Illinois Natural History Survey - University of Illinois

Plant Stress--Its Relationship to Arthropod Pests in Urban Landscapes

Trees and shrubs are important components of urban landscapes because they increase the aesthetic value of any property. Properties that contain an assortment of trees and shrubs are more likely to sell faster than properties that contain no trees or shrubs. In addition, the presence of a diversity of trees and shrubs may provide refuge for animals such as birds, rabbits, and squirrels.

Unfortunately, trees and shrubs growing in urban landscapes are subject to a variety of stress factors that may increase their susceptibility to insects and other opportunistic arthropod pests. This generally involves plants located in residential and commercial landscapes. However, it also includes plants located along streets, walkways, and in shopping center parking lots where they are surrounded by asphalt or concrete ("hardscapes").

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Urban tree being stressed by a number of environmental factors including loss of roots, soil compaction, burial of crown under gravel, and encroachment by hardscape.

Plants that are growing along streets and walkways are subject to pollutants from automobile exhausts and dust. This may not only increase plant stress, but may also reduce the abundance of natural enemies (beneficial predators and parasitoids) because dust has been shown to be detrimental to the natural enemies. The absence of natural enemies may result in plants experiencing higher populations of arthropod pests. In addition, plants growing near streets and walkways are susceptible to disturbance, especially construction such as walkway replacement and repair or the installation of new piping. This generally results in severe root injury, which can compromise the ability of plants to defend themselves and increase their susceptibility to arthropod pests.

Another factor that may lead to increased plant stress is the amount of hardscape, such as parking lots and buildings, surrounding plants. An increase in heat absorption, light reflection, or an inadequate water supply may create a microclimate that is stressful to plants. This environment may be conducive for pest development and deleterious to natural enemies. In addition, plants located in these isolated microclimates may also make it difficult for natural enemies to find pests.

Plants growing in residential or commercial landscapes are subject to stress factors from mechanical injury and improper cultural practices. Mechanical injury can occur when lawn mowers or weed-whackers are used to trim turfgrass growing along the base of trees or shrubs. Lawn mowers or weed-whackers may inadvertently remove bark (cambium) tissue and girdle plants, creating plant stress and increasing susceptibility to wood-boring insects. Proper cultural practices can reduce a plant's susceptibility to wood-boring beetles; however, improper use of irrigation, fertilizers, or mulches may alter the host-pest balance in favor of the pest.

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A thriving tree that has been properly situated and mulched.

Over- or underwatering can create a series of physiological changes that lead to plant stress and greater opportunity for insect attack. Plants stressed from overwatering may result in more resources allocated toward growth and fewer resources allocated toward defense, which makes it easier for opportunistic insects to attack plants. Underwatering may also lead to stress because plants are unable to take up enough water to maintain normal metabolic functions. Wood-boring beetles and other insects take advantage of this situation. It has been demonstrated that plants under water stress are unable to produce oleoresins, which normally act to repel beetles. Pine trees, for example, were more susceptible to pine bark beetles during periods of water stress.

The use of rapid-release fertilizers, such as those used for turfgrass, may increase a plant's susceptibility to piercing-sucking insects such as aphids, leafhoppers, and scale. Overfertilization results in insect problems because plants may allocate more energy into growth and less into defense. The level of chemical defenses necessary for resistance to insects decreases in rapidly growing trees. For example, birch (Betula sp.) and quaking aspen (Populus tremuloides) are more susceptible to leaf-feeding insects when fertilized. In addition, this often leads to the production of soft, succulent growth that has higher amounts of protein and a thinner cuticle that is easier for aphids, mites, and leafhoppers to penetrate with their mouthparts.

Proper mulching can lead to healthy plants due to a reduction in weed competition, higher soil moisture retention, and prevention of damage to the base of trees and shrubs from lawn mowers and weed-whackers. However, too much mulch or mulch that covers the plant crown (base) can cut off oxygen and suffocate plants.

Research at the Illinois Natural History Survey, the University of Illinois, and in other midwestern states is evaluating the impact of plant stress on susceptibility to arthropod pests. This will continue to be an important research consideration as suburban expansion and development proceed at an accelerated pace.

Raymond A. Cloyd, UIUC Department of Natural Resources and Environmental Sciences

Charlie Warwick, editor



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