Data from: Experimental insect suppression causes loss of induced, but not constitutive, resistance in Solanum carolinense

Spatiotemporal variation in herbivory is a major driver of intraspecific
variation in plant defense. Comparatively little is known, however, about
how changes in herbivory regime affect the balance of constitutive and
induced resistance, which are often considered alternative defensive
strategies. Here, we investigated how nearly a decade of insect herbivore
suppression affected constitutive and induced resistance in horsenettle
(Solanum carolinense), a widespread herbaceous perennial. We allowed
replicated horsenettle populations to respond to the presence or absence
of herbivores by applying insecticide to all plants in half of 16 field
plots. Horsenettle density rapidly increased in response to insecticide
treatment, and this effect persisted for at least four years after the
cessation of herbivore suppression. We subsequently grew half-sibling
families from seeds collected during and shortly after insecticide
treatment in a common garden and found strong effects of insect
suppression on induced resistance. Feeding trials in field mesocosms with
false Colorado potato beetles (Leptinotarsa juncta), a common specialist
herbivore, revealed that multi-year herbivore suppression drove rapid
attenuation of induced resistance: offspring of plants from
insect-suppression plots exhibited a near-complete loss of induced
resistance to beetles, while those from control plots incurred ~70% less
damage after experimental induction. Plants from insect-suppression plots
also had ~40% greater constitutive resistance than those from control
plots, although this difference was not statistically significant. We
nonetheless detected a strong trade-off between constitutive and induced
resistance across families. In contrast, the constitutive expression of
trypsin inhibitors (TI), an important chemical defense trait in
horsenettle, was reduced by 20% in the offspring of plants from
insect-suppression plots relative to those from control plots. However,
TIs were induced to an equal extent whether or not insect herbivores had
been historically suppressed. While several defense and performance traits
(prickle density, TI concentration, resistance against false Colorado
potato beetles and flea beetles, biomass, and seed mass) varied markedly
across families, no traits exhibited significant pairwise correlations.
Overall, our results indicate that, while the divergent responses of
multiple defense traits to insect suppression led to comparatively small
changes in overall constitutive resistance, they significantly reduced
induced resistance against false Colorado potato beetle.