Testing for local monophagy in the regionally oligophagous Euphydryas aurinia (Lepidoptera: Nymphalidae)

Regionally oligophagous insects are often host plant specialists at smaller geographical scales, so conservation planning should preferably rely on locally derived information. Host use of the endangered butterfly Euphydryas aurinia was investigated close to the northern limit of its European distribution. We experimentally studied host preference of ovipositing females and growth performance of pre-diapause larvae on different hosts, as well as made field observations on host use by surveying larval webs. In the field, Succisa pratensis has remained the only confirmed host of E. aurinia in Estonia. Also, in the laboratory, the females consistently preferred S. pratensis as oviposition substrate. Larval performance was the highest on S. pratensis though the insects were also able to develop on Knautia arvensis. By contrast, species of Plantago, or Valeriana officinalis, were not suitable for larval development. E. aurinia appears thus to be a host specialist in Estonia, even if the regional list includes a higher number of plant species. Our results confirm a pattern repeatedly recorded in Melitaeine butterflies, i.e. geographic differences in host use and local monophagy. As a methodological contribution, we showed that different lab-based methods yield consistent information on host use, though tend to somewhat overestimate the actual host range of the species in the field. Using larval growth rate as an index of host suitability appears problematic for species not time-stressed during their larval development, and survival is a more appropriate parameter to record.