The Effects of Pulsed Water Stress on Cotton-Pest Interactions

Plant physiology is altered dramatically when plants are water stressed. Nitrogen (N) availability, digestible carbohydrates, and nutrients all change when plants are water limited and changes in the availability of these resources may affect insect herbivores. Currently, we are unable to accurately predict herbivore response to host water-deficit stress. The literature suggests contrasting responses for even the same herbivore guild, such as phloem feeding aphids and cell feeding thrips. Huberty & Denno (2004) conducted a meta-analysis of herbivore response and concluded that pulsed stress increases host plant quality for a variety of herbivores including Thripidae and Aphididae. They hypothesized that N availability was increased by water stress and that plant turgor increases when the plants recover from stress. The increased turgor pressure associated with plant recovery from stress allows herbivores to access the excess N. In this study, we examined the effects of pulsed and continuous stress on cotton (Gossypium hirsutum) physiology and herbivore abundance. Cotton physiology (i.e. photosynthesis rate, turgor pressure), herbivore defense compounds, and herbivore abundance were quantified on continuously stressed, pulse stressed, and non-stressed plants. We found that pulsed stressed plants possessed a greater number of nodes and bolls. Furthermore, we determined that several physiological parameters of cotton may be indicators of herbivore performance. Pulsed treated plants also hosted significantly more pest thripids, pentatomids, and chyrsomelids than continuous and non-stressed treated plants. This study will help us more accurately predict herbivore response to water stressed plants and provide insight into how water availability influences plant-insect interactions.