Abiotic induction affects the costs and benefits of inducible herbivore defenses in Datura wrightii.

We evaluated the costs and benefits of continuous high-level expression of defenses relative to naturally-induced defenses in field-grown Datura wrightii in the presence and absence of herbivores. We induced D. wrightii plants with monthly applications of the plant hormone methyl jasmonate (MeJA) and assessed levels of inducible proteinase inhibitors (Pins). MeJA application increased Pin production by 124 %, whereas the increase in Pins due to herbivory was more modest (36 %). Pin induction was costly and significantly reduced plant fitness compared to unmanipulated plants both in the presence and absence of herbivores. Although MeJA-treated plants exposed to herbivory suffered significantly less herbivore damage than unmanipulated plants exposed to herbivory, this was not accompanied by a corresponding fitness benefit. In contrast to glasshouse studies in which protected plants never expressed Pins, Pin induction occurred in field-grown plants not treated with MeJA and completely protected from herbivory. Subsequent experiments confirmed that putative herbivore defenses can be induced abiotically in D. wrightii as: 1) Pin levels did not differ significantly between field-grown plants protected from herbivory and plants exposed to chronic herbivory over the full season; and 2) plants exposed to ambient UV-B light in the absence of herbivory expressed low levels of Pins after two wk of exposure, whereas plants protected from UV-B remained uninduced. The costs of induced responses may be relatively easily determined under field conditions, but there may be many inducing agents in the field, and the benefits of induction may be difficult to associate with any single inducing agent.

Indirect cost of a defensive trait: variation in trichome type affects the natural enemies of herbivorous insects on Datura wrightii.

The costs and benefits of defensive traits in plants can have an ecological component that arises from the effect of defenses on the natural enemies of herbivores. We tested if glandular trichomes in Datura wrightii, a trait that confers resistance to several species of herbivorous insects, impose an ecological cost by decreasing rates of predation by the natural enemies of herbivores. For two common herbivores of D. wrightii, Lema daturaphila and Tupiocoris notatus, several generalized species of natural enemies exhibited lower rates of predation on glandular compared to non-glandular plants. Lower rates of predation were associated with reductions in the residence time and foraging efficiency of natural enemies on plants with glandular trichomes, but not with direct toxic effects of glandular exudate. Our results suggest that the benefit of resistance to herbivores conferred by glandular trichomes might be offset by the detrimental effect of this trait on the natural enemies of herbivores, and that the fitness consequences of this trichome defense might depend on the composition and abundance of the natural-enemy community.

Costs of glandular trichomes in Datura wrightii: a three-year study.

Models accounting for genetic variation for resistance to herbivores within plant populations often postulate a balance between the costs of that resistance and its benefits. The production of glandular trichomes by Datura wrightii was shown to be costly in a previous one-year study because plants producing glandular trichomes (sticky plants), a factor conferring resistance to some insect herbivores, also produced 45% fewer seeds than plants producing nonglandular trichomes (velvety plants) when grown in a common garden. Because sticky plants tended to be larger than velvety plants but produced fewer seed capsules, we postulated an allocation trade-off in which velvety plants are more reproduction-dominated whereas sticky plants are more growth-dominated. If a greater commitment to vegetative growth eventually allows sticky plants to compensate for reduced seed production, we would expect a reduction or elimination of the cost of resistance over time in this perennial plant. We monitored growth, survival, and seed production of plants from defined crosses of local populations for three years in a common garden when exposed to and protected from herbivores, and with and without supplemental water. The majority of plants exposed to herbivores had died by the end of the study. We used standard life-table methods to determine the net reproductive rate (R0) and the finite rate of increase (lambda) of plants of each trichome type. After three years, when plants were protected from herbivores, sticky plants were 187-245% larger than velvety plants, depending upon irrigation treatment, but sticky plants continued to be less efficient in producing seeds per unit of canopy volume. Even though the total seed production of sticky plants eventually equaled that of velvety plants, the advantage of earlier reproduction by velvety plants increased lambda by 55-230% over that of sticky plants, depending upon herbivore and irrigation treatment. Exposure to herbivores reduced lambda by 69-83%, depending upon plant type and irrigation treatment, whereas supplemental irrigation increased lambda by 29-175%, depending upon plant type and exposure to herbivores. Although there was a large allocation trade-off between growth and reproduction, the benefits of such a trade-off did not emerge before most plants were killed by herbivores. The cost of producing glandular trichomes strictly for herbivore resistance continued to exceed its benefits, and in the absence of other, unmeasured benefits from the suite of life-history characters associated with glandular trichome production, natural selection is expected to eliminate this costly resistance trait from D. wrightii populations.