Mean plant toxicity modulates the effects of plant defense variability

Plant trait variation is thought to suppress herbivore performance, but
experiments are usually done with a single mean level of the trait. We
manipulated the mean and variation of glucosinolate toxin concentration on
Arabidopsis thaliana leaves and fed them to Trichoplusia ni in three field
and greenhouse experiments. Plants painted with a greater mean
glucosinolate concentration had higher fitness and resistance to
herbivores; however, at high mean concentrations, variation reduced the
defensive effect, while at lower mean concentrations, variation enhanced
it. This reversal is consistent with models that include herbivore food
selectivity, but our simulations revealed that the benefit of food
selectivity to herbivores was minuscule. Instead, nonlinear averaging and
physiological tracking effects likely drove patterns in plant fitness and
resistance to herbivores. We suggest that high defense variation in plants
may be a widespread convergent defensive phenotype, but for well defended
plants, variation may inadvertently promote herbivore niche expansion.