The effects of pulsed plant stress on herbivore abundance and plant defense

Plant physiology is altered dramatically when 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 benefit 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 thrips and aphids. 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. We examined the effects of pulsed and continuous stress on cotton (Gossypium hirsutum) physiology and herbivore abundance. Anti-stress enzymes, herbivore defense compounds, and herbivore abundance were quantified on continuously stressed, pulse stressed, and non-stressed plants. Several destructive herbivore families such as Thripidae, Pentatomidae, and Chrysomelidae were significantly more prevalent on pulse stressed plants versus continuously and non-stressed plants. In addition, stress induced changes in cotton structure may reveal how plant architecture influences herbivore preference in conjunction with stress. This study will help us more accurately predict herbivore response to water stressed plants and allow for insight in the design of irrigation regimes to avoid pest outbreaks.