Plants have access to limited resources that must be allocated amongst competing demands. As a result, plant investment in defense is dynamic, varying over the life of the individual as a function of its changing physiological needs and in response variable levels of herbivory. Such ontogenetic variability in defense phenotypes shapes plant-insect interactions, and has implications for plant fitness (e.g., by syncing the deployment of costly defenses with predictable outbreaks of herbivory). We explored ontogenetic change in chemical and structural defense in juveniles of an introduced weed, Verbascum thapsus (common mullein) after noting that chewing herbivores preferentially feed on old rather than young leaves in the field. We grew plants from seed and harvested them weekly for four weeks to measure changes in iridoid glycosides (a chemical defense against generalist insects), trichome length, and leaf toughness. We also conducted feeding trials with the generalist caterpillar Trichoplusia ni to determine whether larvae prefer young or old leaves when given a choice.

Results/Conclusions

We found that investment in the two structural defenses, trichome length and leaf toughness, changes significantly over time intervals ranging from just one to three weeks, with consistent decreases in trichome length paralleled by increases in leaf toughness. Laboratory assays with  T. ni corroborate damage patterns observed in the field, with larvae consuming significantly more tissue from old than young leaves when given a choice. These findings show that mullein’s defense profiles vary over brief periods of time and suggest that chewing herbivores are more strongly deterred by trichomes than leaf toughness. The relative role of trichomes and iridoid glycosides in deterring the T. ni larvae from feeding will be clearer when the chemistry data are in hand.