Nancy A. Schellhorn

Movement of Entomophagous Arthropods in Agricultural Landscapes: Links to Pest Suppression

Entomophagous arthropods can provide valuable biological control services, but they need to fulfill their life cycle in agricultural landscapes often dominated by ephemeral and disturbed habitats. In this environment, movement is critical to escape from disturbances and to find resources scattered in space and time. Despite considerable research effort in documenting species movement and spatial distribution patterns, the quantification of arthropod movement has been hampered by their small size and the variety of modes of movement that can result in redistribution at different spatial scales. In addition, insight into how movement influences in-field population processes and the associated biocontrol services is limited because emigration and immigration are often confounded with local-scale population processes. More detailed measurements of the habitat functionality and movement processes are needed to better understand the interactions between species movement traits, disturbances, the landscape context, and the potential for entomophagous arthropods to suppress economically important pests.

Forest Biodiversity and the Delivery of Ecosystem Goods and Services: Translating Science into Policy

Biodiversity is integral to almost all ecosystem processes, with some species playing key functional roles that are essential for maintaining the value of ecosystems to humans. However, many ecosystem services remain nonvalued, and decisionmakers rarely consider biodiversity in policy development, in part because the relationships between biodiversity and the provision of ecosystem services are not generally appreciated. To date, the majority of work in which the functional importance of biodiversity has been examined has been conducted in relatively species-poor systems. Focusing on forest and agroforest systems, we synthesize recent research on the role of biodiversity in the provision of ecosystem services and provide examples of biodiversity science that informs ecosystem management and policy. Finally, we highlight barriers to the transfer of knowledge from scientists to decisionmakers and suggest that scientists can be much more effective at informing policy and improving resource management by asking policy-relevant questions and providing timely and consistent information to decisionmakers and the public on the linkages among biodiversity, ecosystem services, and their value to people.

Managing ecosystem services in broadacre landscapes: what are the appropriate spatial scales?

Over the past 200 years agriculture has expanded throughout Australia. The culmination of clearing and cultivating land at the farm scale has resulted in highly modified landscapes and a perceived loss of ecosystem services from pest control and pollination. We examine the literature: (i) to identify the appropriate spatial scale for managing pests, natural enemies and pollinators; and (ii) for evidence that farm-scale changes (due to agricultural intensification) across a landscape have resulted in a tipping point favouring pests and hindering pollinators. Although there is limited information to draw firm conclusions, the evidence suggests that actions undertaken on individual farms have an impact both on their neighbours and regionally, and that the culmination of these actions can lead to changes in population dynamics of pests, natural enemies and pollinators. For major pest species, there is reasonable evidence that grain growers may benefit from improved management and higher yields by implementing area-wide pest management strategies on a landscape scale in collaboration with growers of other crops that also share these pests. As yet, for natural enemies and pollinators there is little direct evidence that similar area-wide initiatives will have a greater effect than management strategies aimed at the field and farm level. Managing pests, natural enemies and pollinators beyond the scale of the field or farm is technically and socially challenging and will required a well defined research agenda, as well as compromise, balance and trading among stakeholders. We highlight critical knowledge gaps and suggest approaches for designing and managing landscapes for ecosystem services.

Spatial variability in ecosystem services: simple rules for predator-mediated pest suppression

Agricultural pest control often relies on the ecosystem services provided by the predators of pests. Appropriate landscape and habitat management for pest control services requires an understanding of insect dispersal abilities and the spatial arrangement of source habitats for pests and their predators. Here we explore how dispersal and habitat configuration determine the locations where management actions are likely to have the biggest impact on natural pest control. The study focuses on the early colonization phase before predator reproduction takes place and when pest populations in crops are still relatively low. We developed a spatially explicit simulation model in which pest populations grow exponentially in pest patches and predators disperse across the landscape from predator patches. We generated 1000 computer-simulated landscapes in which the performance of four typical but different predator groups as biological control agents was evaluated. Predator groups represented trait combinations of poor and good dispersal ability and density-independent and density-dependent aggregation responses toward pests. Case studies from the literature were used to inform the parameterization of predator groups. Landscapes with a small nearest-neighbor distance between pest and predator patches had the lowest mean pest density at the landscape scale for all predator groups, but there can be high variation in pest density between the patches within these landscapes. Mobile and strongly aggregating predators provide the best pest suppression in the majority of landscape types. Ironically, this result is true except in landscapes with small nearest-neighbor distances between pest and predator patches. The pest control potential of mobile predators can best be explained by the mean distance between a pest patch and all predator patches in the landscape, whereas for poorly dispersing predators the distance between a pest patch and the nearest predator patch is the best explanatory variable. In conclusion, the spatial arrangement of source habitats for natural enemies of agricultural pest species can have profound effects on their potential to colonize crops and suppress pest populations.

Time will tell: resource continuity bolsters ecosystem services

A common suggestion to support ecosystem services to agriculture provided by mobile organisms is to increase the amount of natural and seminatural habitat in the landscape. This might, however, be inefficient, and demands for agricultural products limit the feasibility of converting arable land into natural habitat. To develop more targeted means to promote ecosystem services, we need a solid understanding of the limitations to population growth for service-providing organisms. We propose a research agenda that identifies resource bottlenecks and interruptions over time to key beneficial organisms, emphasising their resulting population dynamics. Targeted measures that secure the continuity of resources throughout the life cycle of service-providing organisms are likely to effectively increase the stock, flow, and stability of ecosystem services.

Habitat functionality for the ecosystem service of pest control: reproduction and feeding sites of pests and natural enemies

1 Landscape management for enhanced natural pest control requires knowledge of the ecological function of the habitats present in the landscape mosaic. However, little is known about which habitat types in agricultural landscapes function as reproduction habitats for arthropod pests and predators during different times of the year. 2 We studied the arthropod assemblage on six crops and on the seven most abundant native plant species in two landscapes over 1 year in Australia. Densities of immature and adult stages of pests and their predators were assessed using beat sheet sampling. 3 The native plants supported a significantly different arthropod assemblage than crops. Native plants had higher predator densities than crops over the course of the year, whereas crops supported higher pest densities than the native plants in two out of four seasonal sampling periods. Crops had higher densities of immature stages of pests than native plants in three of four seasonal sampling periods, implying that crops are more strongly associated with pest reproduction than native plants. Densities of immature predators, excluding spiders, were not different between native plants and crops. Spiders were, however, generally abundant and densities were higher on native plants than on crops but, because some species disperse when immature, there is less certainty in identifying their reproduction habitat. 4 Because the predator to pest ratio on native plant species showed little variation, and spatial variation in arthropod assemblages was limited, the predator support function of native vegetation may be a general phenomenon. Incentives that maintain and restore native remnant vegetation can increase the predator to pest ratio at the landscape scale, which could enhance pest suppression in crops.

Connecting scales: achieving in-field pest control from areawide and landscape ecology studies.

Areawide management has a long history of achieving solutions that target pests, however, there has been little focus on the areawide management of arthropod natural enemies. Landscape ecology studies that show a positive relationship between natural enemy abundance and habitat diversity demonstrate landscape‐dependent pest suppression, but have not yet clearly linked their findings to pest management or to the suite of pests associated with crops that require control. Instead the focus has often been on model systems of single pest species and their natural enemies. We suggest that management actions to capture pest control from natural enemies may be forth coming if: (i) the suite of response and predictor variables focus on pest complexes and specific management actions; (ii) the contribution of “the landscape” is identified by assessing the timing and numbers of natural enemies immigrating and emigrating to and from the target crop, as well as pests; and (iii) pest control thresholds aligned with crop development stages are the benchmark to measure impact of natural enemies on pests, in turn allowing for comparison between study regions, and generalizations. To achieve pest control we will need to incorporate what has been learned from an ecological understanding of model pest and natural enemy systems and integrate areawide landscape management with in‐field pest management.

Interactions between conventional and organic farming for biocontrol services across the landscape

While the area of organic crop production increases at a global scale, the potential interactions between pest management in organic and conventionally managed systems have so far received little attention. Here, we evaluate the landscape-level codependence of insecticide-based and natural enemy-based pest management using a simulation model for parasitoid-host interactions in landscapes consisting of conventionally and organically managed fields. In our simulations conventional management consists of broad-spectrum or selective insecticide application, while organic management involves no insecticides. Simulations indicate that insecticide use can easily result in lose-lose scenarios whereby both organically and conventionally managed fields suffer from increased pest loads as compared to a scenario where no insecticides are used, but that under some conditions insecticide use can be compatible with biocontrol. Simulations also suggest that the pathway to achieve the insecticide reduction without triggering additional pest pressure is not straightforward, because increasing the proportion of organically managed fields or reducing the spray frequency in conventional fields can potentially give rise to dramatic increases in pest load. The disruptive effect of insecticide use, however, can be mitigated by spatially clustering organic fields and using selective insecticides, although the effectiveness of this mitigation depends on the behavioral traits of the biocontrol agents. Poorly dispersing parasitoids and parasitoids with high attack rates required a lower amount of organically managed fields for effective pest suppression. Our findings show that the transition from a landscape dominated by conventionally managed crops to organic management has potential pitfalls; intermediate levels of organic management may lead to higher pest burdens than either low or high adoption of organic management.

Propagating sinks, ephemeral sources and percolating mosaics: conservation in landscapes

We present a framework that uses both sources and sinks as elements in the construction of a landscape matrix. We propose that the matrix be conceived as a collection of temporary habitats, some of which are sources, others of which are sinks, that form a landscape mosaic. The key element in this framing is that the sources are ephemeral and the sinks are propagating. A mean field approach is used to modify the classic metapopulation model, taking this new framework into account. Additionally a spatially explicit approach reveals different scaling rules for the percolation probability and the propagating probability.

A perspective on management of Helicoverpa armigera: transgenic Bt cotton, IPM, and landscapes

Helicoverpa armigera is a major pest of agriculture, horticulture and floriculture throughout the Old World and recently invaded parts of the New World. We overview of the evolution in thinking about the application of area-wide approaches to assist with its control by the Australian Cotton Industry to highlight important lessons and future challenges to achieving the same in the New World. An over-reliance of broad-spectrum insecticides led to Helicoverpa spp. in Australian cotton rapidly became resistant to DDT, synthetic pyrethroids, organophosphates, carbamates and endosulfan. Voluntary strategies were developed to slow the development of insecticide resistance, which included rotating chemistries and basing spray decisions on thresholds. Despite adoption of these practices, insecticide resistance continued to develop until the introduction of genetically modified cotton provided a platform for augmenting Integrated Pest Management in the Australian cotton industry. Compliance with mandatory resistance management plans for Bt cotton necessitated a shift from pest control at the level of individual fields or farms towards a coordinated area-wide landscape approach. Our take-home message for control of H. armigera is that resistance management is essential in genetically modified crops and must be season long and area-wide to be effective.