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How Does Drought Stress Influence Plant–Insect Interactions?

August 31, 2005

The recent warm and dry weather conditions and lack of rainfall throughout the state may result in more problems with plant-feeding insects and mites due to drought stress. First of all, the excessive heat accelerates insect and mite development so that it takes less time for them to complete their life cycles or generation time. Also, a major control for caterpillars, aphids, beetle larvae, and many other insects are natural fungi present in the environment. These fungi are more prevalent and “aggressive” when the weather conditions are cool and moist. However, when dry conditions are prevalent, more insects survive.

Drought stress, which is usually considered a temporary state, is the lack of sufficient moisture to maintain plant turgor and reduces the plant’s ability to conduct biochemical processes that allow cells to function. This primarily occurs when the rate of transpiration from plant leaves and evaporation from the soil exceeds the capacity of roots to absorb water, due to a lack of sufficient rainfall, leading to decreased plant water potential.

The effect of drought stress varies, depending on the feeding behaviors of insects and mites. For example, insects with piercing–sucking mouthparts such as aphids, whiteflies, scales, and plant bugs typically benefit more from dry conditions than insects with chewing mouthparts, including beetles, caterpillars, and sawflies. Plant stress, due to a lack of soil moisture, often increases susceptibility to wood-boring insects, such as bronze birch borer (Agrilus anxius), twolined chestnut borer (Agrilus bilineatus), and bark beetles. Plants under drought stress decrease production of compounds such as oleoresin, which act to deter feeding by wood-boring insects, thus increasing susceptibility. Additionally, water-deficient plants emit volatile chemicals, such as ethanol and alpha-pinene, that attract these types of insects. Wood-boring insects use these chemical cues to help them locate plants whose natural defenses have been compromised by lack of water. For example, a lack of moisture in the upper tree canopy may result in localized areas of cambial and phloem tissue degradation, which are very attractive to wood-boring insects such as bronze birch borer females for egg laying. Also, the colonization success of bark beetles increases when trees are stressed due to lack of moisture. Bark beetles depend on water stress to weaken the defenses of their target.

Inadequate soil moisture can also lead to higher populations of twospotted spider mite, Tetranychus urticae, because there is less moisture in the air from evaporation. These lower relative humidities and drier conditions tend to favor twospotted spider mite development. Also, twospotted spider mites tend to feed more under dry conditions because the dry air or low relative humidity allows them to easily acquire excess water in plant leaves, which is then excreted by the spider mites.

Insects are affected by drought or dry conditions through a number of mechanisms. First, dry conditions provide a favorable thermal environment of growth and development for plant-feeding insects and mites. Second, drought-stressed plants are more attractive and acceptable to insects. As plants lose moisture via transpiration, the water columns in the xylem cavitate or break apart, producing ultrasonic acoustical emissions that are sensed by many bark beetles, attracting them to stressed plants. Third, drought-stressed plants are more suitable for insects. Water-deficient plants are more favorable for insect growth (that is, increased larval weight), survival, and reproduction because plant nutrients are more concentrated and/or better balanced. However, the primary reason why water-deficient plants are more susceptible to insects is due to a decline in the production of secondary metabolites or defensive compounds, which increases susceptibility to attack. Fourth, drought or dry conditions increase insect detoxification systems. It has been hypothesized that insects feeding on drought-stressed plants are better able to break down certain plant allelochemicals or defensive compounds that would normally have a negative effect on them. The bottom line is that drought stress induces changes in the quality of plants, which improves the performance, in most cases, of plant-feeding insects and mites.