Generalising about generalists? A perspective on the role of pattern and process in investigating herbivorous insects that use multiple host species

Host repertoires and changing insect–plant interactions

“…While one could argue that the Hutchinsonian realised niche (Hutchinson, 1957) is the part of the fundamental niche that an organism is squeezed into by the presence of competitors (and to some extent, the rest of the community), the fundamental host repertoire instead represents excess abilities that are currently not used, but that could be realised if conditions change. Even the insects with the most extensive host repertoires tend to commonly use only a fraction of their potential hosts (Rafter & Walter, 2020). A good example is the Painted Lady butterfly ( Vanessa cardui ), which is able to use plants from 25 families as hosts (Scott, 1986).…”

Behavioural adaptations of the generalist herbivore Helicoverpa punctigera (Lepidoptera: Noctuidae) with respect to primary and secondary hosts

“…By contrast, the consistent pattern of primary host use among generalists strongly suggests that the insects are differentially attracted to plant species, and this depends on the similarity of the associated cues of these host plants to the cues they evolved to recognise and localise (Rafter and Walter 2020). Rather than the insects making choices, plant species with more recognisable cues, such as attractive volatile compounds, have a lower threshold of stimulation required for the insects to oviposit than a plant with only some of these cues (Rajapakse et al 2006;Chapter 4).…”

“…Given that generalists can still be seen as specialised on their primary hosts (Rafter and Walter 2020) and the lack of evidence that multiple host use compromises adaptive oviposition behaviour (Chapter 2), a realistic understanding of why narrow host ranges are the norm needs to consider the processes of species formation. In sexually reproducing organisms, species can be defined as populations of organisms that share in a specific-mate recognition system (SMRS) or fertilisation mechanism (Paterson 1981;, Lambert et al 1987).…”

“…Despite such generalists often having extremely long lists of host plant records, increasing evidence from the field shows these insects are found frequently and in high numbers on only a small minority of plant species in a host list (primary hosts). Many other host plant species (secondary hosts) are used far less often and by fewer individuals, with plant species used only sporadically termed incidental hosts (Walter and Benfield 1994;Manners et al 2011;Rafter and Walter 2020). From this perspective, primary and secondary hosts are relative categorisations (Walter and Benfield 1994), and when frequency of host use is plotted against abundance of larvae at sample sites, the points fall on a continuum, and may appear as distinct clusters (e.g.…”

“…The host recognition process of many insects restricts them to a narrow range of related plant species, or even a single species or genus, whereas others accept a much wider variety of plants (Bernays 1991). Generalist species do not, however, respond to all their host plants equally, but typically have a few primary hosts on which juvenile development mostly takes place, while numerous secondary host plants are used to a lesser extent (Walter & Benfield 1994;Rajapakse et al 2006;Rafter and Walter 2020). This pattern is probably also true for specialists (Wiklund 1981), although their secondary hosts would be fewer in most cases, and closer relatives of the primary host(s).…”