Testing the pulsed stress hypothesis with cotton aphids

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. However, the degree and duration of these benefits depends upon the severity of the experimental stress employed. 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. Several studies suggest an increase in aphid survivorship under stress, while others report the opposite. Huberty & Denno (2004) conducted a meta-analysis of herbivore response and concluded that pulsed stress increases host plant quality for phloem-feeding insects such as 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 aphids to access the excess N. In contrast, continuous stress produces a constant decline in turgor pressure, thereby reducing aphid feeding efficiency. We examined the effects of pulse and continuous stress on cotton (Gossypium hirsutum) physiology and cotton aphid (Aphis gossypii) performance over a stress gradient. Anti-stress enzymes, herbivore defense compounds, and aphid performance were quantified on continuously stressed, pulse stressed, and non-stressed plants. Aphid performance was higher on pulse stressed plants versus continuously stressed plants. 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.