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Sound Cultural Practices Reduce Problems with Wood-Boring Insect

September 2, 2002

The eventual loss of pest-control materials such as chlorpyrifos (Dursban), lindane, and dimethoate (Cygon) means limited options for managing wood-boring insects (beetles and caterpillars). As a result, serious consideration should be placed on implementing sound cultural practices, as opposed to relying on pest-control materials.

Properly implementing sound horticultural practices has the greatest impact in maintaining healthy plants in the landscape. Cultural practices such as watering, mulching, pruning, fertilizing, and plant selection and placement--when properly performed--reduce plant stress, which is a cause of most insect and mite problems.

Water. Under- or overwatering trees often leads to stress, increasing susceptibility to wood-boring insects. For example, stressed plants emit volatile chemicals that attract wood-boring beetles. These beetles use the chemicals emitted to help them easily locate plants whose natural defenses have been compromised by improper watering practices. Underwatering plants may lead to higher populations of spider mites (especially two-spotted spider mite) because there is less moisture in the air from ground and foliar evaporation, resulting in lower relative humidities and drier conditions. These conditions favor spider mite development.

Extreme fluctuations of wet and dry soil conditions such as we have experienced in Illinois may be detrimental to plant health and promote stress. Extended periods of wet soil kill plant roots due to lack of oxygen. These roots are not available later to take up water when dry conditions persist. The symptoms resulting from this kind of stress may not be expressed on large trees for 3 or 4 years; however, the trees are susceptible to attack from wood-boring insects. This often leads to confusion as to the cause of plant death. In this case, the wood-boring insects are secondary, whereas the stress due to extreme wet and dry soil conditions was the primary cause of plant death.

Mulch. Proper mulching moderates soil temperatures, conserves moisture in the soil, reduces competition from other plants, reduces weed pressure, prevents soil compaction, and minimizes soil erosion. However, improper use of mulches can often lead to increased plant susceptibility to insects. For example, applying too much mulch or covering the plant crown prevents the bark from exchanging oxygen, and the plant suffers from asphyxiation. This leads to plant stress and a higher probability of attack from wood-boring insects. It is best to keep mulch at least 4 inches away from a tree trunk. Thick mulches (more than 6 inches) also provide a moist, protective environment that voles find attractive. Voles feed on the bark (cambium) and can potentially girdle plants, thus killing them.

Pruning. Proper pruning during the growing season generally involves removing dead, diseased, damaged, and weakened growth to maintain plant health and vigor. However, excessive pruning during the growing season, such as removing large portions of the plant canopy, results in spurts of succulent growth highly susceptible to insects. Suckers produced from heavy pruning are also susceptible to aphids and other insects because this succulent tissue lacks a protective waxy covering. Besides, improper pruning cuts, such as stubs, emit volatile odors that attract insects and provide easy entry sites for wood-boring insects.

Pruning trees or shrubs at certain times of the year may increase problems with some wood-boring insects. For example, it is generally recommended to avoid pruning birch trees, especially white birch, from May through August because bronze birch borer adults are flying around looking for places to lay eggs. Pruning during this time creates wounds that emit odors, which attract adult females.

Fertility. Over- and underfertilizing plants often lead to stress or the production of susceptible growth. An excessive application of highly soluble nitrate fertilizers (generally used for turf) generates lush, weak growth that is susceptible to attack by insects. Besides, excessive amounts of fertilizer cause plants to allocate more resources to leaf production, diverting resources from the production of defensive compounds, thus increasing susceptibility to wood-boring insects. Conversely, plants unable to obtain sufficient quantities of nutrients are also more prone to insect and/or mite attacks because their natural defense system has been compromised.

Besides the cultural practices mentioned, other practices may increase problems with wood-boring insects. First, avoid injuring the base of trees and shrubs with lawn mowers or weed-whackers because this removes essential cambium tissue that is responsible for transporting food upward to the leaves. This type of injury, which can be easily avoided, places undue stress on plants. Many wood-boring insects are opportunistic and thrive on stressed plants. Second, newly planted trees and shrubs are highly susceptible to wood-boring insect attack. For example, the flat-headed appletree borer attacks recently planted trees or shrubs because they are initially stressed. It is important to properly water plants, provide adequate drainage, and mulch young plants to minimize any stress.

Plant selection and placement. Proper selection of a planting site results in healthy plants that are better able to defend themselves and less susceptible to pest attack. For example, white birch trees located on the south side of a white house are more susceptible to bronze birch borer attack because white birches cannot tolerate the extreme sunlight conditions and the reflective heat projected. It is best to use a tree or shrub that tolerates these conditions. Be sure to consult available resources for information on proper planting zone, light requirements, soil conditions, soil pH, mature plant size, and other factors before selecting and planting any plant material.

Proper implementation of sound horticultural practices such as watering, mulching, pruning, fertilizing, and plant selection and placement is important in maintaining plant health. However, when these practices are improperly performed, problems with insects, mites, or both are likely, which may require pest-control materials. Therefore, maintaining sound cultural practices can often alleviate the need for using pest-control materials.

Author: Raymond A. Cloyd


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