The first category includes aphids and other leaf eaters that prey on any tree that looks appetizing to them, healthy or not. Insects such as borers, beetles and gypsy moths, along with fungi and root diseases, are opportunists. They are attached to weakened or stressed trees. To trees at that stage, insects and diseases are usually life-threatening. Choosing the right tree All too often, stress begins when a tree is planted.
Urban foresters list poor selection as a major cause of street-tree problems.
London plane trees, for example, were once favored in Seattle because of their attractive shape and resistance to air pollution. To cite other examples, sycamores and tuliptrees are resistant to gypsy moths but attract aphids. Most conifers are prized as pest-resistant, but the Colorado blue spruce has an aphid problem, resulting in the heavy loss of its beautiful needles. University of Maryland researchers found that in Washington, DC, metropolitan area ornamental cherries and plums, dogwoods, and flowering crabapples suffer severe pest problems. The thornless common honeylocust does well in the eastern U.S. but in the Pacific Northwest it is subject to the midge pod gall, for which there is no effective control.