Biological Control in the Urban Forest

With public and regulatory concern mounting about pesticide use in urban areas, biological control is likely to become an increasingly important tool for pest managers. Fortunately for urban foresters, the economics and ecology of the urban forest are quite favorable for increased adoption of biological control. Relative to traditional forests and Christmas tree plantations, the dollar value of the standing forest is high. As such, the budget for pest control is greater and can more easily absorb the cost of adopting the new technology of biological control. From an ecological perspective, the urban forest has a great potential to provide a sustainable habitat for natural enemies. The standing forest is long-lived and species diverse. Landscape composition can readily be manipulated to increase the species and structural diversity to make the habitat more favorable to natural enemies.

Effective use of biological control must target the specific needs of the urban forest. Unlike traditional forests and Christmas tree plantations, the urban forest is filled with people as well as trees. Because of this, trees need to be more than just healthy. Trees also need to create an environment that is aesthetically pleasing and safe to the people who use the spaces that they create. Insect and mite pests detract from the aesthetics of the urban forest when they alter tree form or color, or when they are so numerous that pests, their webs, body parts or excrement are easily noticed. When these pests kill portions of trees and reduce the structural integrity of large limbs, they can endanger people and property.

Biological control solutions in difficult situations

The urban forest is a tough place for trees. Many problems that urban foresters encounter are caused, at least in part, by factors that are beyond their control. Trees may have been planted or established at the site. Landscape and construction projects occur without consultation. New pests come and regulations are passed to slow the spread of their movement. To help pest managers implement biological control in these difficult situations we have constructed a series of six situations that represent the range of problems foresters are likely to encounter. The six situations are as follows:

The first two of these situations are reviewed in this article.

Pests of an over-utilized problem tree species

To create a unifying sense of space in the landscape, urban designers will repeatedly use a single plant species. Although the effect is pleasing to the eye, it often has the unintended consequence of creating conditions that are favorable to pest outbreaks. The loss of American elms from the urban forest to Dutch elm disease is an extreme example of the problems that can occur in designed monocultures. Landscape designers replaced the elms with a relatively pest-free species, thornless honeylocust, Gleditzia triacanthos 'inermis'. Use of honeylocust spread rapidly because its vase-like shape was reminiscent of elms and its small leaves did not need to be collected from streets in the fall. Since that time 3 pests, honeylocust plant bug (Diaphnicorus chlorionis), mimosa webworm (Homadaula anicocentra), and honeylocust spider mite (Platytetranychus multidigitali) have flourished on this tree, making it one of the most problematic species of the urban forest.

Early in the season honeylocust plant bug distorts leaves as they unfold from their buds. In late June and early August, mimosa webworm ties together leaves with webs. Honeylocust spider mite outbreaks can occur at any time from July through September when natural enemies have been killed by early season pesticide applications. Adopting a plant health care approach is the only way to maximize opportunities for managing honeylocust pests with biological control.

The mid-season pest, mimosa webworm, can be effectively managed with applications of one of two insecticides that do not harm the natural enemies of spider mites. These are Bacillus thuringiensis, or spinosad. Applications are most effective when applied to young caterpillars before extensive webbing occurs. Pest managers are urged to use binoculars and elevated vantage points, to inspect the tops of trees to improve pesticide timing against each of the two generations. In this way, the populations are likely to be kept low enough to prevent a significant accumulation of unattractive brown webs. This will also minimize the late-summer nuisance problem of late stage caterpillars dangling from silken threads into the faces of people using sidewalks below the forest canopy.

In contrast, applications of foliar insecticides against outbreaks of honeylocust plant bugs, can wreak havoc with spider mite natural enemies and result in outbreaks. Soil applications of imidacloprid directed against honeylocust plant bug have also been found to kill enough predaceous minute pirate bugs to cause spider mite outbreaks. Applications of reduced rates of short residual materials like oil or soap provide only marginal control.

With only one generation per year, the honeylocust plant bug spends the winter as an egg in plant twigs. When the weather warms, eggs hatch to young nymphs that feed on plants. Nymphs can be found feeding on green buds before leaves have unfolded. Nymphs turn into egg-laying adults by early to mid-June. Although this plant bug is common each year it will only occasionally cause enough defoliation to force trees to refoliate in June. This refoliation severely stresses trees making them more susceptible to severe injury from thyronectria canker.

Honeylocust plant bug provides an excellent example of how keeping records can help managers establish site specific thresholds and conserve natural enemies of late season pests by avoiding the use of early season pesticides. By keeping records as part of a nursery plant health care program in Indiana we were able to find that soon after bud break, blocks of trees that had less than one plant bug per compound leaf did not experience significant leaf distortion. Avoiding the use of broad spectrum pesticides in blocks with pests below the threshold prevented the late season spider mite outbreak that occurred where pesticides were sprayed. Applying this approach to street trees is likely to refine thresholds so that early season pesticides applications can be reduced even further.

When the warm weather finally arrives and spider mites become active, regular monitoring should be used to assess the potential for biological control by the resident natural enemies. Trees in exposed sunny locations that are more prone to mite problems than trees in shaded less exposed areas should be the focus of the monitoring program. By tapping branches over sheets of white paper pest managers can readily count the number of spider mites on a branch. Adult and larval forms of predaceous black lady beetles (Stethorus spp.), green and brown lacewings, and minute pirate bugs can easily be identified in this way. Clear, white to amber predatory mites can be detected as well with a hand lens as they rapidly move across the paper. Individuals releasing predatory mites should track their progress using the tapping method.

When monitoring spider mite numbers, consider the time of year and the relative abundance of pests and predators when making decisions about pesticide use. If numbers increase toward the end of the foliage season, no miticide should be applied since the leaves will be falling along with any injury symptoms in a short period of time. In this case, a dormant application of oil the following spring should reduce the chances of an early spider mite outbreak in the following year. A follow-up application of a selective miticide like hexythiazox, can be helpful if mites begin to flare early in the summer despite early season presence of natural enemies. In mid-mite season (late July-early August) consider using soap or oil sprays to reduce further mite injury when mites and predators are present. Restrict residual miticide use in mid-season to rescue treatments to save those plants with extensive injury and many mites but few natural enemies.

Pests of disturbed habitats

Many pests of the urban forest do not cause problems in natural habitats where natural enemies can keep them under control. Because of its artificial nature, the urban forest is considered disturbed and may lack many of the factors that sustain natural enemies. The challenge for the urban forester is to learn to recognize the kinds of factors associated with the biological control failure so potential problems can be identified during the monitoring process and management practices can be changed to foster increased success. The presence of these factors should be noted as part of the monitoring routine in an IPM program.

From the perspective of biological control, disturbed habitats are those where environmental factors separate a pest from local populations of its natural enemies. Poor soil nutrition, soil moisture, excessive or inadequate light or heat can contribute to insect outbreaks in landscapes. Natural enemies can be kept from finding their associated pests when infested trees are physically separated by long distances of turf or pavement. Effects of physical separation of these trees can be more severe when a general lack of flowering plants reduces the amount of pollen and nectar that could be used to sustain natural enemies that are searching for new hosts. Natural enemies can also be mechanically separated from their pests by excessive road dust, or chemically separated by insecticide residues associated with mosquito abatement programs. Although these environmental conditions may interfere with the effectiveness of a wide variety of pests and their natural enemies, the impact on a given pest is likely to be influenced by the biology of both actual pests and natural enemies involved. We will review how habitat disruption has contributed to the outbreaks of three common pest problems that have been carefully studied: scale insects, lace bugs, and leafminers.

Scale Insects. White peach scale, Pseudaulacaspis pentagona, feeds on a wide range of trees including those in the genus Prunus as well as mulberry, willows, and catalpa. Surveys of pest abundance in Maryland showed that this scale was most likely to be a problem when trees were isolated or planted along roadsides, or in parking lots. In contrast, when host plants were located near other host plants or in areas such as parks, and woodlots where plant diversity was high, the scale was seldom, if ever a problem. Control was attributed to generalist predators, including earwigs, tree crickets, and harvestmen.

Obscure scale, Melanaspis obscura, is most commonly found on oaks. As a relatively recent introduction to California, this pest was the focus of a classical biological control program that imported wasp natural enemies from Texas to control this pest. Populations of this pest declined steadily over four years in areas where natural enemies were released. This population decline was delayed for two years on one northern red oak in the study that was subjected to insecticide drift from sprays applied to a nearby elm. Other studies with obscure scale indicate that wasp natural enemies can be greatly reduced by foliar sprays of diazinon and chlorpyrifos.

Lace bugs. Azalea lace bug, Staphanitis pyroides, feeds on azalea and rhododendron and causes the leaves to become discolored with white stipples. With up to four generations a year, entire plants can appear as if the leaves were bleached. In nature these plants are found in the forest understory. Studies conducted in Maryland indicate that outbreaks were most common when azaleas were isolated, or the landscape was sparseley planted. In more complex habitats, where flowers were present, and plants were shaded by tall trees, natural enemies were more abundant and lace bug problems were far less numerous. Lace bugs also grew from egg to adult more slowly in these shaded sites.

Leafminers. The birch leafminer, Fenusa pusilla is an important introduced pest of birch. It feeds between leaf surfaces leaving brown blotchy mines. Often more than half of the leaves can be browned in areas where this pest occurs. By 1995 the introduction of a parasitic wasp reduced leaf mining to under 3% near the site of its release in 1990. Still, however, in areas where the wasp was present, trees located near roadsides commonly had more than 50% of the leaves mined. Leafminers as a group have many natural enemies. Studies of a native leafminer, the native holly leafminer Phytomyza ilicicola, on American holly indicate that it occurs most commonly when trees are isolated and located in sun.

- Cliff Sadof, Purdue University and Michael Raupp, University of Maryland (Adapted from a chapter in an upcoming publication on biological control in urban forests)

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