Jactel et al. recently published a meta-analysis of the effects of drought on damage to trees by insects and pathogens. The results were neat. They found that agents that attack plant leaves were enhanced by water stress to plants. Yet, agents that attack the plant through its wood caused less damage to water-stressed plants than to unstressed plants.
The best part of the paper was linking the degree of water stress to the severity of damage (shown above). Their metric was the reduction in plant water potential relative to the water potential at which conductance is reduced by 50%. The greater the severity of water stress, the greater the damage.
Jactel, H., J. Petit, M.-L. Desprez-Loustau, S. Delzon, D. Piou, A. Battisti, and J. Koricheva. 2012. Drought effects on damage by forest insects and pathogens: a meta-analysis. Global Change Biology 18:267-276.