DOI: 10.5061/dryad.x69p8czf0
title: The contribution of semi-natural habitats to biological control is dependent on sentinel prey type

description: It is widely recognized that landscape factors affect the biological
 control of weed seeds and insect pests in arable crops, but landscape
 effects have been found to be inconsistent between studies. Here, we
 compare six different types of sentinels (surrogate prey that was either
 live insects or seeds) to measure the effects of semi-natural habitats at
 field to landscape scales on levels of biological control in winter wheat
 in the UK. Sentinels were located in fields adjacent to three boundary
 types: grassy margin, hedgerows or woodland to study local scale effects
 and in landscapes of varying heterogeneity in study areas of 1 km radius.
 Overall mean levels of predation were higher for most insect prey (60.8%)
 located on the ground compared to the crop (12.2%) and was lower for seeds
 (5.8%). Predation of sentinels on the ground was attributed to generalist
 predators. Semi-natural habitats had both positive and negative effects at
 field and landscape scales, but the response varied with the sentinel
 type. Herbaceous linear semi-natural habitats had positive effects at
 local scales for Calliphora vomitoria and Sitobion avenae sentinels and
 provides evidence that farmers can introduce linear herbaceous features to
 benefit biological control. In contrast our distance weighted kernel
 models identified a positive relationship between woody habitats and the
 predation of Caliphora vomitoria and Chenopodium album. Natural aphid
 infestations were lower in landscapes with more semi-natural habitat.
 Synthesis and applications. Sentinels may be sensitive enough to detect
 variation in levels of biological control influenced by semi-natural
 habitats, but this study confirms that landscape effects differ for
 different types of sentinel prey. This implies that it may not be possible
 to categorize landscapes as pest suppressive using a single sentinel type.
 Future studies should therefore consider using multiple sentinels to give
 a better perspective on predation intensity. The resulting recommendations
 for farm management include planting woodland adjacent wheat fields
 infested with seed predators and positioning herbaceous linear habitats
 adjacent wheat fields infested with Sitobion Avenae, particularly if
 fields are bordered by woody liner habitats due to their association with
 decreased Sitobion Avenae predation.
description: <b>Study
 area</b> 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).
   <b>Sampling design</b>
 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
 m<sup>2</sup>. Herbaceous habitats comprised &lt;30%
 shrub/tree cover and includes sown habitats, whilst woody habitats had
 &gt;30% shrub/tree cover. <sup> </sup>Total SNH was
 calculated as the sum of these habitats.  
 <b>Sentinel Sampling</b> The
 sentinels were deployed on two occasions in June/July 2014 and 2015. They
 comprised <i>Calliphora
 vomitoria</i> larvae (Diptera: Calliphoridae), <i>Ephestia kuehniella</i>
 eggs (Lepidoptera: Pyralidae), <i>Drosophila melanogaster
 </i>pupae (Diptera: Drosophilidae, one round in 2014
 only)<i>,</i> <i>Poa trivialis </i>seeds (Poaceae)
 and <i>Chenopodium
 album</i> seeds (Amaranthaceae), which were all located on
 the ground,<i> </i>and<i> Sitobion avenae
 </i>adults<i> </i>which were artifically attached to the
 crop (see below). <i>Calliphora vomitoria</i> larvae were
 pinned live to strips of 6 mm thick plastazote (1 x 10 cm), 10 per strip.
 To mimic Lepidoptera egg-laying, <i>E. kuehniella</i> 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 <i>D. melanogaster</i> pupae were attached to dry
 stick card and placed on the ground. Ten live adult wingless apterate
 cereal aphids <i>S. avenae</i> 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 <i>E. kuehniella</i>, 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.