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Plant Stress and Wood-Boring Insects

June 14, 2000

At this time of year, there are a number of wood-boring insects that attack a wide diversity of plants growing in nurseries and landscapes. The major wood-boring insects may be classified as either beetles (for example, bronze birch borer) or clearwing moths (for example, peach tree borer). Both types have larvae that tunnel and feed within the plant tissues.

The main difference is in the feeding behavior and appearance of the adult stage. Adult clearwing moths resemble bees, but the wings are nearly transparent (it appears as if parts of the wing are missing). They don’t normally feed on plants, whereas adult beetles can cause plant injury by feeding on plant foliage. Wood-boring insects can kill plants directly due to larvae feeding within the phloem tissue, which reduces the plant’s ability to obtain nutrients. In addition, wood-boring beetles can kill plants indirectly by serving as vectors of diseases. For example, Scolytid beetles are the primary vectors of Dutch elm disease. In addition, some wood-boring beetles (Ambrosia beetles) introduce a blue-stain fungus that clogs the water transport system of plants.

Most of the wood-boring insects in Illinois are gen-erally a problem when plants are stressed. So, why do wood-boring insects predominantly attack stressed plants? One reason is that healthy plants are normally able to allocate available resources (food) for both growth and defense. Plants can then protect themselves by either sealing off (compartmentalizing) insects, or they are able to tolerate insect populations while still retaining their aesthetic appearance. However, when plants are predisposed to stress, which may occur due to environmental (drought) or physical (weed-whackers or mowers) factors, it increases their susceptibility to wood-boring insects. Stress may cause plants to allocate more available resources toward growth, leaving fewer resources for defense. Wood-boring insects take advantage of this imbalance and find it easier to locate and attack stressed plants. Once wood-boring insects locate stressed plants, some insects can emit chemicals that attract other individuals to the plant, thus overwhelming the tree’s defenses by sheer numbers and accelerating the time it takes to kill the plant.

However, this may be a simplified generalization as not all plants respond in the same manner. In fact, studies have demonstrated that in some situations chemical defenses actually increase in response to stress. Improper implementation of cultural practices such as watering, fertility, mulching, and pruning is a major cause of stress. Many trees have evolved to grow in forest environments, which are very different from the areas in which we expect trees to thrive. As a result, plants are highly dependent on cultural practices to ensure their survival. Thus, when conditions are not conducive to survival, plants are predisposed to wood-boring insects.

Over- or underwatering sets off a series of physiological changes that lead to plant stress and greater susceptibility to wood-boring insects. When a plant is stressed from overwatering, resources may be allocated toward growth (that is, maintenance growth) to survive. In this case, fewer resources are allocated toward defense, which facilitates attack by wood boring insects. Underwatering also leads to stress, as plants are unable to take up enough water to maintain normal enzymatic and metabolic functions. Plants may then allocate resources toward maintenance growth, leaving fewer resources for defense. It has been demonstrated that plants under water stress are unable to produce oleoresins, which normally act to repel beetles. Ponderosa pine (Pinus ponderosa) trees, for example, are more susceptible to pine bark beetle during periods of water stress. Plants that are properly watered are better able to reduce or restrict insect injury. For example, conifer trees (that is, pine and spruce) can compartmentalize, or seal off, insects and prevent them from causing severe injury. A very serious stress factor that predisposes plants to attack by wood-boring insects is prolonged drought.

Overfertilization also results in greater problems with wood-boring insects 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.

Proper mulching can lead to healthy plants. However, too much mulch or mulch that covers the plant crown (base) can cut off oxygen and suffocate plants. This increases their susceptibility to wood-boring insects. Again, plants allocate less energy into defense. Mulch should not be thick enough to cover the plant crown.

Two types of improper pruning can lead to problems with wood boring insects: poor practices and improper timing. Examples of poor practices include leaving “stubs,” topping trees, or cutting too far back on the branch collar. These practices often make it difficult for plants to properly heal themselves. As a result, these open wounds are attractive to wood-boring insects for egg laying, or they are easy entry sites for the larvae. Improper timing includes pruning plants when wood-boring insects are most active and/or pruning when plants are most likely to produce volatile chemicals that attract wood-boring insects. For example, birch trees should not be pruned from May 1 to August 1 as this is the flight period of the bronze birch borer females, and research has shown that female birch borers are attracted to fresh pruning wounds.

Another important factor that predisposes plants to attack by wood-boring insects is plant placement. Sun-adapted plants placed in shady locations are more susceptible to wood-boring insects as less chemical defenses are produced. Similarly, shade-adapted environment. Dogwoods, for example, when located in sunny areas are more susceptible to dogwood borer. Birch trees located on the south side of a white build-ing are more susceptible to wood-boring insects because the reflected light magnifies the intensity of the sunlight causing stress and weakening the plants’ defenses. Proper site selection will minimize problems with wood-boring insects.

Understanding why wood-boring insects attack stressed plants should lead to proper implementation of cultural practices in the nursery and landscape. This will result in having to deal with fewer problems with wood boring insects.

Author: Raymond Cloyd


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