Plant Herbivore Interactions and Chemical Ecology

Euaresta aequalis ovipositing on the fruit of a cocklebur, Xanthium strumarium.

 

A flower of Datura wrightii

 

 Lema daturaphila adults feeding on velvety Datura wrightii leaf.

 

Adult Tupiocoris notatus feeding on a sticky Datura wrightii leaf

Glandular trichomes of sticky Datura wrightii. 

Nonglandular trichomes, or plant hairs, of velvety Datura wrightii with a short, lobed trichome

 

My research strives to answer three fundamental questions. The first question asks, "What is the actual impact of herbivores on plants?" The second related question is, "When do herbivores impose selection on plants for herbivore resistance?" The third question asks, "If resistance to herbivores is beneficial, then why aren't all plants resistant?"

The first question tests the assumption that all herbivory is detrimental and suggests that plants may tolerate some damage. Implicit in the second and third questions is the assumption that the benefits of any particular plant resistance mechanism may be limited, and that such benefits can be obtained only at some cost. My goals are to learn how herbivores affect plant fitness and to understand the potential trade-offs between the costs and benefits of host plant resistance against different herbivore species.

My approach toward answering these questions involves detailed study of variation within and among local plant populations in the interactions between plants and their insect herbivores. I am particularly interested in the genetic component of such variation and how such variation is mediated chemically. As a result, I have worked with several different plant chemicals that influence plant-herbivore interactions. These include not only "primary" chemicals such as leaf protein and individual free amino acids that determine the nutritional quality of plants, but also several classes of "secondary" chemicals. These include terpenoids, alkaloids, tannins and other phenolic compounds, and more recently, defensive sugar esters. In some cases, we needed to identify new compounds and to develop new methods for accurate chemical measurement before we could understand their ecological roles.

Although my initial interest was in natural systems, I also have worked in agricultural systems. In many cases, agricultural systems offer distinct advantages over natural systems because both the genetic structure and the growing conditions of agricultural plants are better controlled than in natural systems. Applied systems also provide an opportunity to test and apply our understanding of plant-herbivore interactions for improved, more ecologically sound pest management.

 For the last few years, I have been studying a plant resistance dimorphism in the locally abundant plant, Datura wrightii. This plant is ideal to study the costs and benefits of host plant resistance. A single dominant gene determines the resistance mechanism, glandular hairs. The glands produce a defensive exudate, and plants with glandular hairs feel sticky when touched, while those with non-glandular hairs feel velvety. Sticky and velvety plants occur within local populations. Sticky plants are less suitable for survival and growth of the tobacco hornworm, flea beetles, a leaf-feeding weevil and are completely resistant to whiteflies. In contrast, sticky plants are more susceptible to Tupiocoris notatus, a small bug that has specific structural adaptations to cope with glandular hairs. Thus, the proportion of sticky and velvety plants in different populations may be, in part, the result of natural selection in opposite directions by different pest species.

In addition to the various direct effects of glandular trichomes on different herbivore species, glandular trichomes also inhibit the activity of various generalist predators.  This poses a dilemma for the evolution of plant defenses:  should evolution favor high densities of glandular trichomes that act directly on herbivores or reduced densities to favor the natural enemies of those herbivores?  In my current research, I test the working hypothesis that the indirect effects of glandular trichomes on natural enemies oppose the direct effects on insect herbivores and constrain the increase in the frequency of sticky plants in established D. wrightii populations.

Most recently, our research is expanding into the area of induced defenses.  Like other solanaceous species, D. wrightii can be induced to produce defensive proteins and volatile organic compounds (VOCs) either after attack by herbivores or treatment with the plant hormone, methyl jasmonate.  Both trichome phenotypes are equally inducible, but there is genetically-determined variation in both the quantities and the blends of VOCs produced by different plants.  We are beginning to investigate the interactions between constitutive plant defenses (glandular trichomes) and induced defenses on plant protection against herbivores and the potential value of each in plant populations that differ in the frequencies of plants with glandular and nonglandular trichomes.

Publications

·       Hare, J. Daniel.  2008.  Inheritance of Leaf Geranylflavanone Production and Seed Production within and among Chemically Distinct Populations of Mimulus aurantiacus.  Biochemical Systematics and Ecology 36: 84-91.  Abstract.  Link to article.

·       Hare, J. Daniel.  2007.  Variation in Herbivore and Methyl Jasmonate-induced Volatiles among Genetic Lines of Datura wrightii.  Journal of Chemical Ecology 33: 2028-2043.  Abstract. Link to Article.

·       Hare, J. Daniel and Linda L. Walling.  2006.  Constitutive and Jasmonate-inducible Traits of Datura wrightii. Journal of Chemical Ecology 32: 29-47.  Abstract.  Link to Article.

·       Hare, J. Daniel.  2005.  Biological activity of acyl glucose esters from Datura wrightii glandular trichomes against three native insect herbivores.  Journal of Chemical Ecology 31: 1475-1491.  Abstract.  Link to Article.

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