Stress magnitude matters: different intensities of pulsed water stress produce non-monotonic resistance responses of host plants to insect herbivores

1. Water stress may increase or reduce the suitability of plants for herbivores. The recently proposed 'pulsed stress hypothesis' suggests consideration of stress phenology (pulsed vs. continuous stress) to explain these conflicting effects of plant water stress on herbivore performance. 2. This hypothesis was tested for the effect of differing stress intensity on performance and preference of insect herbivores belonging to different feeding guilds, namely leaf-chewing insects (Spodoptera littoralis caterpillars) and phloem-feeding insects (Aphis pomi aphids), on apple plants (Malus domestica). The plants were non-stressed or exposed to a low or high intensity of pulsed water stress. 3. Plant responses to the different stress levels were generally monotonic. Growth, stomatal conductance (g(s)), leaf water, and old-leaf nitrogen concentration decreased, whereas young-leaf nitrogen concentration and leaf mass per area (LMA) increased with increasing stress intensity. The stable isotope composition of foliar carbon (delta C-13) responded non-monotonically to the drought treatments. The delta C-13 values were highest in low-stress plants, intermediate in high-stress plants, and lowest in non-stressed plants. 4. The preference and performance responses of the caterpillars were also non-monotonic. Non-stressed plants were intermediately, low-stress plants least, and high-stress plants most attractive or suitable. Aphid population growth was highest on non-stressed plants and lowest on low-stress plants. 5. The results highlight the importance of water stress intensity for the outcome of interactions between herbivores and drought-affected plants. They show that pulsed water stress may enhance or reduce insect herbivore performance and plant resistance, depending on stress intensity.