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SYMPOSIUM PAPERS

Use of Arthropod Diversity and Abundance to Evaluate Cropping Systems

^a Saskatoon Res. Cent., Agric. and Agri-Food Canada, 107 Science Place, Saskatoon, SK, Canada S7N 0X2
^b Dep. of Entomol., 333 Leon Johnson Hall, Montana State Univ., Bozeman, MT 59717-3020

* Corresponding author (olferto{at}em.agr.ca)

Received for publication January 24, 2000.

	ABSTRACT

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
Economic viability and soil degradation are major issues facing farmers in the grassland ecozone of the northern Great Plains. Management strategies such as crop diversification, reduced fallow, and reduced inputs are being promoted as solutions. However, knowledge of the impacts of these management strategies on the grassland ecozone is lacking. Studies using a systems approach, applied as the experimental framework with which to monitor and assess alternate input and cropping strategies, are being conducted through the collaboration of crop, pest, economic, and soil scientists. Five examples are presented that highlight the arthropod (insects, spiders, and mites) component of multidisciplinary studies designed to evaluate crop management strategies. They demonstrate that arthropods are the most diverse group of organisms in the ecosystems studied and include beneficial and pest species. These studies attempt to utilize the arthropod assemblages to characterize the ecosystems that they inhabit. Ecosystem-based, baseline arthropod faunas are integral to evaluating existing cropping practices and aid in the redesign of farming systems to make them economically viable and environmentally sustainable.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
MOST OF THE EARTH'S ARABLE LAND is already involved in some form of crop production to sustain world populations (Altieri, 1994). Thus, future gains in production, required to meet the demands of our increasing population, can come only from increased productivity of our current land base. The prairie ecosystem of the northern Great Plains is a major contributor to world food production. This has not come without ecological costs. Arable land of the prairie ecosystem is one of the most altered landscapes on this continent. Only small remnants of native short-grass prairie remain. As a result, society in general and agriculture in particular are being challenged to develop practices that preserve the natural processes that sustain our ecosphere.

Profitability, diminishing land resources, and land degradation are major issues facing farmers in the grassland ecozone of the northern Great Plains in the new millennium. Crop diversification, reduced fallow, and reduced inputs are being promoted in an effort to address these issues (Olfert et al., 1999a, 1999b). Producers are encouraged to diversify away from monocultures, primarily cereals, to reduce the extent of land left in fallow and to reduce inputs, especially those with the greatest negative environmental impact. However, climate and economics restrict what can be grown. Alternative production systems are needed to meet the current demands for food while preserving the land resource base for future generations to meet their needs for food.

While some agricultural practices and production systems have been evaluated for their short-term impact on sustainable production, few have been evaluated for their long-term impacts (Paoletti et al., 1993). The capacity of crop production systems to function within the ecological rules that tend to govern living systems is inherent in the concept of sustainable land management. Smyth and Dumanski (1993)(p. 7) defined sustainable land management as [that which] combines technologies, policies and activities aimed at integrating socio-economic principles with environmental concerns so as to simultaneously: (i) maintain or enhance production/services; (ii) reduce the level of production risk; (iii) protect the potential of natural resources and prevent degradation of soil and water quality; (iv) be economically viable; and (v) be socially acceptable.

Sustainable management strategies, crop loss prevention, and maintenance of soil health are central to our capacity to maintain the biological productivity of agricultural systems. Arthropods, including insects, spiders, and mites, are integral to crop loss and soil health because they include both beneficial and pest species. Cropping systems must incorporate the relationships between farm practices and the ecosystem to create an equilibrium where farm inputs enhance rather than replace natural processes.

This review will focus on two issues: minimizing preharvest yield loss and maintaining soil health relative to the diversity and abundance of arthropods in diverse cropping systems of the northern Great Plains. Studies in which the role of arthropods (insects, spiders, and mites) are being assessed in redesigning farming systems to be economically viable and environmentally sustainable are highlighted.

	PRESERVING YIELD

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
Until recently, controlling insect pests to reduce crop loss was not a particularly perplexing problem because of the wide acceptance of highly effective chemical insecticides. However, despite an estimated 2.5 million tonnes of insecticides used worldwide annually, pests destroy about 35% of all potential crops before harvest (Albert et al., 1992). Approximately 12% of potential crop loss was attributed to arthropods (Pimentel, 1986). This strongly suggests that a significant number of other factors must be considered when developing strategies to preserve crop yield.

Economic and environmental concerns about pesticide use have also stimulated a need for alternative strategies to manage pests, which include monitoring and forecasting population abundance and implementing cultural, biological, and chemical controls. These have been adopted under the banner of integrated pest management where the overall aim is to reduce pest population levels through a combination of methods that reduce the reliance on chemical insecticides. This section will explore the role of farming systems on insect population increase or decrease.

From an agronomic perspective, the major innovations in farming systems in the last decade have been crop diversification and the soil conservation practices of reduced- and zero-tillage. The extent to which these innovations have influenced, or may influence in the future, insect pest population dynamics is not fully documented. The Manitoba–North Dakota Zero Tillage Farmers Association published a manual for integrated management of insect pests in zero-tillage (Ellis, 1994). In it, Ellis lists 11 pest taxa that are of economic concern in zero-till fields; however, the manual does not document the impact that zero-tillage may have on the pest status of these insects. Included are aphids (Homoptera: Aphididae), armyworms (Lepidoptera: Noctuidae), diamondback moth (Plutella xylostella L.), European corn borer (Ostrinia nubilalis Hübner), flea beetles (Coleoptera: Chrysomelidae), grasshoppers (Orthoptera: Acrididae), wheat midge (Sitodiplosis mosellana Géhin), sunflower beetle (Zygogramma exclamationis F.), and plant bugs (Hemiptera: Miridae).

From a pest management perspective, the major factors that influence pest status of insects are weather, habitat, food, and natural enemies. Cropping systems can play a major role in all of these factors. The vegetative canopy of a crop can effectively moderate microenvironments. Daytime temperatures are lower and nighttime temperatures are higher inside a dense crop canopy compared with air temperature. Humidity tends to be higher inside a dense crop canopy in the more arid regions of the northern Great Plains. These features provide arthropods with a range of microenvironments to select from to suit the specific requirements for their optimum growth, development, and survival (Altieri, 1994). In contrast, higher humidity can contribute to increased insect pest mortality by promoting insect pathogens such as fungal disease organisms.

Crop residues, a prominent feature of reduced- and zero-tillage, can greatly influence the habitat of soil-dwelling arthropods. Winter soil temperatures are higher with increased snow cover, and soil temperatures in spring are lower due to increased soil moisture. Insect diapause, growth, and development are most often governed by temperature and moisture. For insects, when environmental conditions are stabilized between the lower and upper physiological thresholds, both pest and beneficial species are able to take advantage of the optimized conditions (Chapman, 1975).

Crop rotation is often promoted as a cultural control strategy to break the cycle of insect pest population increase. This is especially effective in instances where the insect pest of concern has very narrow feeding preferences (Anonymous, 1996). Selection of resistant crops in rotation is more of a challenge when the insect pest of concern has a broad range of suitable food plants, particularly in the northern Great Plains where climate limits the types of crops that can be grown.

Soil tillage has long been a successful management tool for pest control because tillage practices employed to manage weeds destroy alternate food plants for insect pests. In addition, soil tillage can disturb insect life stages that occur in the soil and increase mortality, predation, or both. Thus, reduced- and zero-tillage practices may increase the frequency of economic insect pest populations of some species by providing greater availability of food plants (weeds) and less soil disturbance. However, evidence suggests that there is also a corresponding increase in natural enemy populations in reduced- and zero-till systems. Stinner and House (1990) identified 30 taxa of predators that had both greater density and activity in zero-till plots.

To conclude this section on minimizing crop loss, the role of natural enemies in agroecosystem management is discussed. Natural regulation by predators and parasites is estimated to provide 5 to 10 times more control of pest species than industrially produced pesticides (Pimentel et al., 1992). As stated in the introduction, our natural habitat resources have dwindled significantly on the northern Great Plains. As a result, natural enemies well adapted to life in agroecosystems are likely to play a larger role in pest population suppression than those species associated primarily with natural ecosystems. However, the diversity of beneficial arthropods is often linked to natural habitats, so it is important that these linkages are understood and preserved (Stary and Pike, 1999).

A number of beneficial arthropods show at least some level of adaptation to agroecosystems. These include predatory Hymenoptera (ants and wasps), Coleoptera (carabid, coccinellid, and staphylinid beetles), Heteroptera (pirate, assassin, and ambush bugs), Neuroptera (lacewings), Diptera (syrphid and chamaemyiid flies) as well as mites and spiders. The major advantage of natural enemies is suppression of phytophagous insect pests at little or no cost and minimal harm to humans or the environment. A disadvantage, from a producer's perspective, is that they are better suited for long-term suppression of pest populations than as a reactive control strategy when outbreaks occur.

Much of the knowledge required to fully understand the complexities of relationships between beneficial arthropods and their habitat is still being developed. However, it is understood that cropping systems can play a significant role in the conservation of natural enemies through habitat management, crop structure, and diversity (Altieri, 1994). Habitat diversity can enhance conservation of natural enemies through increased diversity of crop canopy structures, presence of refugia (set-asides), sources of alternative prey, food plants, or other supplementary food sources.

Agricultural landscapes with a richly diverse patchwork of habitats are considered more suited to conservation of natural enemies, but habitat management is also feasible to maintain key beneficial species in specific annual cropping systems (Powell, 1986). However, in some annual crop monocultures, this may not simply be feasible.

	SOIL HEALTH

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
The desire to maintain the capacity of the soil to sustain biological productivity has positively influenced the study of soil properties that contribute to the quality and health of our prairie soils. Acton and Padbury (1993) defined soil quality in terms of measurable soil properties that influence the capacity of the soil to perform crop production or environmental functions. Doran and Safley (1997) defined soil health as a living system that has the capacity to sustain biological productivity; promote the quality of air and water environments; and maintain plant, animal, and human health.

Soil biota are often lumped into three categories, segregated by their relative size into macro-, meso-, and microfauna. Soil macrofauna include soil-inhabiting life stages of insects, spiders, snails, and earthworms. Soil mesofauna include mites, collembolans, and millipedes. Soil microfauna include organisms such as protozoa, nematodes, tardigrades, and rotifers.

Communities of arthropods that live in the soil are influenced generally by the same factors that influence those living aboveground. Species richness and the biological success of specific communities are positively related to the diversity of niches and soil microenvironments (van Straalen, 1997). As a result, the extent to which cropping diversity, rotational regimes, and soil preparation influence the diversity of microenvironments in the soil tremendously impacts arthropod populations (Pankhurst, 1997). Unfortunately, even less is known about most of the soil arthropods and the agroecological role they play compared with those living aboveground (Freckman, 1994).

Soil microfauna are implicated in a number of soil processes such as decomposition of organic matter, nutrient mineralization, microflora regulation (including plant pathogens), decomposition of agricultural chemicals, and improvement of soil structure (Gupta and Yeates, 1997). Thus, protozoa, nematodes, and rotifers have potential as indicators of soil health because they tend to respond quickly to environmental changes in the soil. The constraint with using these groups as indicators is the requirement for considerable technical expertise to identify groups or species.

Soil mesofauna (mites, millipedes, and collembolans; referred to as microarthropods by some authors) are also thought to be involved in processing organic matter and augmenting processes involved in soil structure (van Straalen, 1997). Because soil mesofauna are still relatively sedentary, they do reflect the conditions of the soil habitat more than more mobile macrofauna. Mesofauna are abundant in agricultural soils, but much more needs to be learned about their contribution to soil processes (Crossley et al., 1992). It has been reported that they are sensitive to agricultural chemical inputs and, as a result, may also have potential as biological indicators of chemical impact on the ecosystem (Koehler, 1992).

Soil macrofauna are sometimes involved in predation (spiders and ants) of pest species; however, others tend to play a role similar to mesofauna in that their diet consists of primary and secondary consumers and they process organic matter and contribute to soil structure (van Straalen, 1997; Doube and Schmidt, 1997).

	SYSTEMS APPROACH TO ASSESSING ARTHROPODS

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
Previous sections of this paper suggested that more research is required to better understand the dynamics of arthropod diversity and abundance as a function of cropping systems in the northern Great Plains. Historically, agricultural research has focused on few specific components of cropping systems, most often to determine input–output relationships and their impact on yield (Leaver, 1994). Recently, issues related to sustainable production have been in the forefront, and producers are encouraged to make management decisions by incorporating knowledge of how their agronomic practices interact with ecological processes within their ecosystem. To meet these needs, the complexity and dynamic nature of ecological processes within the agroecosystem requires a systems approach to research. Systems research attempts to study, characterize, and improve agricultural practices within the context of the biotic, abiotic, and social framework of the farming system. This section highlights five studies that have taken a systems approach to assessing cropping systems on the northern Great Plains and have incorporated arthropods as a fundamental biotic component.

Although the studies use slightly different approaches, they share five basic ingredients in that they are comprehensive in scope, producer oriented, multidisciplined, focused on issues identified by producers, and able to adapt to changing circumstances over time.

Evaluation of interactions among the biological, physical, and chemical components of agricultural systems is complex. The design, data collection, and evaluation of these studies are based on the collaborative efforts of multidisciplinary teams of crop, pest, economic, and soil scientists. The evaluations involve long-term monitoring of specific components within the agricultural system to determine rate and direction of change over time. Data from ecological communities, such as farming systems, generally do not comply with the assumptions of statistical theory that drives highly controlled experimentation. Thus, statistical analyses that identify differences and similarities in the overall response of selected variables among the treatments as well as associations among the different variables themselves are more appropriate. For example, the statistical program CANOCO (ter Braak and Smilauer, 1998) offers several analyses [e.g., partial redundancy analysis (RDA) and canonical correspondence analysis (CCA)] that can provide insights into the impact of farming systems on biological assemblages. These analyses have been successfully applied to weed communities associated with farm management systems in Saskatchewan (Leeson et al., 2000) and hold promise for analysis of soil arthropod fauna in similar systems. The objective of such analyses is to quantify the changes in arthropod communities between the year that baseline data was collected and the current year and to determine the degree to which these changes are attributable to the treatments imposed by the different farming systems. In such analyses, it would be important to take into account the initial spatial variation observed in the baseline data. Ordinations resulting from the analyses can then be used to illustrate the relative contribution of these factors to the arthropod assemblages of interest.

Study Number 1: Long-Term Alternate Cropping Study
This ongoing study is located at Scott, SK, Canada (52°22' N, 108°50' W), in the Dark Brown soil zone; the area is categorized as moist mixed grassland.

The study objective is to monitor and assess alternative input and cropping strategies based on three levels of production inputs and three levels of cropping diversity. Specifically, the study will evaluate different strategies over an 18-yr period with respect to (i) biodiversity, (ii) pest dynamics, (iii) farm profitability, (iv) soil quality, and (v) food safety. In addition to the nine cropping systems, the diversity of arthropods is being evaluated in four uncultivated areas: native prairie, a 50-yr-old grass ecosystem, a 30-yr-old alfalfa (Medicago sativa L.)–bromegrass (Bromus inermis Leyss.) ecosystem, and the grassy margins next to the study site.

The study site consists of 16 ha of farmland that has been under cultivation for about 80 yr. The study was initiated in 1994, with the entire area cropped to barley (Hordeum vulgare L.), and an extensive site characterization of physical and biological components was conducted. The experimental framework of the cropping portion of the study is based on a matrix of three levels of input use and three levels of cropping diversity. The input levels—organic, reduced, and high input—represent the main plots. The three levels of cropping diversity are assigned to subplots (15 by 40 m) within each main plot (replicated four times) to enhance detection of diversity-level differences. The levels of cropping diversity follow a 6-yr rotation and are described as low [fallow–wheat (Triticum aestivum L.)–wheat–fallow–canola–wheat], annual grains [canola–fall rye (Secale cereale L.)–pea (Pisum sativum L.)–barley–flax (Linum usitatissimum L.)–wheat], and grain–forage rotation (canola–wheat–barley–bromegrass and alfalfa–bromegrass and alfalfa–bromegrass and alfalfa). All phases of each rotation are present each year.

The native grass site has not supported any livestock for about 30 yr, but the grass stand is cut for hay every 5 to 8 yr. The old grassland site was cultivated approximately 50 yr ago and seeded back to mixed grass shortly thereafter. It also has been managed similarly to the native grass site for the last 30 yr. The alfalfa–bromegrass site was cultivated and seeded to an alfalfa–bromegrass mix about 30 yr ago and is harvested annually. The grass area in the fence line adjacent to the crop study site is about 5 m wide and is primarily crested wheatgrass [Agropyron cristatum (L.) Gaertn.]. The field plot margins were seeded about 40 yr ago and are mowed several times during the summer months.

Progress to Date
A baseline data set containing density and diversity of arthropods found in the crop and in and on the soil was compiled in 1994. This included assessment of crop-inhabiting arthropods (sweep samples), soil-borne arthropods (pitfall traps), and soil-inhabiting arthropods (soil cores). Preliminary results of 1997 soil assessments indicate that the reduced-input system has significantly higher numbers of some soil microfauna (nematodes, rotifers, and protozoa) as well as some mesofauna (Oribatida, Actinedida, and Gamasida mites) compared with the baseline data. These results will continue to be monitored to determine their value in assessing soil health. Processing of the pitfall traps indicated that Collembola were the most numerous insect order, followed by Coleoptera, Hymenoptera, and Homoptera. To date, insect control measures have been restricted to high-input plots where insecticidal dressings were applied to canola seed to protect the emerging crop from flea beetles (Phyllotreta spp). Future plans are to assess the direction and rate of change over time that is occurring in these components as a function of the different cropping systems (treatments). Evaluations will be conducted on a cyclical basis (6 yr). The design, data collection, and evaluation are based on the collaborative efforts of crop, pest, economic, and soil scientists (Frick, 1995–1998). These studies are intended to guide development of sustainable cropping systems that provide a stable food supply, do not increase inputs of nonrenewable resources, and preserve soil and environmental quality while maintaining or improving cropping potential.

Study Number 2: Correlates of Biodiversity on Prairie Farmland
This study was initiated in 1996 and is being conducted in the Aspen Parkland–Boreal Transition ecoregion of Saskatchewan. A set of 12 cluster sites was identified within the ecoregion, and each cluster contains an organic farm, a conventional farm, a conservation tillage farm, and a wildlife habitat site geographically close to one another. Each component of the cluster also has a permanent wetland associated with the farm or wildlife site.

The study objectives are to assess the composition and productivity of selected groups of organisms within conventional, conservation, and organic farming systems relative to wildlife habitat.

The multidisciplinary team involves entomologists, weed scientists, wildlife specialists, water quality chemists, and agronomists. The study was designed to assess a number of biotic and abiotic variables, including water chemistry in wetlands, vegetation around wetlands, weeds in the cropland, invertebrates in wetlands and in margins of wetlands and cropland, amphibians around wetlands, and avian fauna in wetlands and in uplands.

Progress to Date
A preliminary synopsis indicates that differences in land use have had an impact on the variable measured in the study (Thomas et al., 1999). Wildlife areas tend to have a richer plant diversity, a distinct arthropod community, and a higher density and diversity of bird life than farms. Differences in arthropod and bird communities among farm types were not as apparent. Plant and arthropod diversity did not differ among farm types nor did frog abundance. However, there was a trend toward a greater density and diversity of upland birds in conservation tillage farms relative to conventional or organic farms. Because arthropods are a major source of food for upland birds, predation of pest species will contribute to preservation of crop yield in conservation tillage systems. These are issues that are going to require further study to enhance our understanding of the effects of farm management practices on biodiversity.

Study Number 3: Predatory Insects in Prairie Farming Systems
This study was also based on commercial farms. Eight fields in Saskatchewan were sampled three times during the summer (June, July, and August), and the study was replicated over 3 yr (1994–1996).

The objective was to examine effects of cropping systems and agrochemical input systems on the predatory insect fauna in four Saskatchewan farming systems. Two cropping systems (wheat–wheat–fallow and diversified grain forage) and two input systems (high level and organic) were assessed. Scientific expertise included entomologists, weed scientists, soil scientists, agronomists, and economists. Pitfall traps were used to collect the ground-dwelling predatory insects, and sweep nets were used to collect the predatory insects present in the crop foliage.

Progress to Date
A total of 38 genera of ground-dwelling predators and eight genera of foliar-dwelling predators were collected. The collections were dominated by five genera of Carabidae (ground-dwelling predators) and two genera of foliar predators (Anthocoridae and Nabidae) (Melnychuk, 1999).

Analysis of abundance, richness, and dominance of the two predator communities revealed that cropping system had the largest impact on the abundance and diversity of the predatory insect fauna. The least diverse rotation (wheat–wheat–fallow system) had a significantly higher abundance and diversity of ground-dwelling predators while the more diverse rotation (diversified grain forage system) had a higher abundance and diversity of foliar-dwelling predators (Melnychuk, 1999). The results suggest that the extent to which the two predator guilds will contribute to preserving crop yield will depend somewhat on the attributes of the pest complex. Because no consistent differences in insect abundance and diversity were observed between the two input systems, more specific identification of the specimens may be required. A comprehensive synthesis incorporating all components of the different farming systems is underway.

Study Number 4: Influence of Fragmented Grasslands on Arthropod Diversity
The study, conducted in 1995, included sampling seven pastures of native mixed-grass prairie in southwestern Saskatchewan ranging from small (7 ha) to large (17800 ha). All pastures had relatively low populations of nonnative vegetation within a predominantly cereal crop–based agricultural matrix.

The loss and fragmentation of natural grasslands on the prairies has created a patchwork of grassed areas surrounded by cropland. The objectives were to quantify the patterns of distribution and abundance of arthropods, notably beetles and spiders, in the fragmented native grasslands of southwestern Saskatchewan and to evaluate the impact of replacing the natural disturbance regime with a human disturbance regime in the prairie region of Saskatchewan. Beetles and spiders were targeted because of the availability of taxonomic expertise for these groups. Arthropods were collected using pitfall traps and identified to species level. Diversity was calculated as a function of species richness and abundance (Pepper, 1999).

Progress to Date
Species richness of both spiders and beetles were positively correlated with the size of the grassland (Pepper, 1999). Pepper also showed that there was a high degree of rarity for both beetles and spiders; only two spider species and five beetle species were found on all seven grassland areas. The results have provided baseline data on ground-dwelling arthropods in native grasslands and illustrate the degree of impact that fragmentation of grasslands has had on arthropods. Further research is required to quantify the extent to which cropping diversity and farming practices can ameliorate the decline in prairie arthropod fauna and to quantify the positive attributes of a rich fauna of beneficial arthropods (spiders and beetles) in relation to preserving crop yield.

Study Number 5: Sustainable Pest Management in Dryland Wheat
This project is being conducted in three geographically distinct regions in Montana. Site 1 is in north-central Montana and is a 20-ha parcel that was in the Conservation Reserve Program from 1988–1998. Site 2 is located in central Montana and is 14 ha of cropland that has been in small-grain production for 30 yr. Site 3 is located in northeast Montana at the USDA-ARS Research Farm near Froid. These sites differ climatologically and agronomically from one another yet represent a significant agricultural district where they are located.

The objectives of this project are tailored to fit the agricultural production systems commonly practiced in each farming district. Specifically, diverse crop rotations under different tillage systems and/or levels of input will be evaluated over a 12- to 16-yr period with respect to (i) physical and biological properties of soil, (ii) weed species composition, (iii) presence and impact of plant pathogens, (iv) abundance and diversity of arthropods, (v) economic profitability, and (vi) environmental benefits.

The cropping diversity systems include (i) baseline sites consisting of Conservation Reserve Program and cropped land in continuous spring wheat; (ii) a 2-yr rotation characterized by traditional wheat–fallow–wheat with a cool pulse (pea)–spring wheat; (iii) a 3-yr rotation that includes a diversity of cereal, warm or cool oilseeds, and pulse crops; and (iv) a 4-yr rotation including grain, oilseed, and N-fixing perennial legume, with the latter harvested for seed or forage or used as green manure.

Arthropod density and distribution are monitored using a variety of sampling techniques. These include sweep samples, yellow sticky traps, species-specific pheromone traps, pitfall traps, and soil cores.

Progress to Date
Baseline results from the first year (1998) indicate that significant differences in pest numbers existed among the various crops. Overall, sweep samples from yellow mustard (Sinapis alba L.) had the largest number of pests, including flea beetles and diamondback moths while samples from the Conservation Reserve Program and lentil (Lens culinaris Medikus) samples had high densities of root maggots (Anthomyiidae) and leafhoppers (Cicadellidae). In addition, plant bugs, aphids, and leaf-miners (Agromyzidae) were prevalent in most crops. The vast majority of beneficial insects were small parasitic wasps representing a variety of hymenopteran families, including a polyembryonic encyrtid, which often dominated the samples.

In 1999, most of the crops were in spring wheat. As a result, the differences between destructive and beneficial insect biota in spring wheat following various crops could be assessed. To date, results show relatively insignificant differences. However, spring wheat following both fallow and pea appears to have the highest number of pest species. These results will continue to be monitored to determine their impact on crop yield in subsequent years. At this stage in the study, there appears to be minimal differences in the numbers of pest and beneficial insects between till and no-till management regimes. However, the study is not yet 2 yr old, and a comprehensive statistical analysis of at least one full 4-yr rotation will provide a much greater understanding of the diversity and distribution of arthropods within these cropping systems.

	CONCLUDING COMMENTS

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 
Much of the knowledge required to fully understand the complexities of relationships between beneficial arthropods and their habitat is still being developed for the northern Great Plains. However, it is understood that cropping systems can play a positive role in conservation of natural enemies through habitat management, crop structure, and diversity. Management strategies for control of insect pests have broadened into the concept of ecological pest management and are no longer focused only on the pest species complex. Broader ecological issues at the landscape level, such as the impact of cropping diversity on insect population abundance and species richness, are now being considered. Norris and Kogan (2000) present a thorough review of the many interactions among weeds, insect pests and their natural enemies, and host plants in managed ecosystems. Because interactions among the biological, physical, and chemical components of agricultural systems is inherently complex, the experiment design, data collection, and evaluation of the findings depends on the collaborative efforts of a multidisciplinary team of crop, pest, economic, and soil scientists. In addition, such evaluations often involve a long-term commitment to monitoring specific components within the agricultural system to determine rate and direction of change over time. Arthropods are well suited for characterizing the ecosystems that they inhabit. Ecosystem-based baselines of arthropod diversity and abundance are an integral component in evaluating farming systems and will contribute to our understanding of the impact of cropping systems on sustainability issues.

	REFERENCES

TOP ABSTRACT INTRODUCTION PRESERVING YIELD SOIL HEALTH SYSTEMS APPROACH TO ASSESSING... CONCLUDING COMMENTS REFERENCES

 

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