About Spialia sertorius orbifer (Hübner, 1823)
The red-underwing skipper, Spialia sertorius orbifer, can easily be confused with species in the genus Pyrgus. The underside of its wings has a cinnamon-red to yellowish base color, while Pyrgus species typically have a greenish to brownish base tone. Because this distinct coloration is less noticeable in older butterflies and natural deviations occur, identification should always be confirmed by checking the characteristic arrangement of spots on the underside of the hindwing. On the upper side of the forewing, a series of small, clearly bright spots sit in the submarginal band, which follows a regular, flat curve toward the leading edge. The four spots in the post-discal region closer to the wing base are arranged in a single straight row; by contrast, Pyrgus species only have three of these spots aligned side by side, with the fourth positioned separately. This species has a wingspan of 22 to 26 mm. Identification is especially complicated in Spain, where Spialia rosae, a species formally described in 2016, flies alongside Spialia sertorius at altitudes around 1000 m. Spialia sertorius is widespread across Western and Central Europe. Its range extends south through Southern Europe to North Africa, and east as far as western Poland, the Czech Republic, Slovenia, and Croatia. It is not found in Scandinavia or Great Britain, and it is considered extinct in the Netherlands. In Germany, it does not occur in the northeastern federal states. The species distribution is closely tied to the range of its host plant, Sanguisorba minor, so Spialia sertorius is strongly associated with low-nutrient grasslands growing on limestone or calcareous conglomerates. While the host plant is most common in arid grasslands or gaps in the pioneer stages of semi-dry grasslands, the butterfly strongly prefers host plants growing in small sites with especially favorable thermal conditions. In Central Europe, Spialia sertorius occurs from low elevations up to altitudes over 1000 m, with most recorded observations concentrated disproportionately between 400 and 500 m, compared to other elevation bands. Above 1000 m, the species’ thermal requirements are only met at sites with particularly favorable microclimates, such as south-facing slopes. The host plant’s upper elevation limit in the Bavarian Alps is typically just over 1200 m, but records from warm years after World War II note observations around 1700 m and between 1700 and 2000 m; these butterflies are thought to have drifted up from lower elevations. Adult butterflies fly from April to September, depending on the location. There are usually two generations per year, and adults of the second generation are smaller than those of the first. Spialia sertorius is capable of producing two generations across almost all of its range in Central Europe, though far fewer records exist for the second generation, indicating that it does not appear every year and is less abundant when it does occur. A small second generation is also recorded in the Palatinate, where a large portion of the spring generation’s offspring do not complete development in the same year, and only pupate in April after overwintering. The earliest recorded specimens from Bavaria date to the second half of April during the warm spring of 2007. In most other years, the first observations are made at the end of April to early May. The main flight period of the first generation usually begins in mid-May, peaks at the end of May, and the number of observations drops sharply by the end of July. Around mid-August, observations begin to increase again, corresponding to the emergence of the second generation. Fresh adults have been recorded on the Munich plain in early September. In the Alpine region, observations only occur from mid-May to mid-August. Available data shows that a second generation does not form in all locations or every year. Very few records exist for elevations over 1000 m after mid-July, though this suggests that a second generation can occasionally occur at these altitudes in favorable years. The red-underwing skipper exclusively uses Sanguisorba minor as an egg-laying site and caterpillar food source. First generation females mostly lay eggs on still-closed flower heads, but will occasionally lay eggs on leaves. Because Sanguisorba minor does not regularly produce new flower heads until after summer drought in arid grasslands, second generation females in most areas can only use leaves for egg-laying. As a result, they mostly glue their eggs to the upper surface of leaflets from the host’s pinnate leaves, and prefer leaves that lie close to the ground. Young caterpillars that hatch from eggs laid in flower heads feed on ripening seeds and flower parts during their first two instars; caterpillars from eggs laid on leaves feed on the leaves initially. Later instars live in leaf bags constructed from the host plant’s leaflets. Based on field observations in Switzerland, caterpillars overwinter inside a partial leaf of Sanguisorba minor that they have spun together to form a shelter. Pupation occurs on the ground, inside a cocoon made of woven leaf fragments. Adults have a very fast flight that stays close to the ground. Males are the easiest to observe, as they are territorial and regularly return to spots including flowers of Sanguisorba minor. Flower visitation is not very frequent overall. Multiple observations from Bavaria record adults feeding on Hippocrepis comosa and Lotus corniculatus, and these plants are also important nectar sources for the species in Baden-Württemberg and the Palatinate. Additional observations from Bavaria record the species nectaring on Thymus spp., Globularia cordifolia, Teucrium montanum, Geranium sanguineum, and Pimpinella spp. Males, in particular, frequently feed on moist soil.
