Spider mite resistant maize lines, B75 and B96, maintain resistance under water-stress

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.