 2010, are compared with their deviations from the 30-year average (including 2010). the previous 30 years at a nearby weather station (Yorktown, NY). Temperature and precipitation monthly averages and their deviations from 30-year averages are presented in Figure 1. Treatment Protocols Five of the largest T. chinensis (3.1–4.6 m height, mean of 3.7 m) and an equal number of similarly sized T. canadensis specimens (2.6–5.1 m height, mean of 3.7 m) were identi- fied in each of the three plots, for a total of 30 experimental units. Six individual branches on each study tree were marked with color-coded flagging tape to identify the treatment com- binations in a 3 dosages × 2 spray timings factorial design experiment. Treatments were: 1) 1% horticultural oil appli- cation, 2) 2% horticultural oil application, and 3) untreated check. The first treatment was administered on September 29 and the second spray was applied on November 10, 2010. Lesco Horticulture Oil Insecticide (98.8% mineral oil) was applied with a 7.6 L hand-held pump (garden) sprayer to the point of runoff. The first application timing was selected to be optimal for HWA management. Late September—early October is a window of treatment opportunity to contact sistens (the overwintering generation) before they resume develop- ment and cover themselves with wool. If there was a significant acute phytotoxic response, such as needle loss, then it might occur within one to four weeks after spraying. Later spray tim- ings can be of concern as horticultural oil may affect the tree physiology or leaf cuticle integrity for some weeks (Hodgkin- son et al. 2002), or lead to injury that may occur when sprays are followed by freezing temperatures, possibly resulting in ©2013 International Society of Arboriculture
January 2013
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