Climate variability has major implications for agriculture due to the increase in the frequency and intensity of simultaneous abiotic, namely water-stress, and biotic stresses to crops. Plant water-stress alone harms crops but also can attract outbreaks of herbivores with varied host specialization, and plants succumb to further yield losses dealing with multiple stressors. Host-plant resistance provides a route to lessen yield losses from herbivory; however, our knowledge of the interactions between water-stress and pest resistance is limited, especially for mite herbivores of maize including the generalist two-spotted spider mite (Tetranychus urticae, TSM) and the specialist Banks grass mite (Oligonychus pratensis, BGM). We conducted parallel greenhouse and field experiments whereby a susceptible line (B73) and two TSM-resistant lines (B75 and B96) were subjected to either optimal irrigation or water-stress [50–60% and 5–10% volumetric water content (VWC), and 25–32% and 10–15% VWC, in the greenhouse and field, respectively]. As expected, we found that under optimal irrigation TSM and BGM populations increased readily on B73, while B75 and B96 were largely resistant to the TSM but not BGM. While plant water-stress increased the susceptibility of B73 to both mite species, water-stress did not disrupt initial resistance levels of B75 and B96 maize for either mite species. Elevated protease activity was found in B75 and B96 and may contribute to maize resistance. Our findings that B75 and B96 are highly resistant to the TSM, and maintain resistance to both mite species with water-stress, highlights the importance of including the nuances of multiple stressors within the framework of host-plant resistance.

Water-stress or optimal irrigation levels were quantified by measuring stomatal conductance (mmolm^-2 s^-1) and leaf temperature (°C) using a leaf porometer (Model SC-1, Meter Group, WA, USA), leaf water potential (bar) using a pressure chamber (Model 615, PMS Instrument Company, OR, USA), and stem height (cm) by using a ruler. Leaf temperature, stomatal conductance and stem height were measured at 3 and 7 days post-mite introduction, while leaf water potential was measured after sample collection at 7 days post-mite introduction. 

At 1, 3, and 7 days post mite introduction, leaf samples (leaf areas inside Tanglefoot arenas) from eight plants of four randomly selected replicates (2 plants/replicate) were collected, flash-frozen using liquid nitrogen and stored in a freezer (-20°C) until processing. Each sample was processed by counting the number of eggs and all mite stages and by performing defense protein bioassays. 

The activities of POD, PPO, and CHI were analyzed using a microplate reader (Biotek, EPOCH, VT, USA), while the activity of TI was analyzed by using radial diffusion techniques.

Mite (TSM and BGM) population sizes, assessed as the sum of eggs, nymphs and adults, and plant physiological measurements, and defense protein activity measurements from trials were analyzed using a general linear model (Proc Glimmix; SAS 9.4 M4 University edition). Square-root transformation was used for both mite population growth, plant physiological responses, and defensive protein activities (POD, PPO, CHI and TI) data to conform to the assumption of normality and heteroscedasticity. Data presented here are the raw numbers.