Milkweed trait values associated with aridity gradients and drought-induced changes in hostplant quality

 The response of herbivorous insects to plant drought stress can range from positive to negative, and it has been challenging to understand the causes of this variation. We tested whether plant trait values associated with aridity gradients might underlie this variation and how effects vary among two insect-feeding guilds. Here we propose that plant trait values associated with adaptation to arid environments would result in positive effects of experimental drought on herbivores, with such plant species adaptively shifting resources away from resistance to maintain performance under stress. In contrast, plants with trait values associated with adaptation to mesic environments would result in negative effects of drought because such species lose vigor and thus decline in their host-quality. We tested these predictions using experimental manipulations in 13 milkweed species (genus Asclepias) adapted to a wide range of environmental conditions, and herbivore performance of herbivores. We e..., Trait measurements were obtained from 3-5 newly expanded and undamaged leaves collected from each experimental plant at the conclusion of the greenhouse experiment. Excised leaves were transferred to a paper envelope and immediately frozen at -20 °C for later characterization of plant traits. For water-use-associated traits, we measured leaves from plants in the control treatment as representative of their constitutive trait values. To measure SLA, leaf area (cm2)/dry mass (mg), frozen leaves were scanned, weighed, dried at 60 °C for 2 days, and reweighed to the nearest mg. Leaf water content was calculated as a percentage estimated from the difference between leaf fresh weight and leaf dry weight. Dried leaves were pulverized using a Mixer Mill (Retsch MM 400) for stable isotope and macronutrient analyses. Foliar carbon isotopes ratio (13C and 12C) was estimated from pulverized foliar tissue as an indicator of WUE for a subset of samples (n=46) at the UC-Irvine Mass Spectrometry facili..., Excel