Tremendous strides have been made regarding our understanding of how host plant chemistry influences the interactions between herbivores and their natural enemies. While most work has focused on plant chemistry effects on host location and acceptance by natural enemies, an increasing number of studies examine negative effects. The tritrophic role of plant chemistry is central to several aspects of trophic phenomena including top-down versus bottom-up control of herbivores, enemy-free space and host choice, and theories of plant defense. Furthermore, tritrophic effects of plant chemistry are important in assessing the degree of compatibility between biological control and plant resistance approaches to pest control. Additional research is needed to understand the physiological effects of plant chemistry on parasitoids. Explicit tests are required to determine whether natural enemies can act as selective forces on plant defense. Finally, further studies of natural systems are crucial to understanding the evolution of multitrophic relationships.

Plant chemistry and natural enemy fitness: effects on herbivore and natural enemy interactions.

Plants often release a blend of volatile organic compounds in response to damage by herbivorous insects that may serve as cues to locate those herbivores by natural enemies. The blend of compounds emitted by plants may be more variable than is generally assumed. The quantity and the composition of the blends may vary with the species of the herbivore, the plant species and genotype within species, the environmental conditions under which plants are grown, and the number of herbivore species attacking the plant. Although it is often assumed that induced emission of these compounds is an adaptive tactic on the part of plants, the evidence that such responses minimize fitness losses of plants remains sparse because the necessary data on plant fitness rarely have been collected. The application of techniques of evolutionary quantitative genetics may facilitate the testing of widely held hypotheses about the evolution of induced production of volatile compounds under natural conditions.

Ecological role of volatiles produced by plants in response to damage by herbivorous insects.

Tolerance and resistance are two different plant defense strategies against herbivores. Empirical evidence in natural populations reveals that individual plants allocate resources simultaneously to both strategies, thus plants exhibit a mixed pattern of defense. In this review we examine the conditions that promote the evolutionary stability of mixed defense strategies in the light of available empirical and theoretical evidence. Given that plant tolerance and resistance are heritable and subject to environmentally dependent selection and genetic constraints, the joint evolution of tolerance and resistance is analyzed, with consideration of multiple species interactions and the plant mating system. The existence of mixed defense strategies in plants makes it necessary to re-explore the coevolutionary process between plants and herbivores, which centered historically on resistance as the only defensive mechanism. In addition, we recognize briefly the potential use of plant tolerance for pest management. Finally, we highlight unresolved issues for future development in this field of evolutionary ecology.

The Evolution of Resistance and Tolerance to Herbivores

In recent years, induced plant defenses have received widespread attention from biologists in a variety of disciplines. The mechanisms underlying these defenses and the interactions that mediate them appeal not only to plant physiologists, ecologists, and evolutionary biologists but also to those scientists that search for novel strategies in plant protection. Several recent books (Karban and Baldwin, 1997; Agrawal et al., 1999) and reviews (Baldwin, 1994; Karban et al., 1997; Agrawal and Rutter, 1998; Agrawal and Karban, 1999; Baldwin and Preston, 1999; Dicke et al., 2003) have been devoted entirely to the subject of induced plant defenses. Various forces, ranging from abiotic stresses to biotic factors such as pathogens, arthropods, or higher organisms, may trigger different plant defense responses. Yet, the biochemical pathways that are involved appear to show considerable similarities. This is also true for the so-called indirect defenses. The term indirect defense refers to those adaptations that result in the recruitment and sustenance of organisms that protect the plants against herbivorous attackers. The early published examples of indirect defenses involved intimate plant–ant interactions, in which myrmecophilous plants were shown to have evolved a range of adaptations providing ants with shelter (domatia) and various food sources (Belt, 1874; Janzen, 1966). In return, these plants may obtain a range of benefits because ants can provide nutrition (Thomson, 1981) or more commonly, protection against herbivores, pathogens, and competing plants (e.g. Koptur, 1992; Oliveira, 1997). The well-documented fitness benefits of ant attendance in myrmecophilous plants (Rico-Gray and Thien, 1989; Oliveira, 1997), combined with the fact that domatia and food supplements are difficult to reconcile with other functions, are convincing arguments for the interpretation that these adaptations represent examples

/pdf/recruitment-of-predators-and-parasitoids-by-herbivore-2jbfj3xxe7.pdf

Recruitment of predators and parasitoids by herbivore-injured plants

Both an individual's genotype and environment govern its phenotype, and this phenotype may have extended consequences for species interactions and communities. We examined the importance of plant genotype and environmental factors operating at large (habitat) and small (microhabitat) spatial scales in affecting a multitrophic arthropod community on plants. We planted 926 plants from 14 genotypes of Oenothera biennis into five natural habitats that represent the range of environments in which this plant locally occurs. Genotypic differences among plants accounted for as much as 41% of the variation in arthropod diversity (Simpson's diversity index) and also affected arthropod evenness, richness, abundance, and biomass on individual plants. However, the effects of particular plant genotypes on the arthropod community varied across habitats (i.e., there were significant plant genotype-by-habitat interactions). Plant genotype explained more variation in the arthropod community than did environmental variation among microhabitats, but less variation than habitats, as predicted by the scale-dependent hypothesis. Herbivores and omnivores were more strongly affected by plant genetic variation than predators, consistent with the notion that phytophagous insects undergo stronger reciprocal interactions with plants than do predators. We detected heritable variation in arthropod community variables and the ability for the herbivore community to select on plant traits, suggesting that evolution in O. biennis can lead to changes in the arthropod community. Genetic variation in plant size, architecture, and reproductive phenology were the plant traits most strongly correlated with arthropod community variables. Our results demonstrate that genotype-by-environment interactions are a major determinant of arthropod community structure.

/pdf/plant-genotype-and-environment-interact-to-shape-a-diverse-44l3r71zsd.pdf

Plant genotype and environment interact to shape a diverse arthropod community on evening primrose (oenothera biennis)

Introduction The host plants of herbivores are not neutral substrates upon which interesting herbivores–natural enemies occur. Both the dynamics as well as the outcome of particular herbivore–natural enemy interactions may vary with the herbivore's host plant species, or genotype within species, and understanding such variation is central to the study of tritrophic interactions. The theory of tritrophic interactions implies that plant characteristics that enhance the success of the natural enemies have evolved; plants with traits that encourage the success of natural enemies should have a selective advantage over plants that do not, thus the trait should spread through the plant population (Price et al. , 1980; Fritz, 1992, 1995; Hare, 1992). The primary genetic question underlying this chapter is whether the impact of natural enemies on herbivores is sufficiently strong and systematic to cause changes in gene frequencies in plant traits affecting the impact of natural enemies on those herbivores. In this chapter, I will provide a brief review of several studies showing how genetic variation in plants may affect the outcomes of herbivore–natural enemy interactions. Most of these studies originated from applied studies in biological control. The strengths and weaknesses of those data in developing a general expectation of tritrophic interactions will be evaluated. I will conclude with a list of specific research objectives that may facilitate the development of a more predictive theory of tritrophic interactions.

Multitrophic Level Interactions: Plant genetic variation in tritrophic interactions

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Multitrophic Level Interactions: Plant genetic variation in tritrophic interactions