Common Street Trees and Their Pest Problems in the North Central United States

Tree species

Sugar maple (Acer saccharum), Norway maple (Acer platanoides), and pin oak (Quercus palustris) were among the 10 most common street trees in all subregions except in the Dakotas where they do not grow well (Table 1). American elm (Ulmus americana) was common except in the warmest subregion (IV) where elms were first devastated by Dutch elm disease. Siberian elm (Ulmus pumila) was common in subregion IV, reflecting its position as an early substitute for American elm, and in subregions I and II where it can withstand severe winter temperatures and summer drought. Hackberry (Celtis occidentalis) was common in the 2 cooler subregions but was seldom mentioned in areas with more moderate winter temperatures. Crabapple (Malus spp. hybrids) has been widely planted except in subregion II where cultivars resistant to apple scab could undoubtedly be used more frequently. Honeylocust (Gleditsia triacanthos) was common in the middle subregions but seldom used in the northern and southern extremes of the North Central region.

Ash, probably a mixture of green ash (Fraxinus pennsylvanica) and white ash (F. americana), was the second most common, best adapted (Table 2), and fifth favorite (Table 3) street tree. This is somewhat surprising because ash are commonly attacked by several damaging insects, including oystershell scale (Lepidosaphes ulmi), lilac borer (Podosesia syringae), and banded ash clearwing (Podosesia aureocincta). Their popularity probably relates to ease of transplanting and establishment, rapid growth rate, ability to withstand environmental extremes, and general availability.

Hackberry was considered the favorite and second best adapted street tree, based primarily on responses from the 2 subregions with harsher winters where the tree thrives in spite of the hackberry nipplegall maker (Pachypsylla celtidismamma) and the hackberry blistergall maker (P. celtidisvesicula). Hackberry is seldom used and rather uncommon in subregions III and IV, probably due to numerous cosmetic problems and relatively short service life.

Linden, probably a mixture of American linden(Tilia americana) and little-leaf linden (T. cordata), was the third most popular and third best adapted street tree. However, it was among the 10 most common street trees only in subregion III. Although little-leaf linden initially appeared to be an excellent street tree east of the Mississippi River, many have failed recently from a syndrome sometimes called crown rot (T. D. Sydnor, personal communication).

Sugar maple was ranked fourth as both favorite and best adapted street tree for the region. It was thought to be among the best adapted trees in subregions III and IV and was the favorite tree in subregions II and III. Favorite trees are not necessarily the most commonly planted, and best adpated trees are not necessarily chosen over others.

Thornless honeylocust is thought to be among the best adapted trees in subregions II and III and is a favorite in subregions I and III; however, it is commonly attacked by numerous insect and mite species. Early defoliation usually results in a new flush of leaves that may either enhance the beauty of the yellow-leafed cultivar ‘Sunburst’ or provide a fresh green canopy that gives homeowners and landscape managers a false sense of security about the tree’s vitality. Recent evidence indicates that early defoliation, followed by production of new leaves, reduced the stored energy in the tree, perhaps increasing its suspectibility to borers (Herms 1984).

Honeylocust was among the 10 most common street trees, the 5 best adapted trees, and in subregions I and III a favorite tree. Although it was preceived as undesirable in these subregions, it was also considered the third best adapted tree in subregion III. These contradictory responses may indicate that well-known, readily available trees are sometimes selected for planting without knowledge of or regard for their maintenance problems. Nectria and thyronectria cankers have become common on honeylocust in areas of high drought stress and low winter temperatures.

Callery pear (Pyrus calleryana) cultivars were not considered among the best adapted and were not among the most common street trees, but they were the second favorite tree, being listed by al subregions except the Dakotas. Clearly, the medium-size ornamental pears will become more common as street trees, because they presently have few pests, produce early and showy flowers, and have striking fall color. The only flaw recently reported for some of the more popular cultivars is crotch weakness with advancing age (Haserodt and Sydnor 1983). This problem can be minimized by proper pruning during nursery production and soon after transplanting.

Silver maple was the most common (Table 1) and least desirable (Table 4) street tree, was not included in the top 5 best adapted trees (Table 2) by anyone, but was the fifth favorite tree (Table 3) in subregion I where it was not considered an undesirable tree. These discrepancies reflect the disagreement among even the most noted landscape horticulturists, some of whom champion silver maple as a street and landscape tree if properly pruned, while others condemn it. There can be no disagreement that this species has stood the test of time as a street tree while neighboring American elms and chestnuts have perished. Vital, properly maintained silver maples thrive in the urban environment, have few insect and disease problems and outlive many of their tree neighbors in the landscape. Silver maples cancontinue to be assets in the urban landscape if properly managed.

So-called brittle, lowland, and often short-lived trees, including box elder (Acer negundo), cottonwood (Populus deltoldes) and other poplars (Populus spp.) were commonly listed as undesirable street trees. Except in harsh climates where these are some of the only trees that will grow, they should not be used as street trees or as specimen landscape trees. Siberian elm, a common replacement following removal of dead American elms, is now recognized as an undesirable tree and should not be used. The Chinese elm (Ulmus parvifolia) is a much better choice that is adapted well for the North Central region.

London Planetree (Plantanus acerifolia) and sycamore (Platanus occidentalis) were among the 10 most common street trees in subregions II and IV. The former was the third best adapted tree in subregion IV. Neither was viewed as undesirable by any respondent; both could be used more extensively where large trees are specified, especially in subregions III and IV. Although anthracnose is a problem in some areas, especially during wet, cool springs, trees survive even without direct control measures.

Oaks (Quereus spp.) were not perceived as undesirable, but only pin oak was listed as common. Bur oak (Quercus macrocarpa) (subregion I) and red oak (Quercus rubra) (subregion II) were considered among the best adapted trees; red oak was a favorite in subregions II and IV. Although some oaks, especially those in the white oak group, may be somewhat difficult to transplant, they are sturdy, long-lived trees that commonly do well throughout subregions II, III, and IV.

Tree condition

Respondents were asked to rate the condition of all trees inventoried for their 5 most common street trees. Averaging all estimates from 35 cities throughout the region, 48% of the trees were considered to be in good condition, 31 % fair, 19% poor, and only 2% dead or dying. As expected, condition varied significantly between tree species. Sugar and Norway maples and honeylocust were in similar and better condition than silver maples; there was more variability in condition classes with the latter (Table 5). Most green ash and hackberry were in good or fair condition. Fewer American elms were in good condition and a higher percentage were dead or dying. Many Siberian elms were in poor condition. Nearly 75% of the pin oaks and 96% of crabapples were in good condition; few were in poor condition or dying.

Based on this information, it appears that Siberian elm has not been a good tree and has been a poor substitute for American elm. Sugar and Norway maples and pin oak are perceived to be the best large street trees; honeylocust the best medium-size tree, and crabapples the best small trees in the North Central region. Information on tree condition may be at least partially reflective of urban tree pest management needs.

Insects and diseases

Respondents indicated that only a few species of insects and pathogens cause most of the pest problems on street trees (Table 6). Lepidopterous defoliators, pests that are easily controlled with conventional insecticides or the bacterium Bacillus thuringiensis, were the most common insect pests. Although information about pests was not analyzed according to subregion, bagworm (Thyridopteryx ephemeraeformis) is a problem in subregions III and IV, cankerworms (fall cankerworm, Alsophila pometaria, and spring cankerworm, Paleacrita ver- nata) are common in subregions I and II, and eastern tent caterpillar (Malacosoma americanum) is common throughout the region. Borers, probably including round-headed (Cerambycidae), flat-headed (Buprestidae) and clearwing moth (Sesiidae) species, were next most common, followed by leaf beetles (primarily elm leaf beetle (Pyrrhalta luteola), scales, aphids, and bark beetles. Most of the bark beetle problem is associated with Dutch elm disease; scales are insidious pests that commonly cause damage before they are detected. Borer attack often indicates that the tree is in low vitality as the result of other biotic or abiotic factors or a combination of environmental stresses. Influence of tree vitality on insect and pathogen attack and damage is a current area of research that may someday change the way urban pests are perceived and managed.

Dutch elm disease is still a serious problem, especially in more nothern parts of the North Central region where landscape managers continue the fight to protect and preserve existing American elms. Anthracnose, wilt diseases, and fire blight are chronic problems for which there are limited direct control options (Table 6).

Enviromental stresses

When given the opportunity to assess the occurrence of perceived environmental stresses on street trees, respondents indicated rather divergent but related categories (Table 7). For example, drought was reported as the most common environmental stress problem; winter dieback was also listed but may be drought-related. Soil compaction was listed a distant sixth to drought, but the former may exacerbate or cause drought symptoms even during periods of normal rainfall. In any case, factors that reduce the ability of roots to thrive in soil are recognized as key environmental stress problems for street trees. Research is needed to determine if preplanting practices can alleviate these problems and if soil amelioration around existing trees can improve tree vitality and increase longevity.

Lawn mower damage and vandalism were reported as important environmental stresses that impact street tree vitality. These problems will always occur but can be minimized in many localities through educational progams, use of mower guards and mulch, and planting trees with at least 5 cm (2 inch) caliper to initiate street tree plantings, whenever possible.

Pest Management

A primary goal of our survey was to determine urban forest pest management needs. Although we did accumulate information on the most important pests and environmental stress factors, as perceived by municipal arborists and foresters, respondents were commonly unfamiliar with appropriate control tactics and management strategies for controlling insects and diseases. This was somewhat surprising since Giedraitis and Kielbaso (1982) reported that, nationally, 41 % of municipalities monitor pest levels on their street trees. It would seem that personnel who are trained well enough to monitor trees for pest presence and abundance would also be familiar with pest control options.

Apparently, much insect and disease control work is done by contract arborists. Of those reponsing to the question, “are you familiar with appropriate management techniques for your most common street tree insect problems?,” 36% indicated yes, 39% were familiar with adequate control tactics for some important insect pests, while 1 7% had no knowledge of insect control tactics for street trees. The most frequently used control tactics included: pesticides (56%), sanitation (31%), cultural practices to improve tree vitality (9%), and use of tolerant or resistant trees (4%). Of those responding to the question, “are you familiar with appropriate management techniques for disease problems?,” 23% said yes, 51 % were familiar with control tactics for some of their most important tree disease problems, while 20% had no knowledge of disease control tactics for street trees. The most frequently used control tactics for diseases included: sanitation (53%), pesticides (37%), use of tolerant or resistant trees (10%), and cultural practices to improve tree vitality (4%).

When asked if they were familiar with appropriate management techniques to minimize effects of environmental stress, 14% responded yes, 47% said yes for some stresses, and 31 % were unfamiliar with management techniques for any environmental stress. The most common techniques used to manage environmental stresses were: use of tolerant or resistant trees (22%), improving tree vitality (20%), watering (17%), sanitation (12%), fertilization (12%), proper planting technique (10%), and wrapping and pruning (7%).

Cities with less than 10,000 inhabitants rarely used pesticides or fertilizer on street trees. Those with populations of 10,000 to 49,999 used an average of 8 pounds of insecticides, 3 pounds of fungicides, 5 pounds of herbicides, and 8900 pounds of fertilizer annually. Cities with populations of 50,000 to 249,999 used 243 pounds of insecticides, 687 pounds of fungicides, 16 pounds of herbicides, and 3200 pounds of fertilizer. Cities larger than 250,000 population used 287 pounds of insecticides, 167 pounds of fungicides, 94 pounds of herbicides, and 767 pounds of fertilizer annually.