Do landscape properties mediate the response of herbivorous pest and natural enemy arthropods to local properties?

Intensification of agriculture to maximise crop production leads to disruption of ecosystem services such as of biological control of herbivore pests. Habitat management is a well-established practice at the local field scale to restore natural enemy-mediated pest suppression. Reflecting the vagile nature of arthropod pests and their natural enemies, however, it is crucial to also consider the potential impact on local habitat management strategies of surrounding landscape which may extend up to several kilometres from the focal crops. Recent research on habitat management emphasizes the inclusion of impacts of both local and landscape habitats to elucidate the ecological processes more precisely in herbivore pest management.
This thesis aimed to investigate how landscape properties affect the strength of local field scale properties in mediating the abundance of herbivore pests and their natural enemies in brassica vegetable fields located in diverse landscape types in Australia. The influence of overall landscape structure was explored by extracting data on three landscape properties: landscape composition, edge density, and landscape connectivity. The first part of this work considered the influence of landscape properties on the effects of immediately-adjacent land use on the in-field abundance of pests and natural enemies. The second part of this work explored the extent to which landscape properties influence the success of local-scale habitat management interventions with non-crop flowering plants.
Landscape properties were found to strongly influence the effects of several land-uses immediately adjacent to the focal brassica fields in enhancing natural enemy abundance and pest suppression. Relatively undisturbed or complex landscapes with higher composition and connectivity of non-crop habitats such as woody vegetation and pastures strengthened the effectiveness of similar habitats immediately adjacent to the focal field to suppress specialist brassica herbivore diamondback moth, Plutella xylostella. Landscape-scale edge density of croplands also strengthened the effectiveness of focal fields adjacent to non-host crops to suppress another specialist brassica herbivore, cabbage aphid Brevicoryne brassicae. These effects of landscape properties indicate non-natural enemy-mediated effects of these habitats in suppressing the specialist herbivores. However, the effect of field adjacent woody vegetation on enhance predatory ladybirds showed contrasting effects to different landscape properties; strengthened in crop dominated or relatively disturbed landscapes in terms of landscape connectivity and diminished in complex landscapes with high edge density of woody vegetation.
Field trials of habitat management with non-crop flowering plants suggest that such flowering resources have little effect in complex landscapes. Regardless of the presence of flowering strips, study fields in complex landscapes and with high edge density of woodlands and composition of pastures had a higher abundance of natural enemies and fewer pests, respectively.
Flowering resources enhance in-field abundance of natural enemies only in simple, crop dominated landscapes. However, the strength of benefit rapidly diminished with increasing distance from the flowers to the field interior. Responses of natural enemies to the flowering strips in crop dominated simple landscapes did not support the intermediate landscape complexity hypothesis. In Australia, very simple landscapes with greater area coverage of crop fields increased the strength of local-scale non-crop habitats such as woodlands or flower strips to enhance natural enemy population in focal fields.
Although landscape-scale woody vegetation promoted natural enemies and suppressed pests, inconsistent effects were also revealed. Promotion of in-field abundance of aphids (B. brassicae and Myzus persicae) occurred in landscapes dominated by woody vegetation.
Overall, this thesis highlighted the significance of both local and landscape scales in habitat management and the need for plans to consider both scales in order to maximize the natural enemy abundance and herbivore pest suppression in agroecosystems. Findings suggest the maintenance of resource habitats of natural enemies such as woody vegetation in close surroundings of crop fields in crop dominated simple landscapes to enhance natural enemy abundance. In a similar landscape circumstance, growers can consider the establishment of flowering strips in crop fields to obtain similar ecosystem services in absence of the field adjacent natural or semi-natural habitats. However, in complex landscapes, the establishment of such flowering strips would be redundant.