Ground beetle (Coleoptera: Carabidae) activity density and community composition in vegetationally diverse corn agroecosystems.
Conservation tillage is increasing as a method of controlling soil erosion from agricultural land in the United States. Conservation tillage is a tillage and planting system that includes both reduced and no-tillage systems, and involves leaving a minimum of 30% of the previous year's crop residues on the soil surface after planting (Gebhardt et al., 1985). Abundant plant litter in conservation tillage crop fields has profound influences on the ecology of these agroecosystems, including an increase in organic matter on or near the soil surface and increases in abundance, diversity and activity of soil invertebrates (Stinner and House, 1990). In addition, conservation tillage changes weed density and community composition; for example, no tillage systems often favor perennial weed species over annuals (Triplett and Lytle, 1972). Certain pest species are known to increase in some conservation tillage systems [e.g., stalk borer, Papaipema nebris (Guenee) (Stinner et al., 1984) and armyworm, Pseudaletia unipuncta (Haworth) (Gregory and Musick, 1976); in no-tillage corn]. However, beneficial species such as predaceous carabid, or ground beetles (House and All, 1981; House and Stinner, 1983) and spiders (House and Parmelee, 1985) may also increase. Thus, investigators have been interested in the effects of tillage reduction on both pest and beneficial species in several row crop agroecosystems (e.g., All and Gallaher, 1977; Cheshire and All, 1979; Harrison et al., 1980; Blumberg and Crossley, 1983; House and Stinner, 1983; Brust et al., 1986 a, b; Weiss et al., 1990; Clark et al., 1993, 1994).
Ground beetles constitute an important component of the litter fauna in conservation tillage and other agroecosystems. Most carabid species are considered to be predaceous. Several genera are specialized predators on specific taxa; others, however, are omnivores, feeding on both plant and animal material (Allen, 1979). The predaceous nature of carabids and their prevalence in agroecosystems suggests their possible use as biological control agents (Kulman, 1974). Evidence from field (e.g., Wishart et al., 1956; Coaker and Williams, 1963; Lesiewicz et al., 1982) and laboratory studies (e.g., Frank, 1971; Best and Beegle, 1977a, b) indicates that predaceous carabids consume many pest arthropods.
Investigations have been conducted to determine potential habitat preferences of predatory carabids; results from such studies could lead to particular habitat manipulations for enhancement of biological control by this group of predators. Presence of weedy vegetation in agroecosystems appears to benefit carabid populations (e.g., Dempster, 1969; Barney et al., 1984; Stinner et al., 1984; Thomas et al., 1991; Chiverton and Southerton, 1991), as does the increase in plant litter associated with conservation tillage. House and All (1981) found the abundance and diversity of carabids were greater in conservation tillage than in conventional tillage soybean plantings. They attributed these differences to the well-developed plant litter layer present in the conservation tillage plantings, which presumably provided more habitat for carabids. Similarly, carabid abundance, species diversity and biomass were significantly higher in no-tillage than in conventional tillage soybean agroecosystems (House and Stinner, 1983). Predation rates by carabids on lepidopterous larvae and other pests are often greater in no-tillage than in conventional tillage corn agroecosystems (Brust et al., 1986b).
The objective of this study was to examine the influences of weedy vegetation in reduced tillage corn (Zea mays L.) agroecosystems on the abundance and community structure of carabids. We addressed the following questions: (1) Is the activity density of carabids greater in corn plantings containing weeds than in a weedless planting? (2) Do weeds in corn plantings significantly increase richness and diversity of carabid communities? (3) Do corn plantings having grassy weeds have the greatest activity density of carabids, based on results of previous research in corn agroecosystems (e.g., Stinner et al., 1984; Barney et al., 1984)? (4) Do carabid species respond individualistically to vegetational diversity in corn plantings? (5) Is the potential benefit of larger ground beetle populations offset by lowered crop yields?
STUDY AREA AND METHODS
Research was conducted during 1988 and 1989 at the Ohio Agricultural Research and Development Center, Wooster, Ohio. The experimental field was divided into 16 plots; each plot measured 18.3 m by 18.3 m, with a 3.1 m border containing weeds that were mowed to a height of 6-8 cm on a regular basis. Treatments were replicated four times in a randomized complete block design; the experiment was located in the same field both years. Treatments were randomly assigned to plots within blocks, and these assignments were unchanged during the study. The treatments were: (1) corn monoculture (no weeds present); (2) corn with broadleaved weeds; (3) corn with grassy weeds; and (4) corn with both broadleaved and grassy weeds (mixed weeds). The experiment was a 2 x 2 factorial, with two factors, broadleaved and grassy weeds, and two levels of each factor, present or absent.
The flora of the plot borders was dominated by lambsquarter (Chenopodium album L.), dandelion (Taraxacum officinale Weber ex Wiggers), Muhlenbergia frondosa (Poiret) Fernald, foxtail grass (Setaria spp.), rough pigweed (Amaranthus retroflexus L.) and yellow nutsedge (Cyperus esculentus L.) (Plant nomenclature follows Gleason and Cronquist, 1991.) Areas surrounding the study site consisted of grasses which were mowed occasionally for hay as well as conventional and conservation tillage corn fields. No insecticides were used, but herbicides were applied at the following rates to manipulate naturally occurring weed populations in treatments: corn without weeds - cyanazine (E.I. DuPont, Wilmington, Del.) (3.36 kg [AI]/ha), alachlor (Monsanto, St. Louis, Mis.) (3.36 kg [AI]/ha) and paraquat (ICI Americas, Inc., Wilmington, Del.) (0.56 kg [AI]/ha); corn with broadleaved weeds-alachlor (2.24 kg [AI]/ha) - corn with grasses - cyanazine (2.80 kg [AI]/ha), and 2,4-D amine (Riverside/Terra Corp., Sioux City, Iowa) (0.56 kg [AI]/ha); corn with a mixture of broadleaved and grassy weeds: cyanazine (1.68 kg [AI]/ha), alachlor (2.24 kg [AI]/ha) and paraquat (0.28 kg [AI]/ha). Herbicides were applied to the mixed weeds treatment at low rates to prevent early season competition with the corn; later development of weed communities did not appear to be adversely affected. Plots were hand-cultivated to maintain the different weed communities in the broadleaf weed and the grassy weed plots, and to prevent weed establishment in the weedless plots, because the rates of herbicides used did not control the weeds for the entire growing season.
The entire field was minimally tilled (disked once before planting) and corn was planted with a no-till planter. The corn cultivar 'Pioneer 3780' was planted the 1st yr on 12 May; due to discontinuation of this cultivar after 1988, 'Pioneer 3352' was planted 19 May 1989. Corn yields were determined at the end of the season by cutting plants at four randomly selected 0.5-m-long locations within each plot, drying the seeds at 80 C for 72 h and weighing the seeds to the nearest gram.
Beginning in late May, monthly surveys were conducted on each plot to determine plant community composition in weed treatments. Plant surveys were qualitative, visual estimates of cover by species on a per-plot basis. Surveys were conducted by walking every other corn row in each plot. These visual cover estimates were made to the nearest 5%. Plant species having a mean percent cover value over 10% were considered to be dominant. Weed biomass was determined in late September by clipping four randomly selected 0.25 [m.sup.2] quadrat samples from each weedy plot, oven-drying the samples at 80 C for 72 h, and weighing the samples to the nearest gram.
Six pitfall traps were arranged in a 2 x 3 grid pattern in each plot to sample adult carabids. Pitfall traps are widely used to sample carabids (e.g., Wallin, 1985, 1986; Perfecto et al., 1986; Halsall and Wratten, 1988; Kharboutli and Mack, 1991; Tonhasca and Stinner, 1991; Niemela et al., 1992; Clark et al., 1993, 1994; Spence and Niemela, 1994; Riddick and Mills, 1995). Although numerous studies have demonstrated that catch size of pitfall traps is influenced by a variety of factors apart from population size (e.g., Greenslade, 1964; Luff, 1975; Southwood, 1978; Halsall and Wratten, 1988), pitfall traps continue to be used in ecological studies of epigeal arthropods such as carabids, because the traps are inexpensive and require a minimum amount of labor (Halsall and Wratten, 1988). Traps were set at least 4 m apart, and were placed at least 4 m from the edge of the plot to minimize edge effects. A trap consisted of a 0.5-liter plastic cup, 7.5 cm deep with a 11.0 cm opening. Approximately 120 ml of a 50:50 mixture of water: ethylene glycol was used as killing agent and preservative. Each trap was buffed in the soil so that the opening was level with the soil surface. For each trapping period, traps were left in the field for 1-5 days, and traps were set out every 3-6 wk. Variation in trapping duration and time between trapping sessions was due partly to unstable weather conditions (i.e., the threat of rain) and partly to an effort to find an ideal length for pitfall trapping periods. For most trapping sessions, the duration was 72 h. Trapped beetles were removed and placed in 70% ethanol for later identification. Carabid taxonomy follows Bousquet and Larochelle (1993).
The number of beetles captured per trap per day was calculated for each plot as a measure of carabid activity density. Carabids were identified to species; species richness and the Shannon-Wiener Index were used as indicators of carabid diversity in each treatment. Sorenson's similarity index was calculated for comparison of carabid communities among treatments (Magurran, 1988). The Sorenson similarity index for quantitative data is based on the formula [C.sub.N] = 2jN/(aN + bN), where j = the number of species found in both sites, a = the number of species found in site A, b = the number of species found in site B, aN = the number of individuals in site a, bN = the number of individuals in site b, and jN = the sum of the lower of the two abundances (activity density in our study) of species which occur in both sites. We summed the numbers of each species caught in each sampling period and used this total as the abundance in Sorenson's equation. Activity density and species richness data were log(y+1)-transformed to stabilize the variances; the activity density data were subjected to a repeated-measures ANOVA in order to determine if there was a temporal effect on activity density of carabids. The species richness data for each sampling period were analyzed by two-way ANOVA. Single degree-of-freedom contrasts were used to test for effects of different weed treatments on carabid activity density and species richness. Weed and corn biomass data were log (y+1)-transformed and analyzed by a two-way ANOVA; in the event of a significant result, the means were separated using the Student-Newman-Keuls test.
Weed community composition and biomass. - Weed community composition changed considerably as the growing season progressed; early season communities were dominated by perennial species, and annual species became abundant later (Table 1). The environmental conditions during the 2 growing seasons of this study were very different: 1988 was extremely dry until late July and early August, and 1989 was considerably wetter. A severe drought occurred in 1988; total precipitation for April and May 1988 was 8.8 cm, considerably less than the long-term average of 18.8 cm. June rainfall for 1988 was 1.32 cm, in contrast to the long-term average of 10.1 cm (Wooster Weather Summary, Department of Agricultural Engineering and Statistics Laboratory, Ohio Agricultural Research and Development Center, Ohio State University, and Department of Geography, Miami University). Annual weeds were severely stunted until precipitation increased in early August after which weeds grew rapidly. Conversely, 1989 was much wetter (total rainfall for April and May 1989 was 16.0 cm, which was much closer to the long-term average), and growth of annual weeds was more typical than in the previous year. However, final weed biomass was actually greater in all treatments in 1988 than in 1989 (Table 2), with the broadleaved and mixed weed treatments having significantly greater biomass than the grassy treatment (P [less than] 0.05). Weed biomass values were not significantly different among treatments in 1989 (P [greater than] 0.05; Table 2).
Carabid abundance, species richness and diversity. - The carabid communities in the four treatments (Table 3) were dominated by 10 species in 1988. These species made up [greater than]99% of the total number of carabids caught in the four treatments. In 1989, the most commonly trapped species made up [greater than]97% of the total trap catch in each of the four treatments (Table 3).
Activity density was nearly the same in all treatments early in 1988, but increased dramatically in the broadleaved weed and grass treatments in late August, and declined in late [TABULAR DATA FOR TABLE 1 OMITTED] [TABULAR DATA FOR TABLE 2 OMITTED] September in all treatments [ILLUSTRATION FOR FIGURE 1 OMITTED]. Neither broadleaved weeds nor grasses significantly influenced carabid activity density the 1st yr (F = 0.673, df = 1,9, P [greater than] 0.6 and F = 5.1, df = 1,9, P [greater than] 0.05, respectively); however, the broadleaved weed-grass interaction was significant (F = 7.0, df = 1,9, P [less than] 0.03). Trap captures of carabids were considerably less during 1989, with the number captured dropping in late July and increasing in late August [ILLUSTRATION FOR FIGURE 1B OMITTED]. Significantly more carabids were caught in treatments with broadleaved weeds than in treatments without such weeds in 1989 (F = 23.2, df = 1,9, P [less than] 0.001). Grasses did not have a significant effect (F = 2.0, df = 1,9, P [greater than] 0.1), and the interaction between broadleaved weeds and grasses was also not significant (F = 2.3, df = 1,9, P [greater than] 0.1). Sampling period had a significant influence on carabid activity density in both years (1988: F = 32.3, df = 3,36, P [less than] 0.001; 1989: F = 21.4, df = 4,48, P [less than] 0.001); activity density peaked in late spring and early summer, declined, and then increased again later in the season [ILLUSTRATION FOR FIGURE 1A, B OMITTED].
Treatments containing grasses had a significant positive effect on carabid species richness during one sampling period in 1988 (25-27 August; F = 5.38, df = 1,9, P [less than] 0.05) and during one sampling period in 1989 (23-25 July; F = 9.49, df = 1,9, P [less than] 0.05; Table 4). Plots that included broadleaved weed species had significantly more carabid species than plots without broadleaved weeds during one sampling period in 1989 (23-25 July; F = 7.04, df = 1,9, P [less than] 0.05). A significant interaction effect occurred between grasses and broadleaved weeds twice in 1988 (25-27 August; F = 8.49, df = 1,9, P [less than] 0.05; 27-29 September; F = 5.51, df = 1,9, P [less than] 0.05) and once in 1989 (23-25 July; F = 11.96, df = 1,9, P [less than] 0.05).
Shannon-Wiener Index values were highly variable within each treatment (Table 4). The weedless treatment had the largest index value during the early part of the 1988 season, but the value dropped dramatically by late August. The broadleaved weed treatment had the largest Shannon-Wiener value during three of the five sampling periods of 1989.
Comparisons of species similarity. - Similarity, as measured by Sorenson's Similarity Index for quantitative data, was generally high for all treatments (Table 5). Carabid species appeared to distribute themselves uniformly among treatments. The lowest Sorenson Index values were obtained for the comparison of the no weeds and grasses treatments in 1988, and for the no weeds-broadleaved weeds and no weeds-mixed weed comparisons in 1989.
[TABULAR DATA FOR TABLE 3 OMITTED]
[TABULAR DATA FOR TABLE 4 OMITTED]
[TABULAR DATA FOR TABLE 5 OMITTED]
Responses of individual carabid species to weedy vegetation. - The presence of grasses significantly (P [less than] 0.05) increased the activity density of one species (Harpalus pensylvanicus) and decreased the activity density of another (Cyclotrachelus sodalis) in 1988; no significant effects of grasses were observed for any of the species in 1989 (Table 6; P [greater than] 0.05). Broadleaved weeds had a significant (P [less than] 0.05) negative effect on activity density of Harpalus pensylvanicus in 1988, and a significant (P [less than] 0.05) positive effect on activity density of Poecilus lucublandus and Anisodactylus sanctaecrucis in 1989. Sampling period had a highly significant effect on activity density of most species throughout the study (P [less than] 0.001 for most of the species analyzed). The activity density of each carabid species tended to either increase or decrease as the season progressed, depending on whether the species was an early or late summer breeder.
Effect of weed competition on corn yields. - Weeds had a significant impact on corn yields (Table 2; P [less than] 0.05, Student-Newman-Keuls Test). Yield differences between weedless and weedy corn plantings were large; for example, the weedless corn yielded more than twice as much seed biomass as corn grown with broadleaved weeds (Table 2). These results are not surprising, because weedy plots had very high weed densities, and thus greater competition potentially existed between weeds and corn for resources (e.g., Vengris et al., 1955).
Carabid activity and activity density: effects of weedy vegetation. - Weeds had a significant influence on carabid activity density in 1989. No significant broadleaved or grassy weed effects on carabid activity density were observed in 1988, which was a year of severe drought. Activity of some field-dwelling carabids has since been shown to be directly dependent on soil moisture (Epstein and Kulman, 1990), and the dry conditions in the 1st yr of the study could have obscured potential effects of weeds on carabid activity and community composition. Also interesting was the greater activity density of carabids in 1988 than in 1989; this could be explained by lower weed densities in plots over most of the 1988 season. Honek (1988) observed that trap catches of most carabid species were greater in sparse than in dense cereal plantings; this difference was attributed to both greater activity and possibly greater population density. Sparse cereal plantings had higher temperatures than dense plantings, and this increased temperature could explain higher carabid activity and therefore larger trap captures (Honek, 1988). Some carabid species also appear to thermoregulate by seeking out sunny areas in crop fields (e.g., Honek, 1988). Perfecto et al. (1986) also noted that the emigration rate of two carabid species, Harpalus pensylvanicus and Cyclotrachelus (=Evarthrus) sodalis, was greater in low density tomato and tomato-bean polycultures than in high density plantings. Both of these species were frequently captured in our study. Treatments with broadleaved weeds had greater carabid activity density than treatments without such weeds (P [less than] 0.05) in 1989, a more typical year in terms of precipitation than the 1st yr of the study. The presence of weeds can be favorable for many carabid [TABULAR DATA FOR TABLE 6 OMITTED] species (e.g., Purvis and Curry, 1984; Kromp, 1989); some species, such as H. pensylvanicus and C. sodalis, appear to prefer sites that are wetter and have more shade (Perfecto et al., 1986).
There was a significant positive effect of broadleaved weeds on carabid activity density in the 2nd yr. In addition, the mean numbers of carabids caught in pitfall traps placed in weedy corn plantings were greater than the number caught in traps positioned in the weedless planting on most sampling dates [ILLUSTRATION FOR FIGURE 1b OMITTED]. There are a number of possible explanations for the greater activity density in the weedy corn treatments compared to the weedless corn. Numerous studies have documented the positive influence of weeds in crop fields on carabid activity density and predation efficiency. Speight and Lawton (1976) found that the number of predatory beetles (carabids and staphylinids) was greater in areas of a winter wheat field where Poa annua (speargrass) was abundant than where it was scarce. They speculated that this difference was probably related to better environmental conditions and more abundant prey in areas containing speargrass. Shelton and Edwards (1983) observed greater numbers of Harpalus spp. in pitfall traps located in soybean plots containing either grasses or mixed weeds (broadleaved weeds and grasses) than in soybean plots containing either broadleaved weeds or no weeds at all. Abundant prey species and diverse, favorable habitat conditions present in the grassy and mixed weed plots were cited as possible reasons for these differences (Shelton and Edwards, 1983). In particular, Harpalus spp. prefer soybeans growing with grasses; this preference may be related to the fact that some species of the genus Harpalus readily consume foxtail (Setaria) seeds in the laboratory and in the field (Lund and Turpin, 1977).
Powell et al. (1985) found that the numbers of adults of certain species of carabids and staphylinids, caught in pitfall traps in winter wheat fields, were significantly reduced by herbicide treatments. They proposed that the preference of certain carabids, such as Amara spp., for weedy plots was probably related to diet, because most of these species eat seeds (Lindroth, 1974). Powell et al. (1985) also noted that herbicide treatments drastically reduced overall numbers of carabid and staphylinid larvae; they suggested that the weedy; untreated plots may have provided more suitable conditions for oviposition and possibly higher relative humidifies for the larvae, which are more susceptible than adults to desiccation (Powell et al., 1985). House (1989) demonstrated that predatory soil arthropods were abundant in the root systems of dogfennel, Eupatorium capillifolium (Lam.) Small, and rough pigweed, Amaranthus retroflexus L., present in experimental corn and soybean plantings.
Influence of weedy vegetation on carabid community composition. - Weedy vegetation in corn plantings did have significant effects on carabid species richness during three of the nine sampling periods. In addition, significant interactions between broadleaved weeds and grasses in relation to species richness occurred during two of these sampling periods. It is interesting that these effects occurred later in the season (late July and August), so perhaps a number of the carabid species did respond to the development of certain weed communities, such as the late summer-autumn-breeding Harpalus pensylvanicus. There are a number of reasons why significant weed effects on carabid community composition were not evident over a larger part of the season. Pitfall traps tend to capture larger, more vagile carabids; perhaps significant differences in community composition were detected during only three sampling periods because these larger carabids moved easily among the plots, especially early in the season before the weedy vegetation became dense, and were fairly evenly distributed across the experimental field. As an example of carabid dispersal ability, Best et al. (1981) found that dispersal distances for Poecilus (=Pterostichus) chalcites and Scarites quadriceps (=substriatus) (species also captured in our study) averaged 8.5 and 12.2 m per day, respectively. In a study involving six carabid species in Switzerland, Lys and Nentwig (1991) noted a range of mean daily dispersal distances from 7.7 m to 34.9 m. If these daily dispersal distances are comparable to those of other medium to large carabid species, then it is possible that many of the larger species could have moved easily from one plot to another in our field. The lack of significant differences among treatments for many individual carabid species (Table 6) would seem to provide evidence for exceptional dispersal abilities of these insects.
It is also possible that many of the field-dwelling carabids captured in our pitfall traps have no distinct preference for one habitat type over another in terms of plant species that typically occur in corn fields. Esau and Peters (1975) observed that certain carabids do exhibit habitat preferences, but that others, such as Poecilus (=Pterostichus) lucublandus were collected with nearly equal frequency in corn fields, fencerows and prairies. At least some of the carabids caught in the pitfall traps may have no preference for the broadleaf weed-corn plant community over the grass weed-corn community; as long as the habitats are suitable for a particular species, and prey and/or plant material are available, that species will probably exhibit little or no preference. Species showing no preference, and possessing at least moderate dispersal ability, would tend to move among the different plots extensively, unless large numbers of prey were found in a small area of a single plot. Perhaps the use of larger, more spatially separated plots might have revealed significant differences among the treatments we studied in terms of carabid community structure. In addition, using methods more likely to sample smaller and/or less vagile species may also have indicated genuine differences in the carabid communities inhabiting those different habitats.
Our results, in conjunction with previous studies, suggest that weeds in agroecosystems support large populations of carabid beetles, probably due to a variety of factors. In our study, the observed difference between weedy and weedless corn plantings was likely due to a number of interacting factors, such as ameliorated microhabitats, refuge from predators, and abundance of suitable food. However, the presence of weeds in crop fields, especially at high densities, has major disadvantages that can negatively affect quantity and quality of crop yields, including competition with the crop for resources, interference with harvesting equipment, and contamination of crop seeds during harvest. Nevertheless, the use of weeds as resources for beneficial arthropods remains a viable possibility; maintaining weedy areas around edges of crop plantings (e.g., Chiverton and Sotherton, 1991), and even in strips through the crop field (Kemp and Barrett, 1989; Lys and Nentwig, 1992; Zangger et al., 1994) are alternative methods of weed management that can enhance the abundance and activity of predatory arthropods. Additional research is needed in the area of predaceous arthropod population management because the expanding use of conservation tillage systems, especially no-tillage, increases the possibility of larger weed populations in crop fields. It is imperative that ecological interactions between weeds, arthropods and crop plants be studied more extensively so that pest arthropod populations can be managed below economic injury levels in conservation tillage cropping systems.
Acknowledgments. - We thank David A. McCartney and Joseph P. Reed for field assistance and Bert L. Bishop for statistical advice. We are also grateful to Elizabeth Jacob, Kirk J. Larsen, Richard E. Lee, John D. Peles, Adam Porter and Ann L. Rypstra for their helpful comments and suggestions that greatly improved the manuscript. Salaries and research support were provided by state and federal funds appropriated to The Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio.
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