Urban Shade Trees and Carpenter Ants

Tree species, damage, and nesting

In general, the species densities and diversities of urban shade trees are reduced compared with the native forests. As tree density is generally quite low, and trees tend to be regularly spaced, single shade trees can represent the only available nesting site for carpenter ant colonies, especially if human structures are protected and well maintained.

The propensity of some species of carpenter ants to nest in live standing trees has been well documented (Fitch 1856; Graham 1918; Felt 1928; Sanders 1864). However, the effect of tree species and mechanical damage to the tree on carpenter ant nesting has not been examined. Therefore, we conducted a survey of the shade trees of some urban streets of central New Jersey. We then calculated the percentage of trees of each species having carpenter ant nests, and their relative utilization using Ivlev’s (1961) electivity (E) index, and “nesting” ratio (E’) (Fig. 2). A positive E represents utilization greater than expected if nesting were proportional to abundance, while negative ratios represent the converse. For nesting ratios, indices greater than one indicate preference, while values less than one indicate non-preferred species. Ratios in the neighborhood of one indicate that a constant fraction of the trees available are utilized as nesting sites.

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Figure 2.

Percent of trees with carpenter ant nests for each tree species examined. Given also are Ivlev's indices of electivity (described in text).

The results of our survey (Fig. 2) suggest a rank progression of electivity, or “preference,” from white pine upward through silver maple. White pine is apparently avoided as a nesting site, at least in the area of our survey. Overall, 75% of all trees surveyed had carpenter ant nests, corresponding to a similar finding for the trees of urban parks in central New Jersey (Fowler 1980). White pine was the only species not used as a nesting site by carpenter ants.

To examine the effects of visible damage on the presence of carpenter ants, trees were scored for ant nests and visible mechanical damage, such as collision scars from automobiles, loose bark, pruning wounds, etc. (Table 1). By tree species, only red maple, Norway maple, and red oak demonstrated a significant association (Fisher’s P) between visible damage and carpenter ant nesting. Insufficient sample sizes may have precluded a significant association for other tree species. Nevertheless, over all tree species, the association between visible damage and carpenter ant nesting was highly significant (Fisher’s P = <0.01). This suggests that proper tree maintenance is important in minimizing carpenter ant nesting, as colonization usually occurs via wounds (Herrick 1935).

Effects of carpenter ant nesting on shade trees

Because a high percentage of urban shade trees are used as nesting sites by carpenter ants, their nesting activity poses several problems. In forests, carpenter ant nests have been implicated as contributing to wind breakage (Graham 1918; Felt 1928; Schread 1952). The effect of tree size and carpenter ant nesting on wind breakage has recently been documented for urban trees (Fowler and Roberts 1982). However, unless the tree is highly exposed, and nest excavations are extensive, wind breakage should not present a substantial problem, as the incidence of wind breakage of trunks and major limbs is not high. However, in exposed situations where wind breakage could produce substantial property or bodily damage, trees should be inspected for carpenter ant nest excavations. Large accumulations of sawdust serve as a warning that wind breakage may have to be evaluated. If the risks of property damage are deemed high, the tree should be removed. Treatment of the colony with a recommended pesticide may be warranted to prevent further structural weakening if ant excavations are in an early stage in an exposed tree. In such a case, the treated nest should then be sealed to prevent future colonization by carpenter ants, other insects, or pathogens.

Discussion

The most common effects of carpenter ant colonies located in shade trees is probably that of a nuisance. Throughout the summer, carpenter ants subsist primarily upon honey-dew (Fowler and Roberts 1980). Although their protection of aphid colonies, especially in cherry trees, may lead to extensive leaf-curling in ornamentals and shade trees, these effects probably can be controlled by placing sticky bands around the trunks to preclude ant visitation. Aphid populations are then generally reduced by natural controls, such as predation by coccinelid beetles. When colonies occur in large shade trees in parking lots, over picnic areas, or alongside driveways, and these trees contain aphid populations, the amounts of honeydew produced may be very large, leading to the voiding of copious amounts. In such cases the ants may not be able to harvest all of the honeydew. Honey-dew may then build up under the tree, fall on, and sometimes mar the finishes of automobiles or other unprotected objects. It may also attract a variety of undesirable insects such as yellowjackets. Under such conditions efforts must be made to control the aphids, but not the ants.

One of the most common indirect effects of carpenter ants on shade trees is that of opening up passages for subsequent invasion by other pests and pathogens (Craighead 1950). These effects are obvious, and we will comment on only one type of pathogen that may be associated with carpenter ants: chestnut blight, Endothia parasitica. Anagnostakis (1982) has shown that carpenter ants are capable of transporting chestnut blight, and has suggested (personal communication) that these ants may have been one of the principal vectors of this disease which has all but decimated our native chestnuts.

The effects that we have discussed are generally not major. However, some of these problems can be of economic importance, given the appropriate conditions. Much more research is needed on these ants, which are very common in urban areas of the Eastern U.S. In particular, research needs to be done on risk assessment, and the interaction of these ants with other tree pests and pathogens. We hope, however, that our brief review of carpenter ants in shade trees will provide basic information necessary for decision making on the part of tree care specialists.