Arboriculture & Urban Forestry 43(6): November 2017 Arboriculture & Urban Forestry 2017. 43(6):205–216 205 Relative Susceptibility Among Arborvitae Cultivars to Arborvitae Leafminer, Argyresthia thuiella (Lepidoptera: Argyresthiidae) Fredric Miller and Donna Danielson Abstract. Eighteen Thuja species and cultivars were evaluated for ovipositional preference for, and relative susceptibility to, feeding by the arborvitae leafminer. Thuja occidentalis ‘Hetz Midget’ and ‘Hoopesii’ were least susceptible; T. occidentalis cultivars 'Gracilis', 'Holmstrup', 'Spiralis', and 'Techny', and T. plicata 'Fastigiata', were intermediate in susceptibility; Thuja occidentalis cultivars 'Aurea', 'Douglasii Aurea', 'Globosa', 'Hetz Wintergreen', 'Smaragd', 'Umbraculifera', 'Wagneri', 'Wareana,' and 'Waxen', and species T. occidentalis, and T. plicata, were highly susceptible to ALM feeding. Short trees suffered significantly less feeding compared to tall trees, with number of infested branch tips (IBTs) increasing with tree height. Short trees (<4 m) had significantly more feeding damage in the upper canopy zone and tall trees (>4 m) had greater damage in the lower and middle zones. Trees with very dense canopies had significantly fewer IBTs compared to trees with sparse, medium, and dense canopies, with IBTs decreasing as canopy density increased. There were no significant associations between num- ber of IBTs and canopy aspect. Plant height, growth habit, and canopy zone and density appear to influence infestation levels and possibly ovipositional preference. These findings suggest some important ALM management guidelines for the nursery and landscape industries. Key Words. Arborvitae; Arborvitae Leafminer; Argyresthia thuiella; Insect Ovipositional Preference; Susceptibility; Thuja. The arborvitae leafminer (ALM) [Argyresthia thuiella (Pack.)] is a native insect pest of ar- borvitae (Thuja spp.). Heavy ALM infestations can kill foliage, weaken trees, give the trees a bleached, thin, unattractive appearance, and may make them more susceptible to secondary agents such as wood-boring insects and foliar diseases. Heavily infested trees can be killed. ALM is a univoltine, yponomeutoid moth (Lepidoptera: Argyresthiidae). Adults are small (about six mm long) and light gray with brown and black spots, and have long, slender anten- nae, and pale brown legs and abdomen. The wingspread is about eight mm. The larvae are 0.75 to 3 mm long, green or brown with a black head. In North America, arborvitae leafminer is found from the New England area and in east- ern Canada south to the mid-Atlantic states and west to Missouri, U.S. (Davidson and Raupp 2010; Maier et al. 2011). ALM is also known to occur in Europe (i.e., the Netherlands, Austria, and Ger- many), and an isolated population is also found in British Columbia, Canada (Busck 1907). Other leafminer species attacking arborvitae include the Canadian arborvitae leafminer (A. canadensis) and the brown arborvitae leafminer (Coleotech- nites thujaella), which both feed on northern white cedar and are common in the northeastern United States, and the cypress tipminer (A. cupres- sella), which feeds on cypress in the western U.S. Argyresthia freyella is common to North America and feeds on eastern red cedar (Cranshaw 2004; Davidson and Raupp 2010; Maier et al. 2011). The only known host is arborvitae. Aſter eggs hatch, larvae mine the leaves, basipetally killing up to three centimeters of foliage (Rose and Lindquist 1982). Mined leaves first turn yellow, then brown and dry, and may fall off. Leaf-mining injury begins to appear in summer with extensive damage obvious by autumn and the following spring. ALM overwinters as larvae, and resume feeding in spring. Exit holes are chewed just prior to pupation within mined foliage in late spring, with the pupa facing the exit hole. ALM adults start to emerge in early ©2017 International Society of Arboriculture
November 2017
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