Host range evolution is not driven by the optimization of larval performance: the case of Lycaeides melissa (Lepidoptera: Lycaenidae) and the colonization of alfalfa

Abstract

Herbivorous insects that have recently incorporated novel hosts into their diet provide unique opportunities for understanding factors that promote or constrain the evolution of niche breadth. Lycaeides melissa has colonized both cultivated and feral alfalfa (Medicago sativa) throughout much of North America within the past 200 years. We investigated the quality of the novel host as a resource for juvenile development, and asked if the novel host is a preferred host for oviposition relative to a native host (Astragalus canadensis). Larval-performance and oviposition-preference were examined using L. melissa individuals from a population associated with both M. sativa and A. canadensis, and oviposition-preference was also examined in another population associated exclusively with M. sativa. In addition, we investigated the effects of M. sativa and A. canadensis flowers on both preference and performance. Only one of the hosts, M. sativa, has flowers that are accessible to nectaring butterflies, and we hypothesized that the presence of flowers could affect female behavior. We find that the novel host is a relatively poor larval resource: adults that were reared as larvae on M. sativa were roughly one-third the size of adults that were reared on the native host, A. canadensis. The native host, Astragalus canadensis, is the preferred host in choice experiments involving only foliage. However, when flowers were included in preference assays, the native and novel hosts received similar numbers of eggs. Thus, the presence of flowers on hosts in the field might influence the utilization of a novel and inferior larval resource. These results are consistent with a model in which host shifts are driven by adult behavior that does not directly optimize larval performance.

Ant association facilitates the evolution of diet breadth in a lycaenid butterfly

“…Caterpillars used in experiments were the progeny of approximately 25 wild-caught females from Beckwourth Pass. Eggs were collected and larvae reared in the laboratory (pooled from different females), following methods described in Forister et al [24], in groups of 20-40 on both A. canadensis or M. sativa foliage through the first instar. As experiments were set up in the field, second instar larvae were moved to the same host species on which they had been previously reared.…”

“…In addition to the simple survival and fecundity values, we included ant-tending, variable host availability (the proportion of the host pool composed of M. sativa) and variable host flowering. The different hosts have distinct effects on larval growth and survival, and the availability of flowers affects not only larval performance (flowers improve larval performance on M. sativa, but have little effect on the native host), but also oviposition [24]. Without flowers, the native host is preferred by ovipositing females; with flowers, plants are chosen with equal frequency [24].…”

“…The different hosts have distinct effects on larval growth and survival, and the availability of flowers affects not only larval performance (flowers improve larval performance on M. sativa, but have little effect on the native host), but also oviposition [24]. Without flowers, the native host is preferred by ovipositing females; with flowers, plants are chosen with equal frequency [24]. Flower availability can also affect secretions produced by caterpillars [34], although this has not yet been studied in L. melissa, and is not included in our models.…”

“…We parameterized our demographic simulations with a combination of sources, including our previous work with L. melissa [24,35], the work reported here, and other sources as described below. Our goal for demographic modelling was not to generate quantitative, demographic predictions.…”

“…Lycaeides melissa [26] is found throughout much of the continent in association with leguminous larval hosts, and notably with both cultivated and feral alfalfa, Medicago sativa, that L. melissa has colonized within the past 200 years [27]. Previous work has suggested that, under certain conditions (such as the absence of flowers that larvae will consume), M. sativa is a poor larval resource (being associated, at least under laboratory conditions, with reduced survival and smaller adult size; [24]). We focus here on a population of L. melissa using two sympatric, larval host plants: a native host, Astragalus canadensis and the novel host, M. sativa.…”