Published in Agron. J. 95:1542-1549 (2003).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
PRODUCTION PAPERS
Weed and Insect Communities in Wheat Crops with Different Management Practices
Elba B. de la Fuente*,a,
Susana A. Suáreza,b and
Claudio M. Ghersaa,c
a E.B. de la Fuente, Dep. de Producción Vegetal, Facultad de Agronomía, Universidad de Buenos Aires, Avenida San Martín 4453, (1417) Buenos Aires, Argentina
b Dep. Ciencias Naturales, Facultad de Ciencias Exactas Físico-Químicas y Naturales, Univ. Nacional de Río Cuarto, Ruta 36, km 601, (5800) Río Cuarto, Córdoba, Argentina
c IFEVA Fac. de Agronomía, Univ. de Buenos Aires, Avenida San Martín 4453, (1417) Buenos Aires, Argentina
* Corresponding author (fuente{at}agro.uba.ar).
Received for publication September 25, 2002.
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ABSTRACT
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Biotic adjustments to changes in crop management practices are reflected in the presence or absence of weed and insect populations. The pattern of response and the causes that drive them are important to reveal the main factors involved in molding the structure of an agroecosystem and its dynamics. In this study, weed and insect communities were characterized in wheat (Triticum aestivum L.) crops from the Rolling Pampas with different cropping histories. Surveys were performed in fields that were selected randomly from those located on highlands with typical Argiudol soils and cultivated with conventional tillage. Fields, weeds, and insects were classified with cluster analysis, and weed and insect associations were determined using agronomic variables with canonical correspondence analysis (CCA). The classification of the data resulted in six floristic groups and seven insect groups that characterized different weed and insect communities. Three weed and insect communities associated with the number of years with annual cropping after a pasture that lasted for several years, the duration of wheat crop cycle, and soybean as a preceding crop were identified.
Abbreviations: CCA, canonical correspondence analysis
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INTRODUCTION
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STRUCTURAL AND FUNCTIONAL CHANGES in the ecosystems of the Rolling Pampas occurred during the 20th century (Ghersa and Martínez-Ghersa, 1991; Ghersa and León, 1999). For example, the landscape changed from grassland to an agricultural mosaic, and the natural factors that regulated the ecosystem composition and function lost their importance in relation to agricultural management factors. Wheat was one of the pioneer crops introduced to the region, and it had and will continue to have a significant economic and ecologic impact (Parodi, 1926; 1930; Hall et al., 1992).
Land use in this region has been intensified since 1970, after wheat–soybean [Glycine max (L.) Merr.] double cropping expanded throughout the region, increasing soil cultivation and generating a heterogeneous environment associated with the cropping histories of the fields (Michelena et al., 1989). This heterogeneity implies heterogeneity in resource distribution (Pastor et al., 1997), weed community structure (Ghersa and Martínez de Ghersa, 1991; Ghersa et al., 1996; Ghersa and León, 1999; de la Fuente et al., 1999; Suárez et al., 2001), and plant tissue chemistry and associated plant growth traits (Pastor et al., 1997; Gil et al., 2002).
Changes promoted by land use in the abiotic (physical and chemical environment) and in the biotic (crop and weed) components of the agroecosystem can considerably influence other biotic components of the agroecosystem, e.g., insect numeric balance, population dynamics, and species diversity (Thies and Tscharntke, 1999; Norris and Kogan, 2000).
Taxonomically diverse plant habitats often provide microclimates, greater availability of food sources (prey, pollen, and nectar), alternative hosts, and shelter sites that encourage colonization and population buildup of natural enemies (Coll and Bottrell, 1995; Dicke, 1999). In wheat fields, for instance, predator abundance, species richness, and species diversity increased with an increase in vegetation diversity, the amounts of noncultivated land, and patchiness in the surrounding landscape (Elliott et al., 1999).
The characterization of the causes that drive changes in number of weeds and insects in agroecosystems has been recognized as a very important factor in acquiring knowledge on how to manage agronomic production (Harper, 1977; Cox and Atkins, 1979; Norris and Kogan, 2000). Besides, there is very little empirical information that may be useful to characterize the relationship between cropping history and structure of the biotic communities in the agroecosystems. This kind of information would be an important step toward the understanding of the causes and patterns of change in the structure of the biotic community in cropped lands. Therefore, our objective was to identify weed and insect communities in wheat crops with different cropping histories and to find relationships between management practices and the structure of these communities.
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MATERIALS AND METHODS
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Description of the Area
The Rolling Pampas is a subregion of the grassland of Río de la Plata in Argentina (between 34 and 36° S and 58 and 62° W) (Fig. 1)
. The climate is mild and humid with hot summers. Annual average rainfall is 950 mm, annual average temperature is 17°C, and the prevailing type of soil is Argiudol (Soriano et al., 1991; Hall et al., 1992). In those areas where the main activity is agriculture, there is a significant loss of the A horizon, with a marked reduction of organic matter, total N, and available P (Michelena et al., 1989).
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Fig. 1. Map of the area under study. The gray-shaded area indicate the departments (Rojas, Pergamino, and Salto) of the central Rolling Pampas included in the survey. The map in the inset shows the location of the area under study (black square) in South America.
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Sampling Procedure
In 1996 and 1997, 29 surveys were performed in wheat fields cultivated with conventional tillage to identify weed and insect communities. Surveys were performed in late spring, between 15 November and 15 December, in both years, in similar environments located in the central area of the Rolling Pampas, covering a surface of approximately 5 x 104 ha. Fields were selected randomly on highlands of typical Argiudol soils (INTA, 1974). The sampling area of each survey included the whole central part of the field, with an average area of 70 ha, and borders were avoided. All weeds found in the sampling area were recorded. Most weeds in the surveyed crop fields were subordinated to the crop, presenting low cover abundance values and low height. Insects were captured with sweep net (Tonkyn, 1980). Fixed net sizes (30 cm diam.) and sweeping patterns were used (three points with four net sweepings in each one). The sampling was performed under similar climatic conditions at the same time between 1000 h and 1600 h each day. Once the crop cycle had finished, farmers provided information concerning the following agronomic variables: sowing dates, fertilization, weed and insect management, yields, previous crops, and rotation.
Data Analyses
Weed and insect data were analyzed in terms of species composition. Insect determination was done at order level in all cases and at superfamily, family, or species level when possible. The determination work at these last levels is usually time-consuming, if not impossible, due to lack of taxonomy expertise. Besides, sometimes detailed taxonomy does not improve the results in detail, despite the long time required for acquiring proper taxonomic knowledge (Paoletti and Bressan, 1996). The insect specimens that document our observations are available in our personal insect collection at the Faculty of Agronomy, University of Buenos Aires. The presence or absence of data corresponding to weeds or insects and fields were classified using a cluster analysis. A Sorenson coefficient (CC) version modified by Bray and Curtis (Magurran, 1988) was used for species and surveys as distance measure:
where W is the number of species shared between fields and A and B are the total number of weeds or insects in each sample. Farthest neighbor (complete linkage) was used for weed or insects where the distance between two clusters is given by the maximum distance between any pair of numbers of both clusters. Group average linkage was used as similarity measure (van Tongeren, 1987) for fields. Classifications of weeds or insects and fields were combined in a table where groups of weeds or insects are shown in rows and communities are shown in columns. Constancy (proportion of fields in which a given species occurs in the survey) was calculated for every weed or insect in the community. Weeds or insects with constancy values <10% were eliminated from the analysis because species with lower constancy may be considered as more or less accidental occurrences (Mueller-Dombois and Ellenberg, 1974). Arthropod habits and food preferences were determined using anatomic characteristics and bibliography (Richards and Davies, 1984; Arroyo Varela and Viñuela Sandoval, 1991).
Weed and insect heterogeneity was related to the agronomic variables. The ordination of weeds and insects in fields, obtained from the reciprocal averaging, was constrained to the agronomic variables by multiple regressions with CCA (ter Braak, 1987). Canonical correspondence analysis constructs those linear combinations (axes) of environmental variables along which the distribution of the species is maximally separated. To determine association between data and agronomic variables, a biplot from CCA was prepared by overlaying a vector diagram. It was based on coefficients from the canonical functions describing each canonical axis. The direction of vectors indicates the association of fields with agronomic variables, and the length indicates the discriminating power of the association.
Crop grain yield and the number of years since the last pasture were analyzed with ANOVA, and differences among the means were tested with Tukey test.
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RESULTS
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Weed Community
The classification of 38 weed species with total constancy values 
10% resulted in six floristic groups (Table 1). Two groups, I and III, presented high average total constancy (63 and 48%, respectively); another two groups, II and IV, presented intermediate average total constancy (33 and 26%, respectively); and finally, Groups IV, V, and VI presented a low average total constancy (16 and 13%). Groups I and III were common to all weed communities, whereas Groups II, IV, V, and VI characterized different weed communities.
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Table 1. Constancy values of weed species of wheat in the central Rolling Pampas, Argentina, and units (groups and communities) resulting from the cluster analysis.
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The classification of 29 fields resulted in three weed communities: A, B, and C, with a species richness of 32, 32, and 34, respectively. Community A was characterized mainly by Groups I, II, III, and VI; Community B was characterized mainly by Groups I, II, III, IV, and VI; and Community C was characterized mainly by Groups I, III, IV, V, and VI (Table 1).
Insect Community
The classification of 63 insect species with constancy values 10% resulted in seven insect groups (Table 2): one group, Group I, with a high average total constancy (59%); four groups, II, III, IV, and VI, with an intermediate average total constancy (22, 27, 28, and 23%, respectively); and two groups, V and VII, with a low average total constancy (14 and 20%, respectively). Group I was common to all insect communities, whereas Groups II, III, IV, V, VI, and VII characterized different insect communities.
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Table 2. Constancy values and function, herbivorous (h) and beneficial (b), of wheat insect species in the central Rolling Pampas, Argentina, according to the identified weed communities.
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The classification of fields resulted in three insect communities (Table 2): A, B, and C, with a species richness of 47, 53, and 50 respectively. Community A was characterized mainly by Groups I, II, IV, and V; Community B was characterized mainly by Groups I, II, III, and VI; and Community C was characterized mainly by Groups I, III, IV, and VII.
The habits and food preferences were established in only 42 cases (63%) out of a total of 63 insects. In Group I, for instance, species such as Cicloceraia sp., Caeporis stigmula, and Diabrotica speciosa are herbivorous while others, such as Eriopis connexa and Noconocephalus argentinus, can be classified as beneficial insects as they are predators of the herbivorous ones. No differences among communities were detected in the proportion of herbivorous and beneficial insects.
Relationship between Weed–Insect Communities and Management Practices
The data concerning management practices revealed some differences among communities (Table 3). Years since the last pasture and yield for the long-crop-cycle wheat cultivars were significantly different among communities. When relationships between the structure of weed–insect communities and management information were studied, Community A presented low species richness (32 weed species and 47 insect species) and the presence of Insect Group V. As to the management practices, fields that characterized this community had a high number of years since the last pasture (19 yr), low yields (3.2 Mg ha-1 for long crop cycle and 3.0 Mg ha-1 for short crop cycle), and a high proportion of fields cultivated with short-crop-cycle cultivars, having soybean as preceding crop. Community B presented low weed species richness (32 species), the highest insect species richness (53 species), and the presence of Insect Group VI. This result coincided with those fields with an intermediate number of years since the last pasture (13 yr), high crop yields (4.3 Mg ha-1 for long crop cycle and 3.6 Mg ha-1 for short crop cycle), and a high proportion of fields cultivated with short-crop-cycle cultivars, treated with insecticides and having corn (Zea mays L.), soybean, and wheat–soybean as preceding crops. Finally, Community C presented high weed species richness (34 species), intermediate insect species richness (50 species), and the presence of Weed Group V and Insect Group VII. This data coincided with fields with the lowest number of years since the last pasture (3 yr), low crop yields (3.0 Mg ha-1 for long crop cycle and 3.0 Mg ha-1 for short crop cycle), a balanced proportion of fields cultivated with short- and long-crop-cycle cultivars, having corn and soybean as preceding crops.
The CCA provided results with regard to the most important factors among the measured variables affecting the occurrence of weeds and insects, which explain 53.9% of the total variance in weed–insect data. The three communities were ordered in relation to the three main axes. Eigenvalues were high, considering that variation was restricted to measured variables. The agronomic variables affected weeds and insects. The intraset correlation of management variables with the first three axes from CCA showed that Axis 1 (eigenvalue = 0.49) was related to years since the last pasture and to crop cycle, Axis 2 (eigenvalue = 0.41) was related to crop cycle and soybean as preceding crop, and Axis 3 (eigenvalue = 0.27) was associated mainly with crop cycle (Table 4 and Fig. 2) . Weed Group V is placed to the right of the CCA diagram, coinciding with a low number of agricultural cycles.
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Table 4. Intraset correlation of management variables with the first three axes from canonical correspondence analysis.
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Fig. 2. Ordination diagram of weeds (filled circles) and insects (open circles) in the two principal axes of canonical correspondence analysis (CCA). Communities A, B, and C are enclosed by dashed lines. Vectors represent crop cycle (cycle) and years since last pasture (years). The diagram in the inset shows the weed (w) and insect (i) groups included in each community.
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DISCUSSION
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Taking into account management factors, the main agronomic variables related to weed and insect distributions were the number of years since the last pasture was plowed, crop cycle, and preceding crop. As the number of years with annual cropping since the last pasture increases, the physical and chemical soil environment is impoverished by effects of soil erosion and compaction as well as by crop nutrient exploitation (Michelena et al., 1989). Community A had the lowest diversity of weeds and was present in the fields with the longest period of cropping with annual species. On the other hand, when the number of years since the last pasture was low, Weed Group V was present (Community C, Table 1). Therefore, wheat weed communities might be related to soil environmental conditions. This relationship has been previously described for summer crops and soil condition, which in turn, was also related to crop yields (León and Suero, 1962; de la Fuente et al., 1999; Suárez et al., 2001). In this study, Communities A and C were not associated with a particular wheat yield level. This difference between winter and summer crops may be explained by the following: (i) Soil degradation is masked by a better water balance during winter as the water demand is reduced by lower temperatures (Hall et al., 1992), and a better water supply may compensate for soil degradation caused by cropping (Arce-Díaz et al., 1993); (ii) the proportion of fields cultivated with short and long crop cycles was different among communities, and yield potential is related to crop cycle; and (iii) the lower number of annual crops in fields with Community C might reflect poor environmental conditions that farmers try to reverse by rotating with pastures.
Soybean as preceding crop could also be related to the impoverishment of soil variables caused by the number of years with annual crops. In fields where Community A is present, farmers introduce more soybean than corn in the rotation because soybean yield is less affected by low N availability in the soil than the yield of other summer crops such as corn (Liebhardt et al., 1989; Karlen et al., 1994).
Although Weed Community B was not the richest in species, it was associated with the highest richness in insect species. This community was also associated with high wheat yields in short-cycle varieties. Food quality and quantity have been recognized to control insect richness (Howe and Westley, 1988a). Thus, it is possible that variations in wheat yield may have caused changes in the insect community. Another possibility related to crop cycle is the phenological synchrony in pest–crop interactions (Metcalf, 1999). Other factors related to the weeds or management that may have promoted high richness cannot be disregarded. Fields with Community B presented high temporal crop diversity (corn, soybean, and wheat–soybean), and it is recognized that crop rotations can increase wildlife populations (Karlen et al., 1994). Ecologists have recognized that peaks of diversity occur at intermediate levels of biological production. This is so because low habitat fertility reduces diversity through nutrient stress and also because high fertility removes the limitations imposed by nutrient stress, thus resulting in simplified communities as the outcome of competitive exclusion (Schluter and Ricklefs, 1993). Therefore, taking this into consideration, it can be speculated that Weed Community B was structured by a soil environmental condition that was intermediate between that in Community A and the one for Community C.
The presence or absence of some groups may be related to the abiotic and biotic environment (Howe and Westley, 1988b). Soil conditions affected weed composition, and they could also have affected weed and crop biomass. The absence of Insect Groups II, V, and VI in Community C and Insect Group III in Community A may be related to the presence of Weed Groups V and VI in Communities C and A, respectively. Many weeds of these groups produce volatile oil compounds that may either act as repellent or attract some insects (Salto et al., 1993), e.g., wild celery [Apium leptophyllum (Pers.) F. Muell. ex Benth.], toothpick ammi [Ammi visnaga (L.) Lam.], and beggarticks (Bidens subalternans de Candolle) from Group V and absinth wormwood (Artemisia annua L.) and mayweed chamomile (Anthemis cotula L.) from Group VI.
Besides, several interactions among weeds, arthropod pests, and their natural enemies have been reported. Species such as common lambsquarters (Chenopodium album L.), spurred anoda [Anoda cristata (L.) Schltdl.], and curly dock (Rumex crispus L.) are sources of recolonization for herbivores (aphids) in winter crops while prostrate knotweed (Polygonum aviculare L.) maintains numerous beneficial insects, including Geocoris sp., which migrates to the crop to feed on herbivores (Norris and Kogan, 2000).
Chemical control was very effective on weed and insect abundance, weed cover was always less than 1%, the crop dominated the canopy, and there was no significant crop damage caused by herbivorous insects. It is interesting to note that although weed and insect control is cited as one of the factors explaining their distribution (Norris and Kogan, 2000), our results showed that herbicide and insecticide use had a lower impact on weed and insect distribution than other agronomic variables. Moreover, fields where Community B was present had the highest insect richness and the highest proportion of fields with insect control (Table 3).
The present weed flora and insect fauna are dominated by a few species (species from Groups I and III for weeds and Group I for insects). This community structure is very common in arable lands (Clements et al., 1994), and the species in these dominating groups frequently cause serious problems for winter crop production in this area (Hall et al., 1992; Froud-Williams, 1999).
In agreement with Lawton and Brown (1993) and Lawton (1994), we observed that food web functional structure was maintained regardless of differences among communities in species composition. It can be argued that the number of species in which we were able to describe functional structure was not enough, but because the data of insect functions was obtained randomly, our data can be accepted as supporting this hypothesis. However, we guess the insect functions that we were able to determine were those of the most investigated insects.
Major changes in the agroecosystem of the Rolling Pampas caused by land use history are reflected in the weed and insect communities in wheat. Three communities were distinguished that may be associated with the number of years with annual cropping after a pasture that lasted for several years, the duration of wheat crop cycle, and soybean as a preceding crop.
A wide range of biotic and abiotic factors have been shown to impact the structure of communities. The evaluation of the importance of agroecosystem management in crop–weed–insect community has been hampered because of the difficulties inherent in most experimental designs using naturally occurring situations. This approach provides a useful methodology to address this kind of study and is a first step to understand how agroecosystems are structured by management decisions.
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ACKNOWLEDGMENTS
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The authors acknowledge Dr. Diego Medán, Dr. Marta Loiacono, and Dr. Fabiana Gallardo for their help with insect determination. They also acknowledge the farmers who provided management information. This work was supported in by PIP-CONICET 01/G933 and FONCYT BID 08-00116-02093.
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ABSTRACT
INTRODUCTION
