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.