Forest Biology Lab.

Variation in plant traits that affect plant resistance to herbivory is assumed to be maintained because of the balance between the benefits of increased resistance to herbivores and the costs associated with resistance.
Populations of Datura wrightii vary in the frequency of plants that produce glandular trichomes, a resistance trait under the control of a single gene.
In the absence of herbivores, plants with glandular trichomes (gstickyh plants) produce 45% fewer viable seeds than plants with nonglandular trichomes (gvelvetyh plants), but grow to a larger size. After three years in an experimental garden, sticky plants remain larger but continue to reproduce less than velvety plants, never making up for initial costs of glandular trichome production. Although sticky plants have greater resistance to at least six species of herbivores, they are more susceptible to a mirid bug. Glandular trichomes also reduce predation rates by natural enemies of the herbivores that attack D. wrightii, an additional ecological cost. We predicted that the frequency of sticky plants should decline in nature because of these costs, and a survey of plants in 11 natural populations over 5 years suggest this occurs.
The maintenance of the trichome dimorphism in natural populations thus remains to be explained; there may be rare selective episodes we have not measured (herbivore outbreaks, extreme El Nio events) that favor sticky plants.