Can plant resistance to specialist herbivores be explained by plant chemistry or resource use strategy?

Abstract

At both a macro- and micro-evolutionary level, selection of and performance on host plants by specialist herbivores are thought to be governed partially by host plant chemistry. Thus far, there is little evidence to suggest that specialists can detect small structural differences in secondary metabolites of their hosts, or that such differences affect host choice or performance of specialists. We tested whether phytochemical differences between closely related plant species are correlated with specialist host choice. We conducted no-choice feeding trials using 17 plant species of three genera of tribe Senecioneae (Jacobaea, Packera, and Senecio; Asteraceae) and a more distantly related species (Cynoglossum officinale; Boraginaceae) containing pyrrolizidine alkaloids (PAs), and four PA-sequestering specialist herbivores of the genus Longitarsus (Chrysomelidae). We also assessed whether variation in feeding by specialist herbivores is attributable to different resource use strategies of the tested plant species. Plant resource use strategy was quantified by measuring leaf dry matter content, which is related to both plant nutritive value and to plant investment in quantitative defences. We found no evidence that intra-generic differences in PA profiles affect feeding by specialist herbivores. Instead, our results indicate that decisions to begin feeding are related to plant resource use strategy, while decisions to continue feeding are not based on any plant characteristics measured in this study. These findings imply that PA composition does not significantly affect host choice by these specialist herbivores. Leaf dry matter content is somewhat phylogenetically conserved, indicating that plants may have difficulty altering resource use strategy in response to selection pressure by herbivores and other environmental factors on an evolutionary time scale.

Incorporating citizen science, museum specimens, and field work into the assessment of extinction risk of the American Bumble bee (Bombus pensylvanicus De Geer 1773) in Canada

“…As B. pensylvanicus is a grassland species, the extensive loss of grasslands in Ontario may be one reason why this species is in decline (Colla and Dumesh 2010;Ontario Biodiversity Council 2010Hatfield et al 2015c). A visual assessment using Google Maps and Google Streetview (Google 2018) of the habitat around the recent sites documented in this study found that 73% were located in, or within 500 m of, a grassland/old field area (including one pasture site), with an additional 18% of sites in, or within 200 m of, a large city park and/or a large riparian area.…”

“…The loss of forage plants, particularly for specialist species, may result in a decline of the pollinator species (León-Cortés et al 1999;Johnson and Steiner 2000;Memmott et al 2007;Burkle et al 2013). As a generalist species that has been known to feed on over 200 plant species (Macfarlane 1974;Colla and Dumesh 2010;Williams et al 2014), B. pensylvanicus could be thought resilient to the loss of some plant species, as a result of disturbances such as habitat loss or climate change. However, although B. pensylvanicus was the bee species with the most observed unique plant species interactions in an Illinois study in the late 1800s, it was only observed once, on one plant species a century later at the same area, despite an extensive survey effort and with many of the same plants being in existence (Burkle et al 2013).…”

“…Bombus pensylvanicus is a grassland species associated with open fields and farmlands (Colla and Dumesh 2010;Williams et al 2014). In its Canadian range, it is a late-emerging species that tends to nest on the surface of the ground amidst long grasses, occasionally nests above ground, and only rarely nests underground (Wyatt 1970;Macfarlane 1974;Macfarlane et al 1994;Colla and Dumesh 2010;Williams et al 2014). This species tends to have smaller colonies (average of 33 new queens and 132 workers and males produced per colony) relative to congeners (Macfarlane et al 1994).…”

A phylogenetic analysis and new delimitation of Senecio sect. Crociseris (Compositae: Senecioneae), with evidence of intergeneric hybridization

“…Modern phylogenetic methods now provide opportunities to review and revise these morphology-based infrageneric classifications of Senecio. Such studies contribute to arriving at a sectional classification that reflects evolutionary history and can therefore provide a useful basis for studies aimed at understanding various aspects of the diversity of Senecio species, such as their biochemical diversity (e.g., Pelser & al., 2005;Langel & al., 2011) and ecology (e.g., Kirk & al., 2012). These phylogenetic studies have resulted in a new delimitation of Senecio and the identification of major lineages within the genus (Pelser & al., 2007).…”

Improved Understanding of Weed Biological Control Safety and Impact with Chemical Ecology: A Review

“…Nevertheless, Bernays et al ( 2004) conclude that it is likely that other compounds play a role in host selection because PAs of the type tested also occur in other genera and families. Adult feeding by the two chrysomelids Longitarsus jacobaeae Waterhouse and Longitarsus flavicornis (Stephens) (Coleoptera: Chrysomelidae), another two biological control agents of tansy ragwort, were not correlated with the PA profile of ragwort plants (Kirk et al 2012). Because adult feeding damage in L. jacobaeae is correlated with larval attack rate (Rapo et al 2010), the lack of correlations between adult feeding and PA profile suggests an absence of a significant relationship between plant PA profile and overall attack of ragwort species by L. jacobaeae.…”