Determining oviposition preferences to inform population reinforcement of the specialist chequered blue butterfly ( <i>Scolitantides orion</i> )

Abstract Many Lepidoptera species are declining to the point that active measures such as reinforcements and reintroductions are necessary to maintain viable populations. To maximise success of such efforts, we need to understand the factors influencing habitat preferences for oviposition. In a Swedish island population of the red‐listed chequered blue butterfly ( Scolitantides orion ), we characterised oviposition preference by documenting whether the presence and abundance of eggs (i) increased with size and local density of its host plant, orpine ( Hylotelephium telephium ), and (ii) was associated with nectar plant abundance, degree of shading, and ground cover composition within small and large circular plots (1 m vs. 5 m radius) around focal plants. To reinforce a tiny population on a neighbouring island, we released 127 pupae and 46 imagos and recorded oviposition for two seasons. The probability of receiving eggs increased with focal host plant size, and also with mean size of other host plants and cover of lichens within the large plots. The abundance of eggs within large plots increased with host plant density and amount of lichen cover. Habitat characteristics of the large plots predicted oviposition better than did those of small plots, and both outperformed models including host plant size only. In the reinforcement population, all released pupae eclosed, and we found a total of 507 eggs on 272 host plants during the release season. Eggs were present on 5% of examined host plants, and median distance to a release spot was 19.3 m. In the following year, we found 420 eggs on 209 plants, and median distance to a release spot increased to 35.7 m, indicating diffusion through the habitat. Practical implication. The chequered blue butterfly prefers to oviposit on large host plants in patches with high host density and high cover of lichens. To identify suitable release habitats, environmental conditions should be considered in an area of ca 80 m 2 . The reinforcement shows that small releases can be successful and suggests that similar low‐effort approaches can be effective to boost local populations of many threatened butterflies, provided suitable habitats are available.