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

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.