Insect-plant interactions involving the cultivated tomato and its relatives in the genus Lycopersicon have been intensively studied for several decades, resulting in one of the best documented and in-depth examples of the mechanistic complexities of insect-plant interactions, which encompass both herbivores and their natural enemies. Trichome-mediated defenses are particularly significant in L. hirsutum f. glabratum and have been extensively implicated in negative tritrophic effects mediated by direct contact of parasitoids and predators with trichomes, as well as indirect effects mediated through their hosts or prey. Both constitutive and inducible defense traits of L. esculentum exert effects on selected parasitoids and predators. The effects of any particular plant defense trait on parasitoids and predators depend on the specific attributes of the plant trait and the details of the physical, biochemical, and behavioral interaction between the natural enemy, its host (prey), and the plant.

Annual cropping systems consist of a shifting mosaic of habitats that vary through time in their availability and suitability to insect pests. Agroecosystem instability results from changes that occur within a season with crop planting, development, and harvest. Further instability results from continuous alterations in biotic and abiotic insect life system components and from agricultural inputs. Changes to agroecosystems occur across seasons with changing agricultural practices, changing cropping patterns, and technological innovations. Much of this instability is a result of events unconnected with pest management. The abilities of polyphagous pest species to move among and utilize different habitat patches in response to changes in suitability enable the pests to exploit unstable cropping systems. These pest characteristics determine the location and timing of damaging populations. Habitat suitability is influenced by plant species and cultivar, crop phenology, and agricultural inputs. Pest movement is affected by a suite of intrinsic factors, such as population age structure and mobility, and extrinsic factors, including weather systems and habitat distribution. The life systems of three selected polyphagous pests are presented to demonstrate how an understanding of such systems in agricultural ecosystems improves our ability to predict and hence manage these populations.

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The potential of natural enemies to influence the rate of herbivore adaptation to resistance factors in plants is examined using conceptual and mathematical models. Results indicate that natural enemies could increase or decrease the rate of herbivore adaptation. The specific behavioral and physiological effects of a resistance factor on the herbivore, as well as the behavior of the natural enemy, and the population dynamics of the natural enemy/herbivore system are important in assessing the extent to which the natural enemies will affect the rate of herbivore adaptation to a resistance factor. Herbivore adaptation to partial resistance in a host‐plant is generally expected to be slower than adaptation to high levels of resistance, even in the presence of natural enemies, if genetic variance is not limiting. Résumé Influence des ennemis naturels sur le taux d'adaptation d'herbivores à des plantes‐hôtes résistantes Des modèles conceptuels et mathématiques ont servi à examiner l'influence du potentiel d'ennemis naturels sur le taux d'adaptation d'herbivores à des facteurs de résistance des plantes. Les résultats montrent que les ennemis naturels peuvent augmenter ou réduire le taux d'adaptation des herbivores. Les effets spécifiques (comportementaux et physiologiques) d'un facteur de résistance sur l'herbivore, aussi bien que le comportement de l'ennemi naturel et la dynamique de population du système ennemi naturel/herbivore permettent de déterminer le niveau de l'effet des ennemis naturels sur le taux d'adaptation de l'herbivore au facteur de résistance. Une adaptation de l'herbivore à une résistance partielle à une plante est généralement envisagée comme plus lente qu'une adaptation à des hauts niveaux de résistance, même en présence d'ennemis naturels, mais il peut y avoir des exceptions.

Populations of the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae), from the east coast of the United States differ in their ability to survive on a wild host, Solanum carolinense (Solanaceae), but not on their most important cultivated host, Solanum tuberosum. On the wild host, the North Carolina population survived best, while populations from Virginia, New Jersey, and Connecticut exhibited uniformly low survival. Formal genetic studies of populations from Connecticut and North Carolina demonstrated heritable variation in the ability to survive on S. carolinense both between and within populations; the North Carolina population had the higher heritability for this trait. Overall, there was no genetic variation between populations or within the North Carolina population for survival on S. tuberosum, but such variation existed within the Connecticut population. Hybrids and backcrosses between these two lines all survived at intermediate levels, although survivorship did not appear to be inherited additively. Differences in survival were greater than differences in adult weight at emergence and development time of the survivors. Leptinotarsa decemlineata was first reported from North Carolina less than 100 years ago. The rapid expansion of L. decemlineata's host range in North Carolina is attributed to the poor synchrony between the insect and S. tuberosum compared to more northerly locations. In contrast to the prediction of a strong negative correlation in fitness on different host species, the ability of L. decemlineata to survive on S. carolinense was not correlated with that on S. tuberosum. Adult weight and female development time were significantly positively correlated across hosts. Our results are in accord with most previous studies in which strong negative correlations in fitness of specialized phytophagous insects feeding on different hosts were expected, sought, but not found.