Increase in toxicity of an invasive weed after reassociation with its coevolved herbivore

Abstract

The ability of weeds to proliferate into nonindigenous habitats has been attributed to escape from their native natural enemies, allowing reallocation of resources from chemical defense into growth and reproduction. Many invasive weeds, however, eventually encounter their native, coevolved enemies in areas of introduction. Examination of herbarium specimens of an invasive phototoxic European weed, Pastinaca sativa , through 152 years reveals phytochemical shifts coincident in time with the accidental introduction of a major herbivore, the parsnip webworm, Depressaria pastinacella . Plants collected before the introduction of webworms in North America and during the earliest stages of establishment (1850–1889) are lower in toxic furanocoumarins than all plants subsequently collected in North America and lower than European plant samples collected before 1889. Thus, introduction of a major specialist herbivore can increase noxiousness of a species in its area of introduction, illuminating a potential consequence of classical biocontrol programs involving insect herbivores and poisonous weeds.

Development of an Insect Herbivore and its Pupal Parasitoid Reflect Differences in Direct Plant Defense

“…In addition to the wild B. oleracea populations, genetic variation in the expression of allelochemicals has been studied extensively in only a few species of wild plants, including both forbs and trees (Fritz et al 2003;Lawrence et al 2003;Ode et al 2004;Zangerl and Berenbaum 2005;Loney et al 2006). Interpopulation differences in the expression of allelochemicals may be driven by the intensity of selection from herbivores, particularly if natural enemies are scarce or absent (Ode et al 2004;Zangerl and Berenbaum 2005).…”

“…Interpopulation differences in the expression of allelochemicals may be driven by the intensity of selection from herbivores, particularly if natural enemies are scarce or absent (Ode et al 2004;Zangerl and Berenbaum 2005). Reciprocally, this can have profound effects on plant selection and performance in generalist herbivores, although studies have focused mainly on mammals, such as brush-possums in Australia (O'Reilly-Wapstra et al 2005;Loney et al 2006).…”

A Comparison of Two Nicotiana attenuata Accessions Reveals Large Differences in Signaling Induced by Oral Secretions of the Specialist Herbivore Manduca sexta

“…This suggests that plants of the AZ accession have adapted to their native environment via as-yet unidentified defense or tolerance mechanisms. Coevolution has long been considered to have shaped the diversity of both plants and herbivores (Ehrlich and Raven, 1964;Mauricio, 2001;Zangerl and Berenbaum, 2005). Further ecological studies could provide the necessary data to connect diversity in herbivoreinduced plant responses to differences in native herbivore population structures and their dynamics and to determine whether different selection forces imposed by diverse insect populations could have caused polymorphisms in defense compound production between native plant populations.…”

Selection for chemical trait remixing in an invasive weed after reassociation with a coevolved specialist

“…Before the appearance of webworms in North America, parsnips exhibited reduced furanocoumarin defenses in comparison with contemporaneous European parsnips, as determined by examination of herbarium specimens collected in North America over 152 years spanning the period before introduction of webworms to the present, and European specimens from the 19th century to the present, for webworm damage and furanocoumarin content (24). Concomitant with the rise in webworm infestation between 1890 and 1909, levels of all five furanocoumarins (those previously associated with resistance as well as imperatorin and isopimpinellin) increased significantly and continued to increase thereafter.…”