The contribution of semi-natural habitats to biological control is dependent on sentinel prey type

Study area The study was conducted in 18 focal fields of winter wheat per year surrounded by a 1 km radius landscape circle in the South East and South West regions (counties Dorset and Hampshire) of the UK in 2014 and 2015 (Fig. 1). A 1 km radius was used because this was the distance over which many previous studies had detected an infuence on biocontrol in cereal crops (e.g. Rusch et al., 2016). The area has a temperate climate, summers are warm and humid (Kottek et al., 2006). Due to crop rotation, focal fields sown with winter wheat in 2014 were put into a different crop in 2015 which prevented the use of the same focal fields and their surrounding landscape circles in 2015. Between years, landscape circles were, however, selected to be close together and are defined as landscape circle pairs. The central point of landscape circle pairs were seperated by a distance of 90 m to 3 km (mean 1.15 ± 0.05 km). Sampling design A standardised QuESSA protocol was followed that examined the impact of three different boundary types (local effect) on sentinel predation. From the 18 focal fields studied each year, six had a field boundary for each of three categories, 1) herbaceous strip between fields (control), 2) woody linear habitat (hedgerow) or 3) woody areal habitat (woodland) (Table A.1). Within each field, sentinels were placed at 2, 25, 48 and 71 m from the adjacent SNH, along two transects 10 m apart (Fig. A1). To minimize the influence of the other field boundaries on transects, the distance between the end of the transects and non-focal field boundaries had to be at least 1.25 times the length of the transect (i.e. 88.75 m). All SNHs within a 1 km radius of the transect centres were digitised in Arc GIS V.10.5 (ESRI, 2011), using the Lambert Azimuthal Equal Area geo-referencing system. SNH present in landscape circles were mapped using the Customer Land Database (CLAD) and in-field observations, SNH were then categorised as either herbaceous linear, herbaceous areal, woody linear, woody areal or fallow (land which has been plowed and left unseeded for at least one season). Areal habitats were defined as having a minimum width of 25 m, whereas linear habitats were between 1.5 m and 25 m wide, both areal and linear habitats were defined as having a minimum length of 50 m and minimum surface area of 75 m2. Herbaceous habitats comprised <30% shrub/tree cover and includes sown habitats, whilst woody habitats had >30% shrub/tree cover. Total SNH was calculated as the sum of these habitats. Sentinel Sampling The sentinels were deployed on two occasions in June/July 2014 and 2015. They comprised Calliphora vomitoria larvae (Diptera: Calliphoridae), Ephestia kuehniella eggs (Lepidoptera: Pyralidae), Drosophila melanogaster pupae (Diptera: Drosophilidae, one round in 2014 only), Poa trivialis seeds (Poaceae) and Chenopodium album seeds (Amaranthaceae), which were all located on the ground, and Sitobion avenae adults which were artifically attached to the crop (see below). Calliphora vomitoria larvae were pinned live to strips of 6 mm thick plastazote (1 x 10 cm), 10 per strip. To mimic Lepidoptera egg-laying, E. kuehniella egg masses were exposed on four corners (0.25 x 0.25 cm each) of dry-stick paper (1 x 2 cm; supplied by Oecos Ltd.). The percentage of eggless surface on each corner of dry-stick was then estimated in the laboratory using microscopes). For the seed sentinels, fine sandpaper (5 x 10 cm) was attached to plastazote of the same size to provide some rigidity (Westerman et al., 2003). Twenty seeds of each species were then attached to the sandpaper using M3 spray mount in two blocks with a 5 x 4 arrangement. All other sentinels were attached to dry stick card which was then coated with fine sand to allow predatory insects to walk across the surface. Ten D. melanogaster pupae were attached to dry stick card and placed on the ground. Ten live adult wingless apterate cereal aphids S. avenae were attached to dry stick card and two cards were stapled onto flag leaves at each sampling point. Natural cereal infestation by aphids was assessed on 25 tillers at each distance along one of the transects, on one occassion in 2014 and two occasions in 2015 when populations would be peaking in both years. All sentinels placed on the ground were covered with a metal cage (1 cm mesh) to prevent access by birds and rodents. Numbers of all sentinels, except for E. kuehniella, were counted when deployed (in case any were lost in transit) and on collection in the field. Assessments were made after 24 h for animal prey and 7 days for seeds. Seed sentinels were left in the field for longer than animal prey because they are not as perishable as live prey and previous studies recommend that sampling takes place after 2 – 14 days (Westerman et al. 2003). Partially or totally consumed prey items were recoded as predated.

研究区域：本研究于2014年与2015年，在英国东南部与西南部（多塞特郡（Dorset）、汉普郡（Hampshire））开展，每年选取18个冬小麦核心田块，每个核心田块外围以1km半径的景观圈划定研究范围（图1）。选取1km半径作为研究范围，是因为已有多项研究证实该尺度下农田生境可对谷物作物的生物防治产生显著影响（Rusch等，2016）。该区域属于温带气候，夏季温暖湿润（Kottek等，2006）。由于轮作制度，2014年种植冬小麦的核心田块在2015年将更换为其他作物，因此2015年无法沿用同一核心田块及其周边景观圈作为研究样地。不过，研究选取了空间邻近的景观圈组成景观圈对（landscape circle pairs），每对景观圈的中心点间距为90m至3km（平均1.15±0.05km）。
采样设计：本研究遵循标准化的QuESSA实验方案，探究三种不同田块边界类型（局域效应）对哨兵猎物捕食压力的影响。每年选取的18个核心田块中，三类边界类型各对应6个田块：1）田块间草本条带（对照组，herbaceous strip between fields）；2）木本线性生境（树篱，woody linear habitat/hedgerow）；3）木本面状生境（林地，woody areal habitat/woodland）（表A.1）。每个田块内，哨兵样点设置在距相邻半自然生境（Semi-Natural Habitat，SNH）2m、25m、48m和71m处，沿两条间距10m的样带布设（图A1）。为最小化其他田块边界对样带的干扰，样带末端与非研究田块边界的间距需至少为样带长度的1.25倍（即88.75m）。以样带中心点为圆心的1km半径范围内的所有半自然生境，均通过Arc GIS V.10.5（ESRI，2011）进行数字化处理，采用兰伯特方位等面积地理参考系统（Lambert Azimuthal Equal Area geo-referencing system）。景观圈内的半自然生境通过客户土地数据库（Customer Land Database，CLAD）与野外实地观测进行制图，随后将其划分为草本线性生境、草本面状生境、木本线性生境、木本面状生境或休耕地（fallow，即经翻耕后至少一季未播种的土地）。面状生境的最小宽度为25m，线性生境的宽度介于1.5m至25m之间；两类生境的最小长度均为50m，最小表面积均为75m²。草本生境的灌丛/乔木覆盖率低于30%，包括人工播种生境；木本生境的灌丛/乔木覆盖率高于30%。半自然生境总面积为上述各类生境的面积之和。
哨兵采样：哨兵采样分别于2014年和2015年的6-7月开展两次。所用哨兵猎物包括：全部布设至地面的红头丽蝇（Calliphora vomitoria）幼虫（双翅目：丽蝇科，Diptera: Calliphoridae）、地中海粉螟（Ephestia kuehniella）卵块（鳞翅目：螟蛾科，Lepidoptera: Pyralidae）、黑腹果蝇（Drosophila melanogaster）蛹（双翅目：果蝇科，Diptera: Drosophilidae，仅2014年设置一轮）、普通早熟禾（Poa trivialis）种子（禾本科，Poaceae）和藜（Chenopodium album）种子（苋科，Amaranthaceae），以及人工附着于作物上的麦长管蚜（Sitobion avenae）成虫（详见下文）。红头丽蝇幼虫以活体形式固定于6mm厚的聚苯乙烯泡沫条（plastazote，1×10cm）上，每条固定10头幼虫。为模拟鳞翅目昆虫产卵行为，将地中海粉螟卵块暴露于1×2cm的干卡纸（dry-stick paper，由Oecos有限公司提供）的四个角落（每个角落面积0.25×0.25cm）。随后在实验室通过显微镜估算每个角落卡纸表面未被卵覆盖的比例。对于种子类哨兵猎物，将5×10cm的细砂纸固定于同等尺寸的聚苯乙烯泡沫条上以增加硬度（Westerman等，2003）。随后将每个物种的20粒种子以5×4的排列方式分为两组，使用M3喷胶固定于砂纸上。其余所有哨兵猎物均固定于干卡纸上，并在卡纸表面涂布细沙，以便捕食性昆虫可在表面行走。将10头黑腹果蝇蛹固定于干卡纸上后放置于地面；将10头无翅活体麦长管蚜成虫固定于干卡纸上，每个采样点将两张卡纸用订书钉固定于旗叶上。在2014年开展一次、2015年开展两次蚜虫自然侵染调查：在每条样带的各采样距离处，选取25个小麦分蘖统计蚜虫数量，调查时段均选在蚜虫种群峰值期。所有放置于地面的哨兵猎物均用网孔为1cm的金属笼覆盖，以避免鸟类与啮齿类动物的取食干扰。除地中海粉螟卵块外，所有哨兵猎物在布设初期（防止运输过程中丢失）与野外回收时均统计数量。动物类哨兵猎物的回收评估在布设24小时后开展，种子类哨兵猎物则在布设7天后回收。种子类哨兵猎物的野外滞留时间长于动物类，因为其不易腐坏，且已有研究建议种子捕食的采样时长为2~14天（Westerman等，2003）。部分或完全被取食的猎物均记为被捕食。