Tolerance and phenological avoidance of herbivory in tarweed species.

Avoidance and tolerance of herbivory are important components of plant interactions with herbivores. Their relationship to each other and to plant defense is important in understanding how plants maximize fitness in the face of herbivore pressure. Various tarweed species have populations comprised of both early-season and late-season flowering individuals. Late-season flowering individuals employ a recently described indirect defense against herbivores in which the accumulation of dead insects on their sticky surfaces attracts predatory insects that eat herbivores. In two tarweed species (Hemizonia congesta and Madia elegans), we observed that key herbivores rarely interact with early-season individuals in the field, and early-season individuals did not invest in dense glandular trichomes that cause indirect defense. We conducted field and greenhouse bud-removal experiments to assess tolerance of M elegans to herbivore damage. We found that late-season individuals were more tolerant of simulated herbivory than early-season individuals in both the field and the greenhouse. Late-season individuals that were forced into an earlier phenology with a 24-h light cue lost their tolerance to simulated herbivory. One possible mechanism linking phenological avoidance of herbivores with decreased tolerance is that early-season individuals invested less in below-ground biomass than late-season individuals, which may accumulate belowground resources for regrowth at the expense of early flowering.