Arboriculture & Urban Forestry 41(1): January 2015 37 Acer I or D or S Probable Cause ↓ I Gall midges 11 D Gypsy Moth 11 I Honey Locust leafhoppers and Plant bug (Diaphnocoris chlorionis) S Northern cedar bark beetle 1 D Oak Leaf Blister Taphrina caerulescens 1 S Other Native Borers (combined) 11 11 D Pinewood nematode, Bursaphelenchus xylophilus. 11 D Powdery Mildew (combined) 1111 S Shothole borer 1 I Soft scales: Black, Brown, Citricola, Cottony cushion; Fruit lecanium; Nigra 11 11 S Spider mites (combined) 11 I Spruce aphid, Elatobium abietinum. 1 D Tar spot, Rhytisma arbuti. 11 I Twolined Chestnut Borer Agrilus bilneatus 1 D Verticillium wilt, Verticillium albo- atrum, V. dahliae. 11 1 D White Pine Blister Rust Cronartium ribicola 1 I Sirex WoodwaspSerix noctilio (F.) 1 11 11 1 1 11 11 1 11 1 11 11 11 1111 1 11 1 1 11 1 1 1 1 Maple (Acer spp.) Box elder (Acer negundo) Ash (Fraxinus spp.) Honey locust (Gleditsia triacanthos) Spruce (Picea spp.) Acer neg. Fraxinus GleditsiaPicea Pinus 2PopulusPrunusThuja Pine: scots, nigra, sylvestris, resinosa sugar, brutia, jeffrey Poplar, Cottonwood, Aspen Cherry, Plum, Prune, Peach, Nectarine (Populus spp.) (Prunus spp.) Ulmus amer.AcerBetulaCercisCornusEuonymusFagus Cedar(Thuja occidentalis) Elm: American + European (Ulmus spp.) Maple (Acer spp.) Pest count → 12 785379919 12 Birch (Betula spp.) Redbud (Cercis spp.) Dogwood (Cornus spp.) Euonymus (Euonumus spp.) Ginkgo Beech (Fagus spp.) Maiden-hair tree (Ginkgo biloba), Ginkgo Gleditsia Honey locust (Gleditsia triacanthos) PopulusQuercus decSyringa Poplar, Cottonwood, Aspen (Populus spp.) Oak - deciduous (Quercus spp.) 8 5832059 11 TaxusTilia Thuja Lilac (Syringa spp.) Yew (Taxus spp.) Linden (Tilia spp.) Cedar(Thuja occidentalis) 5 031 1 1 11 11 11 it with uncommon or under-utilized genera will help to increase the forest’s overall diversity value. More steps could be taken to further decrease pest vulnerability by addressing the limited presence of coniferous trees. Discounting Thuja, because of their primary use as hedge species, only Picea make up more than 5% of either the street or non-street tree populations citywide. Though not above 5% city- wide in either population, Pinus is also common in one management district of Toronto. Compounding the current low coniferous diversity, the trees most commonly planted by all actors surveyed or inter- viewed are overwhelmingly deciduous. The only coniferous genera noted were Taxus and Thuja by a handful of garden centers and nurseries. In Jactel and Brockerhoff’s (2007) meta-analysis investigating insect herbivory and tree stand diversity, their find- ings showed an increased resistance to herbivory where there is a mixture of broadleaf and coniferous trees. Pest vulnerability potentially could be further reduced in Toronto if more coniferous trees were planted in both street and non-street populations. Another step to help reduce pest vulnerability is to decrease the amount of ornamentals planted. Ornamentals tend to be more susceptible to pests, causing primarily aesthetic issues, but these minor issues can become larger problems due to public concern (J. Ric, pers. comm., July 10, 2013). Ulti- mately, they can cost more in terms of management and care, as well as in mortality and replacement. It is important to acknowledge the limitations of this work. Examining these topics at the citywide scale likely misses important intracity spatial varia- tion that exists both in current composition and planting trends. Additionally, the use of a sample, rather than a complete tree inventory for non-street ©2015 International Society of Arboriculture
January 2015
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