Treatments
Eggs of white-marked tussock moths (Orgyia leucostigma ) were obtained from the Canadian Forest Service (Sault Ste. Marie, Ontario, Canada). Eggs and hatched caterpillars were kept in climate chambers (24 °C, 50–70% humidity, 16:8 L:D light cycle) and were reared on wheat germ based Bell diet (Bell, Owens, Shapiro, & Tardif, 1981) to the fourth larval stadium. On July 17-18, 2018 we placed four fourth-stadium caterpillars onto each of the experimental trees in each of 10 mesocosms. Trees in 12 mesocosms received no larvae. Caterpillars were confined to trees in nylon mesh bags and allowed to feed for two days. Larvae were then removed and the defoliation was manually completed to a consistent 50% level across all leaves on all trees, using pinking shears. This method allowed us to incorporate the cues derived from natural herbivory with a standardized amount of foliar tissue removal (Stevens, Waller, & Lindroth, 2007). Control trees (no defoliation) were covered with empty mesh bags for two days. The drought stress treatment was applied to five mesocosms of defoliated trees and six mesocosms of undefoliated trees. Water was withheld for 7-12 days until most trees began to wilt, whereupon the mesocosm was watered to soil saturation. This procedure was repeated for 37 days, until the end of the experiment. Control mesocosms were watered to saturation every 2-4 days.