Speyeria idalia (Drury, 1773)
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Basis for Listing
The regal fritillary has suffered a catastrophic recent decline in the eastern half of its historical range, from Wisconsin and Illinois east to the Atlantic seaboard. In the eastern part of its range, it is presumed extirpated in many states and is possibly extirpated in most of the remainder, as well as in Ontario. In the few states in this part of its range where it is still extant, it is either imperiled or critically imperiled (NatureServe 2008). It is faring better in the western half of its range, but it is considered apparently secure only in Kansas. In four states (including Minnesota) it is considered vulnerable, and it is imperiled or critically imperiled in the rest (NatureServe 2008). Because the decline in the east is not understood, it is not possible to evaluate the likelihood of this decline spreading to the western part of the range, but the rapidity and severity of the decline strongly suggests that the species is susceptible to subtle environmental change.
The regal fritillary is a large, distinctively marked species and one of temperate North America's most striking butterflies. Forewing length in males is 3.5-4.8 cm (1.4-1.9 in.). Females are slightly larger than males, with a forewing length of 5 cm (2 in.) not unusual. The sexes are similar in color and pattern. Above, the forewings are a rich reddish orange with a number of irregularly shaped black spots and a row of small whitish flecks in a narrow black border along the outer margin. The upper side of the hind wings is a somewhat iridescent blue black, with an inner row of large white spots and an outer row of smaller spots that are orange in males and white in females. The underside of the hind wings is identical in both sexes: a bold pattern of large, triangular silver spots in a dark brown ground color. The only butterflies in Minnesota that resemble the regal fritillary are the monarch (Danaus plexippus), the great spangled fritillary (Speyeria cybele), and the Aphrodite fritillary (S. aphrodite). A fourth similar fritillary, the Atlantis fritillary (S. atlantis), is restricted to the forested northern part of the state and wouldn't be encountered with the regal. Uncertainty of identification for any of the 4 species would only arise for individuals seen at a distance or for extremely flight-worn individuals. None of these has the strong contrast in coloration between the forewings and hind wings above.
In Minnesota the regal fritillary is strongly associated with native prairie habitat. Adults are encountered in both upland prairies and in wet prairies, although larval development may be restricted to upland prairie.
Biology / Life History
The regal fritillary has a single annual generation. Eggs are laid in late summer and hatch after a few weeks. The tiny larvae do not feed after hatching, but hide in the duff and enter dormancy until the following spring, when feeding and growth begin. Larvae complete their growth in June and pupate, with emergence of adults beginning in mid-June and continuing into July. Males emerge on average a week or more before females and quickly begin searching for newly emerged females. Females mate soon after emergence but appear to remain concealed in the vegetation for a period of several days after mating, becoming active only after male numbers have greatly declined (Kopper et al. 2001). They visit flowers but may not begin egg laying until August. Females rarely mate more than once; they are capable of laying up to 1,000 eggs, which are matured gradually and placed singly, not in clusters or masses. Adult life expectancy in the wild has not been reliably determined, but individuals have been observed to survive a month or more (Nagel et al. 1991). Throughout their adult lives, both sexes feed frequently on floral nectar.
Conservation / Management
Frequent fire was an integral part of the prairie landscape, and prescribed burning is an important tool in managing the remnants to prevent woody succession and to help control invasive non-natives. However, burns probably cause high mortality in all immature stages of the regal fritillary (Swengel 1998). Wherever practical, only a fraction of a site should be burned during a butterfly generational cycle, especially in sites that appear to be effectively cut off from immigration. It is unknown whether females will lay eggs in prairie the first growing season after a burn when there is little or no duff; until it is determined that they do, care must be taken to maintain enough unburned habitat through two or more successive seasons to maintain an adequate reservoir of butterflies in a site. Haying can be a useful complement to prescribed burning in sites where constraints on burning would make it difficult to achieve management objectives with fire alone. It is not known whether females will lay eggs in a recently mowed prairie. Delaying mowing until most egg laying has occurred would remove this concern, but it creates another, as hay raking could result in significant larval mortality. Haying only part of a site each year is therefore advisable.
Conservation Efforts in Minnesota
Several survey efforts in Minnesota have included the regal fritillary as one of their target species. Documented occurrences are recorded in the Rare Features Database maintained by the Natural Heritage and Nongame Research Program, and these data are consulted in environmental review of projects that could affect the species or its habitat. Many prairie sites that support this species are protected and managed to maintain the prairie community by the Minnesota DNR, the U.S. Fish and Wildlife Service, and The Nature Conservancy. Land managers have been provided with guidance on the importance of subdividing sites and burning the units in a rotation that reduces the likelihood of extirpating insects and other small animals restricted to these remnants. The Minnesota DNR also provides assistance to private landowners in managing native prairie on their land. The Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook are available to land managers and landowners to help protect and manage remaining prairie parcels.
References and Additional Information
Debinski, D. M., and Kelly, L. 1998. Decline of Iowa populations of the Regal Fritillary (Speyeria idalia) Drury. Journal of the Iowa Academy of Science 105:16-22.
Henderson, R. A., J. Meunier, and N. S. Holoubek. 2018. Disentangling effects of fire, habitat, and climate on an endangered prairie-specialist butterfly. Biological Conservation 218:41-48.
Kelly, L., and D. M. Debinski. 1998. Relationship of host plant density to size and abundance of the Regal Fritillary Speyeria idalia Drury (Nymphalidae). Journal of the Lepidopterists' Society 52:262-276.
Kopper, B. J., S. Shu, R. E. Charlton, and S. B. Ramaswamy. 2001. Evidence for reproductive diapause in the Fritillary Speyeria idalia (Lepidoptera: Nymphalidae). Annals of the Entomological Society of America 94:427-432.
Nagel, H. G., T. Nightengale, and N. Dankert. 1991. Regal Fritillary butterfly population estimation and natural history on Rowe Sanctuary, Nebraska. Prairie Naturalist 23:145-152.
NatureServe. 2008. NatureServe Explorer: An online encyclopedia of life [web application]. Version 7.0. NatureServe, Arlington, Virginia.
Ries, L., and D. M. Debinski. 2001. Butterfly responses to habitat edges in the highly fragmented prairies of central Iowa. Journal of Animal Ecology 70:840-852.
Swengel, A. B. 1997. Habitat associations of sympatric violet-feeding fritillaries (Euptoieta, Speyeria, Boloria) (Lepidoptera: Nymphalidae) in tallgrass prairie. Great Lakes Entomologist 30:1-18.
Swengel, A. B. 1998. Effects of management on butterfly abundance in tallgrass prairie and pine barrens. Biological Conservation 83(1):77-89.