According to the coevolution hypothesis, herbivory is the main selective force responsible for the evolution of plant defenses. This hypothesis is supported by observations that defense traits exhibit heritable variation and affect reproductive fitness in the presence of herbivory. Selection on defense traits has been demonstrated primarily by examining the effect of herbivory on seed production in plants with varying levels of chemical or mechanical defenses. This study examines the role of trichomes in rapid-cycling Brassica rapa (RBR) in defense against herbivory by Trichoplusia ni larvae.
Heritability of the trait of trichome production (h2 = 0.37) was demonstrated through an artificial selection experiment on the number of trichomes on the right half of the first true leaf. In a multigenerational study, herbivory by T. ni larvae reduced flower and seed production in RBR compared to control populations, but did not result in increased trichome production in the herbivore-exposed populations over three generations. The effect of trichomes on larval feeding behavior was also examined. Trichome densities for each leaf in a RBR population were recorded. Larvae were placed on the first true leaf of each plant. Larval leaf selection was recorded the following day. Larvae were found more frequently on high trichome density leaves than expected by chance. Trichome density did not affect the amount of time the larvae spent moving, feeding, or resting. Finally, larvae were placed in Petri dishes with leaves of varying trichome density, and leaf consumption was measured by digital imaging. Leaf consumption increased with trichome density. Larvae also fed on leaves in which the trichomes had been removed from one half of the leaf, but leaf consumption did not differ between the shaved and intact portions. These results suggest that trichomes in RBR do not defend against herbivory by T. ni, and that T. ni may exhibit a preference for feeding on leaves with higher trichome density. Potential explanations for these unexpected results include 1) if cost constraints cause an inverse correlation between the levels of chemical and mechanical defenses, the larvae may be responding to decreased levels of chemical defenses in high trichome density leaves, 2) trichomes may be more effective in defending against a different herbivore, and 3) the 30-yr selection process creating this rapid-cycling variety may have affected trichome function.