Plant stress favors pests in urban landscapes
Trees and shrubs growing in urban landscapes are subject to stress factors that their country cousins never encounter. Pollution, heat reflecting off paved surfaces and injury from mowers and string trimmers are just a few of the elements than can increase their susceptibility to insect and other opportunistic arthropod pests.
Urban landscapes generally involve plants in residential and commercial landscapes. However, they also include plants along streets, walkways and in shopping center parking lots, where trees and shrubs are surrounded by asphalt or concrete ("hardscapes").
We're not the only ones stressing out over smog and construction Plants growing along streets and walkways are subject to pollutants from automobile exhaust and dust. This not only increases plant stress, but also may reduce the abundance of natural enemies, as dust has been shown to be detrimental to parasitoids and predators. Fewer natural enemies result in higher pest populations.
In addition, these plants are susceptible to disturbance, especially during construction involving walkway replacement, repair or the installation of new piping. Many times, this results in severe root injury, which can compromise a plant's ability to defend itself against arthropod pests.
Another factor that can lead to increased plant stress is the amount of "hardscape," such as parking lots and buildings that surround plants. An increase in heat absorption, light reflection or an inadequate water supply creates a microclimate that is stressful to plants. This environment can be conducive for pest development and deleterious to natural enemies. In addition, plants in these isolated microclimates ("islands") may be difficult for natural enemies to find.
Be careful with those landscaping tools Plants growing in residential or commercial landscapes are subject to stress from mechanical injury and improper cultural practices. Mechanical injury often occurs when you use lawn mowers or string trimmers around the base of trees or shrubs. Lawn mowers or string trimmers can remove bark (cambium) tissue and girdle plants. This creates plant stress and increases the susceptibility to wood-boring insects.
It's only good for them if you know how to do it Proper cultural practices can reduce a plant's susceptibility to wood-boring insects. However, improper use of irrigation, fertility, mulching and pruning may alter the host-pest balance in favor of the pest.
Over- or under-watering may cause a series of physiological changes that lead to plant stress and greater opportunity for insect attack. Plants stressed from overwatering allocate more resources toward growth and fewer toward defense, which makes it easier for opportunistic insects to attack plants. Underwatering may also lead to stress. Plants are unable to take up enough water to maintain normal metabolic functions, and wood-boring insects and other pests take advantage of this situation. It has been demonstrated that plants under water stress are unable to produce the resins that normally repel beetles. Pine trees, for example, are more susceptible to pine bark beetles during periods of water stress.
Rapid-release fertilizers 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 to growth and less to defense. The level of chemical defenses necessary for resistance to insects decreases in rapidly growing trees. For example, birch (Betula resinifera) 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.
Proper mulching can lead to healthy plants. It reduces in weed competition, helps plants retain more soil moisture and prevents damage to the base of trees and shrubs from lawn mowers and string trimmers. However, using too much mulch or mulch that covers the plant trunk or stem can cut off oxygen and suffocate plants. This increases plant susceptibility to opportunistic insects as well as fungal pathogens.
Two types of improper pruning can lead to insect problems: poor practices and improper timing. Examples of poor practices include pruning in a manner that stimulates succulent growth, which is attractive to insects such as aphids and planthoppers; leaving stubs; topping trees or cutting too far back into the branch collar. These practices often make it difficult for plants to properly heal themselves, which leads to insect problems such as wood-boring beetles.
Improper timing includes 1) pruning plants when insects, especially egg laying females, are most active; and 2) pruning when plants are most likely to produce volatile chemicals that attract insects. Pruning during winter dormancy typically avoids timing problems.
Improper irrigation, fertility, mulching and pruning can have a significant influence on plant health and may increase susceptibility to insects. Understanding how cultural practices are related to a plant's ability to defend itself will allow you to properly conduct cultural practices in the landscape. Doing so will mean having to deal with fewer insect problems in the long term.
Raymond A. Cloyd is an assistant professor and extension specialist in ornamental entomology/integrated pest management at the University of Illinois (Urbana-Champaign, Ill.).
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