Carabid (Coleoptera: Carabidae) biology within diverse agricultural systems of the Southern Great Plains is not well studied. This study was conducted to elucidate carabid dispersal powers, colonization rates, habitat utilization, natal origins, and the impact of tillage on carabid populations in diverse agroecosystems. The effects of tillage regimes and colonization rates from a semi-permanent habitat into an annual cropping system were investigated with standard pitfall trap sampling. Carabid dispersal powers, habitat utilization, natal origins, and diet switching were determined by stable carbon isotope data. For 2006, the average number of carabids per trap in no-till (NT) and conventional till (CT) was similar in Period 1. During Period 2, these averages decreased in no-till and increased in conventional tillage. The differences in the availability and distribution of resources and the density of ground cover in NT and CT impacted carabid foraging activity-density which altered the opportunities for carabids to be trapped. Data analysis for 2007 tillage impact is in process. Colonization data has provided fine scale resolution to beetle movement into the annual cropping system from the semi-permanent habitat. Stable carbon isotope data for 2006 and 2007 show that 631 carabids moved from alfalfa to sorghum and 61 moved from sorghum to alfalfa. This data revealed that there were a total of 305 carabids that did not move from alfalfa and 29 did not move from sorghum. Natal origins were determined for 505 carabids in 2006 and analysis continues for the rest of 2006 and 2007. Dieting switching was confirmed for a total 28 carabids based on stable carbon isotope ratios of the P and R tissue sub-samples from each beetle.
This study has provided information to producers regarding carabid colonization and dispersal abilities in diverse agroecosystems. Tillage and dispersal data demonstrates that alfalfa is utilized as a refuge habitat by carabids during catastrophic environmental disturbances. Isotope results from the selected P and R sub-sample tissues have shown a high degree of resolution for determining movement and diet switching for individual beetles. This data supports the need for diversifying agroecosystems to provide carabids with the various habitats necessary to complete all life stages. Carabid mobility and polyphagous feeding habits demonstrate the need to conserve these biological control agents within the agroecosystems of the Southern Great Plains.
In the Southern Great Plains, natural enemies have a regulating effect on pest populations in winter wheat, alfalfa, cotton, and sorghum (Kring et al. 1985; Rice and Wilde 1988; Giles et al. 2003). This natural regulating effect is an essential component of integrated pest management plans (IPM). Holland et al. (2005) considers carabids one of the most important ground-dwelling consumers of agricultural pests. Carabids are excellent organisms to study in agroecosystems because they react to environmental changes quickly and measurably (Thiele 1977; Fournier and Loreau 2000; Holland et al. 2005). This is due to their reproductive plasticity and flexible behavioral and environmental requirements (Thiele 1977; Holland 2002). These beetles are relatively easy to sample due to their foraging techniques and dispersal characterized by walking rather than flying (Thiele 1977; Fournier and Loreau 1999). Carabids are often abundant and persistent despite catastrophic disturbances in agroecosystems (Thiele 1977; Lövei and Sunderland 1996). Integrated pest management (IPM) practitioners readily recognize the polyphagous nature of carabids and the potential implications of this feeding activity on the consumption of pest species. However, carabid biology within diverse agricultural systems of the Southern Great Plains is not well studied.
Monoculture crops dominate farming practices in the prairies of North America and monocultures can lead to an increase in pest pressures (Elliott et al. 1998; Ahern and Brewer 2002; Brewer and Elliott 2004; Boyles et al. 2004; Men et al. 2004; Ribas et al. 2005). These homogeneous crops increase the isolation and fragmentation of suitable habitats for natural enemies. Predators are generally thought to be more vulnerable to fragmentation of habitat than prey species (Kruess and Tscharntke 1994; Abensperg-Traun and Smith 1999; Kruess and Tscharntke 2000). This vulnerability can be expressed as a breakdown in food chains and loss of trophic structure within ecosystems (Hunter 2002). Additionally, habitat degradation and limited resources within these monocultures can diminish the ability of natural enemies such as carabids to decrease pest populations leading to a loss of crop and forage yields (Lys 1994). For example, yield loss and insecticide costs from an outbreak of greenbug, (Schizaphis graminum) can range from $3.8 to $135 million in grain wheat grown in Oklahoma (Webster 1995). The greenbug can reproduce rapidly often without detection and reach economic injury levels quickly (Starks and Burton 1977). Alfalfa, grown for livestock forage, can produce gross incomes exceeding $100 million for Oklahoma producers and over 480,000 tons annually for Texas alfalfa producers. Alfalfa suffers reduced quality and yield losses from attacks by alfalfa weevils, spotted and blue alfalfa aphids, pea aphids, defoliating caterpillars, and army cutworms (Cooperative Extension Service 2008). Oklahoma and Texas producers plant over three million acres in sorghum annually with an estimated cash value over $315 million dollars, however; this amount exceeds $1 billion when used as livestock feed (NASS 2005). Sorghum producers are also plagued by insect pests, such as greenbugs, and weed encroachment.
Producers have used insecticides to address some pest problems; however, use of these products can cause a breakdown in the life cycle of natural enemies. This breakdown can lead to pest resurgence episodes that require additional insecticide applications, which increases input costs and can cause a greater risk of insecticide resistance in pests. In an effort to reduce the negative effects associated with monocultures and increase net profits, some producers are diversifying their agricultural systems (Brewer and Elliott 2004; Keenan et al. 2005). The concept that diversification of agroecosystems increases and maintains natural enemy assemblages, which in turn increases the efficiency of these biological control agents, is supported by growing data (Parajulee and Slosser 1999; Guerena and Sullivan 2003; Brewer and Elliott 2004). Carabids constitute a major part of the fauna and are an important part of the natural enemy assemblages in agroecosystems (Fox and MacLellan 1956; Rivard 1964, 1965, 1966; Whitcomb and Bell 1964; Frank 1971; Kirk 1971; Esau and Peters 1975). Carabid richness has been positively correlated to small-scale landscape heterogeneity (Weibull et al. 2003). More complex habitats supply carabids with the necessary resources to maintain higher populations allowing colonization of crops before pest species reach economic damage levels (Hunter 2002). Diversity provides increased habitat richness, which increases the abundance and persistence of predators of agricultural pests and consumers of weed seeds. Carabids consume insect pests and weed seeds; however, carabids need a variety of habitats to complete their life cycles, including feeding grounds, breeding habitats, and over wintering refuge. Because carabids react to environmental changes quickly and measurably they may also be useful as bio-indicators as well as biological control agents (Thiele 1977; Norris and Kogan 2000; Fournier and Loreau 2002; Holland et al. 2005).
With the prevalence of monocultures, producers also combat the issue of increased weed production and ultimately weed seed production (Keenan et al. 2005). Carabids are an important consumer of weed seeds due to their polyphagous nature (Lund and Turpin 1977; Thiele 1977). Genera like Amara and Harpalus have species that selectively consume weed seeds once they fall from the parent plant to the ground. This consumption of seeds can have a major influence on seed survival and therefore on plant community composition (Tooley and Brust 2002). In agroecosystems, carabids and rodents are considered the two major weed seed predators (Brust and House 1988; Marino et al. 1997; Westerman et al. 2003). In the Central Great Plains, weeds such as volunteer rye, downy brome, and jointed goatgrass, negatively impact profitable winter wheat production (Lyon and Baltensperger 1995).
Producers need more information regarding carabid feeding behavior and its impact on pest populations in diverse systems. Additionally, producers need a better understanding of carabid dispersal and colonization abilities so that they can integrate carabids into IPM plans and habitat management decisions. Carabids have long been identified as biological control agents in scientific literature; however, recognition of carabid beetles as an important component of natural enemy assemblages is lacking among producers. As producers move toward no-till diverse agricultural systems in an effort to reduce input costs and increase net profits, it is essential that they understand the impacts of these changes on pests and natural enemies. Coupled with this diversification, a reduction of pesticide use within these systems will decrease environmental contamination and protect natural enemies, like carabids, from unnecessary mortality. A reduction in disturbance regimes such as conventional tillage will decrease carabid mortality and increase their effectiveness as biological control agents. By changing farming practices, reducing the use of broad-spectrum pesticides, and diversifying agroecosystems the economic and ecological sustainability of farms in the Southern Great Plains will be improved.
Carabid distribution is affected by abiotic factors such as soil pH, soil type, and soil moisture (Baker and Dunning 1975; Hengeveld 1979; Gruttke and Weigmann 1990; Holland et al. 2007). Evidence suggests soil moisture has the greatest influence on carabid distribution (Holopainen et al. 1995; Sanderson et al. 1995; Luff 1996). Soil composition may influence microhabitat selection for oviposition and larval burrowing (Holland and Luff 2000). Larvae have higher survival rates in moist soils and are sensitive to soil temperatures (Luff 1994; Holland 2002). Adult carabids of arable lands are susceptible to water loss and are dependent upon microhabitats for added protection from desiccation.
Carabids frequently show aggregation patterns of high and low densities based on vegetation canopy, structure, and density (Speight and Lawton 1976; Hengeveld 1979; Cárcamo and Spence 1994; Holopainen 1995; Thomas et al. 1998; Holland et al. 1999). Carabids have been found in higher numbers in weedy crops. For example, no-till fields encourage weeds thereby increasing organic material on the soil surface altering microclimates (Speight and Lawton 1976; Purvis and Curry 1984; Powell et al. 1985; Kromp 1989; Pavuk et al. 1997). The amount of crop canopy present over time may influence changes in carabid assemblages by retaining more moisture as the canopy closes. All of these environmental factors alter resource and habitat availability to all carabid life stages and ultimately leads to discrete distributional patterns within and among fields (Holland and Luff 2000; Thomas et al. 2002).
Soil cultivation is another factor that can kill or disturb carabids at all life stages. However, which life stage is most at risk remains unclear. Several reviews on farming practices have described a reduction in abundance and diversity of carabid assemblages due to deep tillage (Thiele 1977; House and All 1981; Luff 1987; Stinner and House 1990; Kromp 1999; Holland and Luff 2000). It has been reported that deep soil cultivation with a moldboard plough had a negative effect on carabid abundance compared to shallow cultivation (Dubrovskaya 1970). This finding was supported by Stassart and Grégoire-Wibo’s (1983) analysis of pitfall data over several years in Belgium where they determined the depth of tillage was a major factor affecting field carabid fauna.
It has been noted that carabids are an important component of natural enemy assemblages consuming agricultural pests. Through the use of stable carbon isotopes the relationship of carabid dispersal and dietary intake over time can be better understood (Teeri and Schoeller 1979; Boutton et al. 1983; Peterson and Fry 1987; Wada et al. 1987; Harrigan et al. 1989; Ostrom and Fry 1993). By determining the differences in isotope ratios between predators, prey, and host plants within agroecosystems the dispersal of carabids can be traced among habitats (Ostrom et al. 1997; Hobson et al. 1999). This dispersal information demonstrates seasonal habitat utilization by carabids as larvae and adults (Ostrom et al. 1997; Hobson et al. 1999). Ratio data can clarify which habitat type, feeding grounds, breeding habitats, over-wintering refuge, and non-cultivated refuge, larvae and adult carabids utilize. The use of stable carbon isotope ratios (SCIR) reflects dietary information over time determining carabid prey selection and dispersal power (Peterson and Fry 1987; Hobson and Clark 1992). By understanding the environmental requirements of carabids, their presence in diversified agricultural habitats can be enhanced.
Isotope Definition and Measurements:
Elements exist in nature as one or more isotopes. Isotopes are defined as atoms of the same element which have the same number of protons and electrons but different numbers of neutrons. These isotopes will have the same charge but different masses. It is this difference in mass that can be exploited for scientific study and since their discovery in the 1920’s, ecological and biological studies have been using isotopic compositions at an increasing rate.
Fractionation is the term applied to isotopic variance and defined as the enrichment or depletion of a heavy isotope relative to a light (low mass) isotope (Broecker and Oversley 1976; Tieszen and Boutton 1989). Fractionation is the proportional difference between the isotopes’ masses. These proportional differences represent very small changes in the physical and chemical properties of each isotope within tissues (Park and Epstein 1960; Broecker and Oversley 1976; Ehleringer and Rundel 1989). Enzymatic discrimination within tissues is the utilization of one isotope and not the other or the use of one isotope before another isotope (Ehleringer and Rundel 1989). Isotopic composition absolute values can be measured accurately within a sample over the short-term; however, reliability over the long term is questionable (Hayes 1983). To provide high accuracy and repeatability over time, differences between a standard and sample must be measured (McKinney et al. 1950; Ehleringer and Rundel 1989). Differential analysis has been a standard procedure in isotope compositions since its introduction (McKinney et al. 1950). The reference material for carbon was the carbon found in the PeeDee limestone (belemnite, PDB); however, this material is now depleted. The current standard for carbon is the equivalent Vienna PeeDee Belemnite (VPDB) standard (Clark and Fritz 1997; Kendall and Caldwell 1998). Use of VPDB indicates the standard has been calibrated to 0‰ according to the International Atomic Energy Agency (IAEA) guidelines (Coplen 1996). Expression of isotopic composition uses differential notation, in other words, terms of ? values (parts per thousand differences from a standard):
?Xstd = [(Rsample/Rstandard) –1] x 1000,
where X is 13C, the isotope ratio reported in delta units relative to a standard; Rsample/Rstandard is the absolute isotope ratios of the sample and standard, 13C/12C (Peterson and Fry 1987; Ehleringer and Rundel 1989; Hobson et al 1994). Multiplying by 1000 (‰) expresses values as “parts per thousand” or “per mil” allowing very small differences between samples to be examined more clearly (Peterson and Fry 1987; Ehleringer and Rundel 1989).
Isotope Relationship between Host Plants, Aphids, and Carabids:
Plants convert sunlight, water, and carbon dioxide to organic materials thereby storing sunlight as usable energy within plant tissues. Plants use two distinct pathways to accomplish this energy conversion. One photosynthetic pathway used by plants (e.g. alfalfa) produces three-carbon molecules and is called the Calvin cycle (C3). Other plants (e.g. sorghum) use the alternative Hack-Slack pathway (C4) which produces four-carbon molecules. C3 and C4 plants have distinctly different carbon isotope ratios that provide a predictive relationship with ?13C values as 13C depletion continues (Bender 1968). In C3 plants, the accumulated levels of 12C are higher than 13C (–20 to –35‰) compared to atmospheric CO2 (ca. –7.7‰). C4 plants have measurably higher 13C levels (–9 to –14‰) compared to C3 plants. Alfalfa (C3) and sorghum (C4) have specific aphid species that only feed on these particular plants and should reflect the isotope ratios of their host plants.
Animal tissues that reflect a predictable carbon isotope enrichment or depletion rate when compared to dietary intake are used to reconstruct diet histories. Evidence of migration between isotopically discrete food webs can be retained in animal tissues for a period of time depending on elemental turnover rates. Stable isotopes are fractionated (enrichment or depletion) through the enzymatic transformation and assimilation of food within animal tissues. DeNiro and Epstein (1978) used mice to demonstrate that ?13C values were similar in whole-animal vs. bulk dietary intake. However, they found that ?13C values in tissues differed in a sequential pattern. Tieszen et al. (1983) verified these results using gerbils. Tieszen et al. (1983) demonstrated that by switching the diet from corn (C4) to wheat (C3) carbon replacement in gerbil tissues was dependent on tissue type (i.e. liver half-life = 6.4 days vs. muscle half-life = 27.6 days). These rates are dependent on fast or slow turnover of isotopic compositions. Carabid flight muscles and soft organs are metabolically active and can reflect recent dietary turnover of carbon isotopes in a short period of time. In contrast, carabid elytra, wings, and pronotal exoskeleton are basically metabolically inactive. Therefore, these tissues retain carbon isotope compositions from the beetles past dietary intake. These inactive components retain larval compositions indicating natal origins.
Examination of carabid movement or dispersal based on carbon isotope ratios can only be done within systems with distinct 13C sources (Prasifka and Heinz 2004). Aphids will exhibit isotopic signatures of the crop type (C3 or C4) they are consuming. Consequently, carabids will reflect the isotopic signatures of aphids preyed upon. Boutton et al (1983) demonstrated termite preferences for C3or C4 plants at two locations in the grasslands of East Africa. This work demonstrated that within colonies termites focused on one vegetation type while between locations the vegetation utilized varied. Using alfalfa (C3) and sorghum (C4) as isotopically discrete habitats, carabid dispersal patterns can be reconstructed.
1.Quantify carabid colonization of annual crops (sorghum/winter wheat) from a semi-permanent habitat (alfalfa) as it relates to disturbance (tillage);
2.Elucidate carabid dispersal powers through prey selection, diet changes within and among habitats, and larval habitat utilization;
3.Provide results of this research to producers and IPM professionals.