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Symposium Papers

Pest Management Implications of Reduced Fallow Periods in Dryland Cropping Systems in the Great Plains

^a Dep. of Bioagric. Sci. and Pest Manage., Colorado State Univ., Fort Collins, CO 80523-1177
^b Texas A&M Res. and Ext. Cent., Amarillo, TX 79106
^c Agric. and Agri-Food Canada, Saskatoon Res. Cent., Saskatoon, SK S7N 0X2, Canada

^* Corresponding author (frank.peairs{at}colostate.edu)

Received for publication January 22, 2004.

	ABSTRACT

TOP ABSTRACT INTRODUCTION PEST MANAGEMENT IN DRYLAND... PESTS FAVORED BY CONTINUOUS... PESTS DISADVANTAGED BY... PEST MANAGEMENT CHANGES UNDER... OVERALL ASSESSMENT REFERENCES

 
The intensification of traditional wheat (Triticum aestivum L.)–fallow production systems may have important consequences for management of insects, pathogens, and weeds in Great Plains dryland production systems. Assessment of these consequences is difficult due to the diversity of production systems, environmental conditions, and pests found in the region. Certain pest groups, such as weeds, traditionally controlled during the fallow period, may be favored by intensified cropping while others, such as those specializing on wheat, should be disadvantaged. Changes in pest and disease complexes will likely be evolutionary rather than revolutionary, as has been the case with other significant changes in production practices. Preventive practices in dryland production systems currently emphasize the control of grassy weeds while intensified systems may have less emphasis on the control of volunteer wheat. Crop rotation will remain a key avoidance strategy for pathogens and will help broaden herbicide options. Pest monitoring provides essential information on pest activity and environmental conditions and will become more complex as production systems are intensified. Important suppressive practices for dryland production systems include conservation biological control, tillage, and chemical controls. Chemical control, in particular, is expected to become more complicated due to drift concerns, rotational restrictions, the possible need for herbicide-tolerant crops, and the development of weed populations resistant to glyphosate. Pest management requirements should be considered during cropping system design and establishment.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION PEST MANAGEMENT IN DRYLAND... PESTS FAVORED BY CONTINUOUS... PESTS DISADVANTAGED BY... PEST MANAGEMENT CHANGES UNDER... OVERALL ASSESSMENT REFERENCES

 
THE INTENSIFICATION of traditional wheat–fallow production systems is accomplished through reductions in the length of the fallow period, perhaps even to the point of its complete elimination. In the Great Plains, this intensification generally involves replacement of some of the fallow period with a variety of crops, ranging from sorghum [Sorghum bicolor (L.) Moench] and cotton (Gossypium hirsutum L.) in the south to oilseed and pulse crops in the north. Such changes may have important pest management consequences. Our purpose is to review pest management as currently practiced for dryland production systems in the Great Plains region, identify pests favored and disadvantaged by the fallow period, and consider likely changes in pest management practices due to reductions in the fallow period.

This assessment is made difficult by the diversity of the dryland production systems employed across this large region. In an attempt at simplification, the main focus of our discussion will be wheat, which is a dominant crop in most of these production systems and has similar pest management approaches throughout the region. We also assume the widespread implementation of reduced tillage or no-till, which is a factor that can have important effects on pest activity (Stinner and House, 1990).

An additional source of complexity is regional variation in pest status. For example, Russian wheat aphid, Diuraphis noxia (Mordvilko), is a rare pest of wheat in the fall and a sporadic pest in the spring in the Southern Plains. In the Central Plains, it is a rare pest in the fall and a key pest in spring while in the Northern Plains, it is sporadic in the fall and rare in the spring. These differences are due in large part to climate (Elliott et al., 1998). Climate and weather often exert greater influence on pest activity than can be achieved with available pest management practices. For example, differences in disease severity on wheat and lentil (Lens culinaris Medik. subsp. culinaris) among years at a site in Saskatchewan were much greater than differences associated with tillage treatment or crop rotation (Bailey et al., 2001) and much greater than the impact of crop rotation on root rot severity of wheat in Washington State (Cook et al., 2002).

The development of pest management practices for dryland cropping systems was reviewed by Holtzer et al. (1996). They recommended an emphasis on low or no-cost pest management tactics, which could be designed into the production system rather than added later as pest problems arose. Given the complexity of this approach, they called for long-term, large-scale research at the cropping system level. Such research would be multidisciplinary (e.g., Olfert et al., 2002; Thomas and Brandt, 2001) and generally would involve producers from the beginning.

In addition, it may be that some aspects of research and implementation of pest management for dryland cropping systems will need to be conducted at the landscape level. This is because many important sources of arthropod pests and pathogens, and, to a lesser extent weeds, are external to the field being managed. Of particular interest are field boundary areas, that is, any margin of an agricultural field including cropped or noncrop areas, native plants, fence rows, and roadsides. Proposed regulations may require these areas to serve as pesticide-free buffers, which would enhance their importance as sources of pests, pathogens, and natural enemies for the crops they border (Bailey and Gossen, 2005).

	PEST MANAGEMENT IN DRYLAND AGROECOSYSTEMS

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Each pest discipline has developed unique terminology to categorize pest management activities. A recent proposal (Anonymous, 2004) has suggested use of unified terminology to categorize pest management practices across entomology, plant pathology, and weed science: prevention, avoidance, monitoring, and suppression.

Prevention encompasses those practices intended to prevent the introduction of pests into a field as well as avoiding production practices that favor the development of pest populations or disease outbreak. Preventive practices in dryland production systems emphasize the control of grassy weeds, which serve as important sources of a variety of arthropod pests and pathogens of wheat. Elimination of volunteer wheat can be of particular importance if its temporal occurrence is such that it might serve as a "green bridge" for the mite and aphid vectors of such important viral diseases as barley yellow dwarf and wheat streak mosaic. Another important preventive consideration in dryland production systems is the use of clean seed free of pathogens such as Claviceps purpurea (Fr.:Fr.) Tul. (ergot), Fusarium graminearum Schwabe (Fusarium head blight), or Tilletia tritici (Berk.) G. Wint. in Rabenh. (bunt) and weeds such as cereal rye (Secale cereale L.) and jointed goatgrass (Aegilops cylindrica Host).

Avoidance occurs when pests are present in the field and crop management practices are used to avoid the development of significant population densities, disease intensity, or crop damage. Crop rotation is a key avoidance strategy for pathogens, particularly those that are residue borne (Krupinsky et al., 2002). This approach often has the additional advantage of broadening herbicide options. Crop rotation is of limited use against arthropod pests, with the important exception of narrow-host-range, limited-mobility soil pests such as the Diabrotica spp. rootworms in corn (Zea mays L.) (Chiang, 1973). Another important avoidance strategy is the use of resistant cultivars, which are deployed against a variety of pathogens and to a lesser extent against arthropod pests, such as greenbug (Schizaphis graminum Rondani) in grain sorghum and Hessian fly (Mayetiola destructor Say) in winter wheat. However, the occurrence of pathogen host races and arthropod biotypes can limit this approach (Bailey et al., 2003; Berzonsky et al., 2003; Haley et al., 2004; Porter et al., 1997; Su et al., 2003). The availability of herbicide-tolerant cultivars is a new aspect to be considered in this area. The modification of planting dates is a third avoidance strategy, often used to minimize synchrony between pests or pathogens and susceptible crop growth stages. For instance, delayed planting can be used to avoid aphid virus vectors (Halbert et al., 1990) or Hessian fly (Cartwright and Jones, 1953) in wheat. Manipulation of planting dates also may be used to enhance the competitive advantage of a crop over weeds or to facilitate the use of herbicides (Holtzer et al., 1996).

The increased plant diversity often associated with reduced fallow periods also may promote pest avoidance, which has been an area of interest to agricultural ecologists (Altieri and Nicholls, 2004). While the influence of increased crop diversity on pest abundance is variable, there are sufficient positive observations reported to document the potential pest management benefits. One explanation for the reduction of pest activity in more diverse cropping systems is the increased abundance of natural enemies due to increased availability of alternative prey and other resources (Sheehan, 1986; Smith and McSorley, 2000). Vandermeer (1989) proposed that the presence of a second crop could affect pest abundance on the primary crop by disrupting the pest's ability to find its host or by attracting the pest away from the primary crop to the second crop.

Many of the observations in this field come from studies of intercropping while the increased crop diversity associated with reduced fallow periods in the Great Plains is generally accomplished by intensified crop rotations. Brewer and Elliott (2004) reviewed the influence of habitat manipulation, including crop diversification, on natural enemies of cereal aphids, with emphasis on the Great Plains region. They concluded that while increased plant diversity in the Great Plains would be best achieved through intensified crop rotations, there was little documentation of the insect pest management benefits provided by such increases. Given that pest movement is often at a landscape scale rather than a within-field scale, such rotations may have similar effects. Parasitism of Russian wheat aphid by Diaeretiella rapae McIntosh and Aphelinus albipodus Hayat & Fatima was greater in wheat grown in rotation with sunflower (Helianthus annuus L.) than in a wheat–fallow rotation in southeast Wyoming and north-central Colorado (Ahern and Brewer, 2002). In a Colorado study, Capinera et al. (1985) compared irrigated sweet corn and dry bean (Phaseolus vulgaris L.) strip intercrops varying in width from 1 to 16 rows, with the aim of assessing the potential for realizing the pest management benefits of intercropping in mechanized production systems. Intercropping influences were observed in strips as wide as eight rows, with specialist pests more affected than generalists. Natural enemies were not affected by crop strip width in this study although natural enemies are commonly more active in field edges (Altieri and Nicholls, 1999) and thus would be expected to be more abundant in narrower-strip configurations.

Monitoring provides essential information on pest activity and environmental conditions. Such information forms the basis for suppression attempts. Of particular importance are those types of information that allow the use of economic thresholds and other treatment decision support tools. The cost of acquiring such information has been problematic, particularly in low-value dryland production systems. Much effort has gone into cost-effective methods for pest monitoring. For example, commercially available pheromone lures contain species-specific attractants that can be used to trap adult males of such important wheat pests as army cutworm (Euxoa auxiliaris Grote) and pale western cutworm (Agrotis orthogonia Morrison) (Metcalf and Metcalf, 1993). Sequential sampling plans for a variety of wheat pests, including greenbug (Giles et al., 2000) and Russian wheat aphid (Legg et al., 1994), also have been developed. Regional cereal aphid monitoring has been accomplished using a network of inexpensive suction traps (Halbert et al., 1990). Remote sensing technology for numerous arthropod pests, diseases, and weeds of dryland crops is under development. In addition, networks of automated weather stations are being developed to make environmental data that is relevant to individual growers or individual fields more available. Diligence will be required to monitor fields so that new weed infestations can be eliminated quickly. Annual grasses and weeds should be controlled before their producing seed. Perennial weeds tend to increase under conservation tillage systems and should be controlled with appropriate herbicides before they become established over significant acreage.

Suppression entails cultural, physical, biological, and chemical controls used to reduce pest activity if prevention and avoidance tactics are unsuccessful. Important suppressive practices for dryland production systems include conservation biological control, i.e., maximizing the effect of existing natural enemies, and a variety of cultural practices, including tillage. Chemical controls should be used only as a last resort and only when their cost effectiveness is assured. However, most of our experience with intensified dryland cropping systems indicates that the use of herbicides is essential under most conditions. For example, in the Texas High Plains, herbicides are essential under most conditions to prevent weeds from using valuable soil moisture before planting. For example, in the Texas High Plains, for every centimeter of water used by uncontrolled weeds, sorghum yield could be increased 89 kg ha^–1 (Unger and Wiese, 1979)

	PESTS FAVORED BY CONTINUOUS CROPPING

TOP ABSTRACT INTRODUCTION PEST MANAGEMENT IN DRYLAND... PESTS FAVORED BY CONTINUOUS... PESTS DISADVANTAGED BY... PEST MANAGEMENT CHANGES UNDER... OVERALL ASSESSMENT REFERENCES

 
Insects and pathogens favored by continuous cropping share several features. Local overwintering tends to favor colonization of nearby crops. Pests that survive the winter in this manner tend to be strongly affected by spatial and temporal arrangements that increase the likelihood of nearby suitable hosts. Pests and pathogens that immigrate from southern overwintering sites tend to colonize across the landscape and are much less affected by cropping systems. In the same manner, pests and pathogens that feature limited dispersal within the growing season would be favored by the stable resources offered by continuous cropping, particularly if they have a broad host range that allows them to exploit several of the crops used in a given cropping system. For example, insect pests that are generalized grass feeders are favored in the Great Plains region because of the predominance of cereal crops.

Pale western cutworm exemplifies this group of pests. Eggs are laid in the fall and hatch early in the spring. Larvae are thus present in the field when spring crops are sown and will remain there until maturity. Pale western cutworm also has a broad host range and is capable of damaging a variety of grass and broadleaf crops (Peairs, 2004). Similarly, Fusarium head blight, caused (predominantly) by F. graminearum, overwinters in plant residue and has a broad host range that includes many crop species (Leonard and Bushnell, 2003). Spores of the pathogen are produced on infected residue. Where susceptible crops are grown in sequence in a field (or adjacent fields), head blight can increase rapidly when weather conditions are conducive for epidemic development, resulting in substantial losses in both yield and quality.

Weed population shifts would be expected to occur in continuous cropping systems because historically, the fallow period has been used to control certain weeds that are difficult to control in-crop (Lyon and Baltensperger, 1995; Dao, 1987; Blackshaw et al., 1994). Johnsongrass [Sorghum halepense (L.) Pers.] can be a major pest in sorghum in the Southern Plains, and no herbicides are available for its selective control. The jointed goatgrass seedbank in wheat can be greatly reduced if this weed is controlled during the fallow period.

The major weed problems of the Great Plains are listed in Table 1. The extent to which these would be expected to increase will largely depend on the crop rotation used in the continuous cropping system.

	PESTS DISADVANTAGED BY CONTINUOUS CROPPING

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	PEST MANAGEMENT CHANGES UNDER CONTINUOUS CROPPING

TOP ABSTRACT INTRODUCTION PEST MANAGEMENT IN DRYLAND... PESTS FAVORED BY CONTINUOUS... PESTS DISADVANTAGED BY... PEST MANAGEMENT CHANGES UNDER... OVERALL ASSESSMENT REFERENCES

 
Pest management practices are expected to change in response to the potential shifts in the pest, pathogen, and weed complex due to the reduction or elimination of the fallow period. Changes can be expected in prevention, avoidance, monitoring, and suppression practices.

Prevention practices will likely have reduced emphasis on or need for volunteer wheat management since this is one weed problem that should be reduced by the trend toward continuous cropping. It also will be important to avoid the many pest management disadvantages of monoculture in continuous cropping. Clean, pest-free seed will remain an essential preventive practice for both diseases and weeds.

Avoidance practices will benefit from increased opportunities for crop rotation, particularly in the area of disease management (Krupinsky et al., 2002). It will be important to consider existing weed problems and available control options when selecting a cropping system for a given field. The use of resistant cultivars will remain an important strategy although there has not been great emphasis on the development of insect-resistant cultivars in regional crops other than wheat and sorghum. Planting dates will likely become less flexible as more crops are added to the production system, particularly where winter wheat is an important system component. This may require the development of alternative management approaches for pests and diseases that had been avoided through the manipulation of planting dates. In contrast, on the northern edges of the Great Plains, where spring wheat predominates over winter wheat, widespread use of zero-till has increased planting date flexibility on most soils.

Monitoring will become more complicated as the number of crops, potential pests, and treatment decisions increases. This, in turn, will likely lead to an increased demand for less expensive and more efficient sampling technology, such as that being developed in the area of precision agriculture (Allen et al., 1999)

Suppression practices also will become more complicated. In particular, the importance of drift and rotational restrictions will increase as more crops are added to dryland production systems. It may be necessary to adjust crop rotations to maximize herbicide options to manage weed problems that previously had been addressed during the fallow period (Derksen et al., 2002). This would include rotation of grass and broadleaf crops to broaden herbicide selection as well as the use of cultivars bred or engineered to be tolerant to nonselective or broad-spectrum herbicides such as glyphosate [N-(phosphonomethyl)glycine]. However, the development of weed populations that are resistant to glyphosate is a potential consequence of overreliance on this technology. A number of Great Plains weed species (Table 2) are relatively tolerant to this herbicide and would be prime candidates to develop resistance problems. Since naturally occurring biological control is expected to assume greater importance in more diverse cropping systems, conservation of natural enemies of insect pests also will become a more important factor in the selection of insecticides and application methods.

	OVERALL ASSESSMENT

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	REFERENCES

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