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

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 characterized oviposition preference by documenting whether the presence and abundance of eggs increased with size and local density of its host plant, orpine (Hylotelephium telephium), and 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 hatched, 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 m2. 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.

诸多鳞翅目（Lepidoptera）物种正面临种群衰退，乃至需采取种群补充（reinforcement）与再引入（reintroduction）等积极保育措施，方可维持其可存活种群规模。为最大化此类保育行动的成效，我们亟需厘清影响其产卵生境偏好的关键因子。针对瑞典某岛屿上被纳入IUCN红色名录的格纹青灰蝶（Scolitantides orion）种群，我们通过如下方式表征其产卵偏好：记录卵的存在与否及数量是否随寄主植物——费菜（Hylotelephium telephium，俗称orpine）——的株型大小与局域密度发生变化，同时考察其是否与焦点植株周边两种尺度圆形样地（半径分别为1米与5米）内的蜜源植物丰度、遮阴程度及地表覆盖物组成存在关联。为增殖邻近岛屿上的极小种群，我们共释放127只蛹与46只成虫，并连续两个季节记录其产卵行为。大型样地内，植株被产卵的概率随焦点寄主株型增大而升高，同时也与其余寄主的平均株型及地衣覆盖率呈正相关；样地内的卵量则随寄主植物密度与地衣覆盖量的增加而提升。大型样地的生境特征对产卵偏好的预测效果优于小型样地，且二者均优于仅以寄主株型作为单一变量的预测模型。在本次增殖种群中，所有释放的蛹均成功羽化，释放季内我们共于272株寄主植物上发现507枚卵。被检测的寄主植物中仅5%带有卵，其与最近释放点的中位距离为19.3米。次年，我们于209株植物上发现420枚卵，其与释放点的中位距离增至35.7米，表明该种群已在生境中完成扩散。实践启示：格纹青灰蝶偏好于寄主密度高、地衣覆盖率高的斑块中的大型寄主植株产卵。为筛选适宜的增殖生境，需以约80平方米的区域为单元评估其环境条件。本次增殖实验证实小规模释放即可取得成功，这表明，若存在适宜生境，此类低投入的保育手段可有效提升诸多受威胁蝶类的局域种群规模。