Herbivore response to plant drought stress in milkweeds

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 plants
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, plant 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 a
specialist leaf-chewer (monarch
butterfly; Danaus plexippus) and sap-feeder (oleander
aphid; Aphis nerii). We exposed plants to species-specific
watering regimes physiologically calibrated to maximize (100%) or reduce
(50%) stomatal conductance and then monitored the performance of the
herbivores. The effects of drought stress on herbivore survival ranged
from strongly positive (50% increase) to strongly negative (80% decrease)
among milkweed species, but these effects were inconsistent between the
two herbivores.  We demonstrated that the indirect
effects of drought on herbivores varied tremendously among closely related
plant species and between co-occurring insect herbivores.