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Influence of farming transition systems on ground beetles (Coleoptera: Carabidae) communities

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Ground beetles (Coleoptera, Carabidae) occur in all temperate agroecosystems, and have been implicated as predators of many pests, including aphids, lepidopterous larvae, and slugs. Most are polyphagous, and some are primarily spermophagous. The species assemblage present in any particular crop is determined by multiple factors, but usually comprises a limited number of abundantly active species, which may be common to many crop types. Abiotic soil factors, especially soil type and moisture status are important in determining the species present. Crop type affects the carabid assemblage indirectly through cultivation practices and microclimatic changes. Any soil cultivation affects the carabid assemblage, but studies comparing ploughing with reduced tillage have shown varying results, according to local conditions. Pesticides, especially insecticides have a localised and short-term effect, as many carabids rapidly re-invade sprayed crops. The long-term effect of pesticide usage at a landscape scale is, however, more difficult to predict, and may have contributed to the observed decline in carabid diversity in the wider countryside. Whilst fertiliser application is generally beneficial to carabids, comparisons of conventional and organic farming systems suggest that localised short-term variations in species’ abundances are more important than the overall farming system used. Non-crop habitats are very important to Carabidae, as many use adjacent hedges and field margins for shelter, breeding or dispersal. But other features such as roads may act as barriers to dispersal. It is concluded that further measures need to be taken if Carabidae are to realise their potential in integrated pest management systems.
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Habitat preferences of generalist predators of Pseudaletia unipuncta [Mythimna unipuncta] and Agrotis ipsilon were evaluated by comparing their abundance among 4 reduced-tillage maize systems in Virginia which differed in the degree of soil disturbance, quantity and structure of the surface mulch due to tillage, and cover crop management practices. Two sampling methods were used to collect predators: pitfall trapping and vacuum sampling. Although there was considerable difference in the composition of species collected with each method, similar trends in overall predator abundance were observed. Generalist predator abundance followed the gradient of ground cover. The treatment with the highest degree of mulch ground cover had the highest overall predator abundance while the treatment which was disked and without surface mulch had the lowest. Although most of the common species preferred those systems with the most ground cover, several species preferred the system with the least amount of ground cover.
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Four different corn, soybean, and wheat cropping systems were established in small replicated plots and simultaneously in larger (4-hectare) unreplicated fields. Each system was subject to distinct tillage practices, fertility programs, and methods of pest control, based on methods currently in use on farms in the Mid-Atlantic region of the United States. Ground-dwelling arthropods (mostly Carabidae and spiders) were sampled during the fourth and sixth growing season after the establishment of the plots, and foliar insects (pest and beneficial) and pest damage were sampled on corn in the sixth growing season. Overall, beneficial arthropod populations were lowest and corn pest insect populations (especially Western corn rootworms, Diabrotica virgifera virgifera LeConte) were highest in the simplest, most intensively managed continuous corn system, which had annual use of soil insecticides. Generally ground-dwelling species were higher in soybeans than in corn, and in no-till than in deep-tilled crops. Growers wishing to enhance populations of beneficial insects should consider predominantly no-till cropping systems with several different crops in the rotation and minimal insecticide use. For both ground and foliar sampling, patterns of abundance among systems and crops in the small replicated plots generally followed those observed in the large fields, but numbers of spiders and carabids collected per pitfall trap were generally much higher in the large fields.
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Ground beetle assemblages were compared in organic, no-till, and chisel-till cropping systems of the USDA Farming Systems Project in Maryland. The cropping systems consisted of 3-yr rotations of corn (Zea mays L.), soybean (Glycine max L. Merr.), and wheat (Triticum aestivum L.) that were planted to corn and soybean during the 2 yr of field sampling (2001–2002). Each year, ground beetles were sampled using pitfall traps during three 9- to 14-d periods corresponding to spring, summer, and fall. A total of 2,313 specimens, representing 31 species, were collected over the 2 yr of sampling. The eight most common species represented 87% of the total specimens collected and included Scarites quadriceps Chaudoir, Elaphropus anceps (LeConte), Bembidion rapidum (LeConte), Harpalus pensylvanicus (DeGeer), Poecilus chalcites (Say), Clivina impressefrons LeConte, Agonum punctiforme (Say), and Amara aenea (DeGeer). Canonical variates analysis based on the 10 most abundant species showed that the carabid assemblages in the three cropping systems were distinguishable from each other. The organic system was found to be more different from the no-till and chisel-till systems than these two systems were from each other. In 2002, ground beetle relative abundance, measured species richness, and species diversity were greater in the organic than in the chisel-till system. Similar trends were found in 2001, but no significant differences were found in these measurements. Relatively few differences were found between the no-till and chisel-till systems. The estimated species richness of ground beetles based on several common estimators did not show differences among the three cropping systems. The potential use of ground beetles as ecological indicators is discussed.
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The infuence of farm management practices on ground-dwelling natural enemy communities and predation of insects and weed seeds was investigated over the first 2 years of the transition from conventional to organic production. Three transition strategies were selected that differed in their management and input intensities, and were characteristic of pasture/ley systems (low intensity), cash grain systems (intermediate intensity), and vegetable production (high intensity). Benefcial arthropods (insectivores and granivores) were monitored using pitfall (arthropod activity) and quadrat (arthropod density) samples. The frequency of predation on restrained larvae of Galleria mellonella and the species observed feeding were recorded. Weekly removal rates of weed seeds representative of abundant species at our site were monitored over a 3-week period during fall. Management intensity affected the activity and abundance of biological control agents. In year two of the transition, biological control agent densities were higher in the low-intensity treatment than in the other two treatments, but activity of insectivores and granivores was reduced in this treatment relative to the higher intensity systems. The patterns in the abundances of biological control agents may be explained by habitat stability within the different cropping systems. Quadrat samples were strongly correlated with the insectivory index, although pitfall samples were not. Insectivory rates were highest (>80% of G. mellonella larvae) in the low-intensity treatment. Predation patterns over a 17-h period differed substantially among the management treatments, indicating behaviorally distinct insectivore communities. Seed removal was also highest in the low-intensity treatment. We conclude that low-intensity cropping systems are most favorable to the abundance and function of benefcial ground-dwelling arthropod communities (insectivores and granivores) during the transition process.
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Many agroecosystems are unfavorable environments for natural enemies due to high levels of disturbance. Habitat management, a form of conservation biological control, is an ecologically based approach aimed at favoring natural enemies and enhancing biological control in agricultural systems. The goal of habitat management is to create a suitable ecological infrastructure within the agricultural landscape to provide resources such as food for adult natural enemies, alternative prey or hosts, and shelter from adverse conditions. These resources must be integrated into the landscape in a way that is spatially and temporally favorable to natural enemies and practical for producers to implement. The rapidly expanding literature on habitat management is reviewed with attention to practices for favoring predators and parasitoids, implementation of habitat management, and the contributions of modeling and ecological theory to this developing area of conservation biological control. The potential to integrate the goals of habitat management for natural enemies and nature conservation is discussed.
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Ground beetles are well known as beneficial organisms in agroecosystems, contributing to the predation of a wide range of animal pests and weed seeds. Tillage has generally been shown to have a negative effect on ground beetles, but it is not known whether this is because of direct mortality or the result of indirect losses resulting from dispersal caused by habitat deterioration. In 2005, field experiments measured direct, tillage-induced mortality, of four carabid weed seed predators, Harpalus rufipes DeGeer, Agonum muelleri Herbst, Anisodactylus merula Germar, and Amara cupreolata Putzeys, and one arthropod predator, Pterostichus melanarius Illiger, common to agroecosystems in the northeastern United States. Three tillage treatments (moldboard plow, chisel plow, and rotary tillage) were compared with undisturbed controls at two sites (Stillwater and Presque Isle) and at two dates (July and August) in Maine. Carabid activity density after disturbance was measured using fenced pitfall traps installed immediately after tillage to remove any effects of dispersal. Rotary tillage and moldboard plowing reduced weed seed predator activity density 52 and 54%, respectively. Carabid activity density after chisel plowing was similar to the undisturbed control. This trend was true for each of the weed seed predator species studied. However, activity density of the arthropod predator P. melanarius was reduced by all tillage types, indicating a greater sensitivity to tillage than the four weed seed predator species. These results confirm the need to consider both direct and indirect effects of management in studies of invertebrate seed predators.
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Ground beetles (Coleoptera: Carabidae) are important in agro-ecosystems as generalist predators of invertebrate pests and weed seeds and as prey for larger animals. However, it is not well understood how cropping systems affect ground beetles. Over a 2-yr period, carabids were monitored two times per month using pitfall traps in a conventional chemical input, 2-yr, corn/soybean rotation system and a low input, 4-yr, corn/soybean/triticale-alfalfa/alfalfa rotation system. Carabid assemblages were largely dominated by a few species across all cropping treatments with Poecilus chalcites Say comprising >70% of pitfall catches in both years of study. Overall carabid activity density and species richness were higher in the low input, 4-yr rotation compared with the conventionally managed, 2-yr rotation. There were greater differences in the temporal activity density and species richness of carabids among crops than within corn and soybean treatments managed with different agrichemical inputs and soil disturbance regimes. Detrended correspondence analysis showed strong yearly variation in carabid assemblages in all cropping treatments. The increase in carabid activity density and species richness observed in the 4-yr crop rotation highlights the potential benefits of diverse crop habitats for carabids and the possibility for managing natural enemies by manipulating crop rotations.
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Best Management Practices (BMPs) have been developed by state regulatory agencies to reduce nitrate non-point source pollution in the St. Johns River watershed from potato production in the Tri-County Agricultural Area (TCAA) near Hastings, Florida. A project was developed to determine whether legumes planted as summer cover crops and/or fall cash crops could supply nitrogen to a spring potato crop reducing the need for inorganic nitrogen fertilizer. The experiment was designed as a 2 x 2 x 5 factorial design with four replications. Summer cover crop main factors were sorghum-sudan grass (Sorghum vulgare x Sorghum vulgare var. sudanese, var. SX17) and cowpea (Vigna unguiculata, var. Iron Clay). Fall treatment factors were green bean (Phaseolus vulgarus, var. Stallion) and fallow. Potatoes (Solanum tuberosum L., var. Atlantic) were planted on 28 February, 2001 and fertilized at five nitrogen rates (0, 112, 168, 224, 280 kg/ha). Potatoes were irrigated with seepage irrigation during the season. Potatoes were harvested and graded with commercial equipment into five size classes on 1 June, 2001. Total potato yield for the sorghum and cowpea main cover crop plots were not significantly different at 31.8 and 31.4 MT/ha, respectively. Total tuber yield for the green bean and fallow fall treatments were not significantly different at 31.8 and 29.6 MT/ha, respectively. However, potato plants in the green bean-0 kg N ha-1 treatment produced approximately 10 Mt ha-1 more total and marketable tuber yield than the fallow-0 kg ha-1 treatment. Although this was the first year of a three-year study and a transitional period between a traditional system and an alternative system, results suggest that legumes planted in sequence with potatoes may help growers reduce dependence on inorganic nitrogen sources without sacrificing production.
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The hypothesis that generalist predators limit associated prey populations was tested using spiders, a group classically thought to be an insignificant component of the agroecosystem. Two habitat manipulations (addition of mulch and flowers) in separate and combined treatments were utilized to enhance spider numbers in a mixed vegetable system. Compared to control plots, significantly higher spider densities were observed in the plots to which mulch alone or both mulch and flowers had been added. Insect damage to the plants was significantly lower in the plots to which mulch had been added as well, which correlates with the lower numbers of pest insects in plots containing mulch. That spiders were the probable cause of the effect was demonstrated in plots containing both mulch and flowers but from which spiders had been systematically removed. Removing spiders from plots with mulch and flowers removed the effect of this treatment on pest numbers and plant damage. In addition, 84% of the predators observed foraging in the study areas during timed watches were spiders and 98% of the predation events observed were by spiders. In a separate experiment, spiders were added to a subset of individually bagged broccoli plants that had been infested previously with known numbers of herbivorous pests. Plant damage in bags lacking spiders averaged 93.3%, whereas damage averaged 31.8% in bags to which spiders had been added.
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(1) Pitfall traps were used to investigate the effect of weeds and farmyard manure application on arthropod activity in sugar beet, giving particular attention to the activity of predators during the seedling stage of the crop. (2) The most marked increases in both species richness and abundance accompanied weed establishment, well after sowing, on plots not treated with herbicides. These increases were greatest amongst detritivore and weed specific herbivore species, although predatory Staphylinidae and parasitic Hymenoptera had similar responses. (3) Carabidae were the most abundant predators caught but their numbers were not affected obviously by weeds. Farmyard manure application increased the activity of species common early in the season such as Pterostichus strenuus (Panzer) and Bembidion lampros (Herbst) and resulted in an immediate but temporary increase in general species diversity. (4) Weed growth in the crop would not help reduce seedling pest losses because predator responses to weed establishment were too slow. Manure application immediately encouraged beneficial arthropods and seems, potentially, a useful and practical means of promoting natural control of injurious species during crop establishment.
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Data of comparative studies about carabid beetles in organically and conventionally managed winter cereal fields of central Europe, using the pitfall trapping method, were collected from the literature and unpublished data sources and were then pooled and analysed. According to an index, which was designed to calculate how much a species benefits from organic management, Carabus auratus turned out to benefit most. Some Amara species (A. familiaris, A. similata and A. aenea) as well as Pseudoophonus rufipes and Harpalus affinis also showed high index values. When analysing the traits of the carabids, the habitat preference was the most important variable for the differentiation of organic and conventional management. The stronger the preference for open field, the more the species are supported by organic agriculture. For the promotion of the agricultural carabid fauna it is suggested that weedier and less densely cropped fields be tolerated.
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The abundance of ground beetles (Coleoptera: Carabidae) was compared on 4 pairs of conventional and organic farms. All farms compared had identical locations, soil types, and cropping histories, but differed in that one farm in each pair was conventionally managed; the other, organically (no use of commercial fertilizers or pesticides). During the summer sampling period, organic farms had significantly greater numbers of carabids captured in pitfall traps in 3 of 4 pairs. Differences in abundance during this time ranged from 20 percent to almost 7-times greater on organic farms. Organic farms also had about twice the number of species found on conventional farms, but had approximately the same level of diversity as measured by the Shannon-Wiener index.
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Metal enclosures were used to confine black cutworm (BCW),Agrotts ipsilon (Hufnagel), larvae in different corn agroecosystems to ascertain the effects of cropping systems (tillage and previous crop), soil-insecticide (phorate) use and predator removal on BCW damage to corn. Absolute-density estimates of predaceous soil arthropods were taken in each treatment. Treatments in the conventional tillage corn following soybean system had significantly more cut plants and fewer predators than plots for the no-tillage corn following soybean system. Treatments where soil insecticide was applied contained significantly more cutworm-damaged corn plants and fewer predators than areas not receiving the soil insecticide. Enclosures where predators were removed had significantly more cut corn plants than enclosures where predators were not removed. The use of tillage and a soil insecticide significantly increased the number of cut plants when predators were not removed. However, the use of tillage and of the soil insecticide did not significantly increase the nl\mber of damaged plants when predators were removed. This study showed that the endemic soil-predator complex is a major factor in reducing BCW damage in corn agroecosystems.
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Rye, Secale cereale L., used as a winter cover crop was killed by the herbicide paraquat or by mowing with a rotary mower. In subsequent no-till corn, Glyptapanteles militaris (Walsh) (Hymenoptera: Braconidae) and Periscepsia laevigata (Wulp) (Diptera: Tachinidae) were the most abundant of twelve species of parasitoids that emerged from field-collected larvae of the armyworm, Pseudaletia unipuncta (Haworth). No effects of cover crop suppression practices were detected for parasitism rates for any individual species or for total armyworm parasitism. Seasonal parasitism rates ranged from 32 to 45%. Higher numbers of Pterostichus spp. and Scarites spp. (Coleoptera: Carabidae), and wolf spiders (Araneae: Lycosidae) occurred early in the corn season in the mowed cover crop treatment compared with the herbicide killed cover crop treatment. Subsequent reduction of larval densities of amryworm in mowed plots following higher predator densities suggests the role of these generalist predators in biological control of armyworm.
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Seasonality and habitat preference of 34 species of carabid beetles were investigated in soybean, Glycine max Merrill, agroecosystems during 1978-79. Habitats included conventional and conservation tillage systems of soybeans and adjacent areas of a fescue pasture, old field (mixed annual grasses and herbaceous plants), and mixed hardwood-pine woodlot. Pitfall trapping indicated greatest carabid abundance in mid-spring and late summer. Populations often were several-fold greater in conservation than conventional tillage soybeans. Harpalus pensylvanicus DeG. was the dominant species in all habitats, except the conventionally tilled soybeans. Mark/recapture experiments indicate carabid movement from field and fescue border areas into soybean fields of either type tillage system.
Article
Winter legume and grain cover crops preceding corn, Zea mays L., grown using conventional and no-tillage methods were investigated for their effect on population dynamics and community structure of soil arthropods. Hairy vetch, Vicia villosa Roth, supported higher below-ground arthropod population densities and a more taxonomically diverse fauna than crimson clover, Trifolium incarnatum L., or wheat, Triticum aestivum L. Pest and beneficial soil arthropods were most abundant in no-tillage corn preceded by hairy vetch. Diversity of soilarthropod species was higher under no-tillage than conventional tillage. Divergences in community structure of soil arthropods among cover crop species, evident early in the season, dissipated by midseason. Arthropod predators were more numerous in no-tillage than conventional tillage systems regardless of previous cover crops. Although no-tillage practices promoted a more trophically balanced soil arthropod community than conventional tillage during early and mid season, in 1987 seedling corn plants in no-tillage vetch treatments sustained significantly higher (P < 0.05) damage from the southern corn rootworm, Diabrotica undecimpunctata howardi Barber, than in other treatments. Tillage system preference was shown by herbivores: Seedcorn maggot, Delia platura (Meigen), occurred in large numbers in conventional tillage, and southern corn rootworm populations were high in no-tillage, especially following legume cover crops.
Article
Giant ragweed seeds have high nutritional value, consisting of 47% crude protein and 38% crude fat, and may be an important food source for rodent and invertebrate populations in agricultural and early successional ecosystems. We investigated temporal patterns of postdispersal giant ragweed seed predation on the soil surface of a no-tillage cornfield as affected by involucre (seed dispersal unit) size and presence or absence of crop residue. Cage exclusion experiments indicated that rodents and invertebrates were the principal predators of giant ragweed seed, and total predation of involucres over a 12-mo period beginning in November was 88%. Rodents were the greatest predators of giant ragweed involucres during fall and winter, and cumulative predation by February 1 in treatments with rodent access ranged from 39 to 43%. In contrast, giant ragweed involucre predation by invertebrates occurred mainly from May 1 to November 1. When rodent access to involucres was prevented, total involucre predation by invertebrates over a 12-mo period ranged from 57 to 78%. Rodents showed an initial preference for large involucres (> 4.8-mm diameter), and invertebrates preferred small involucres (< 4.8-mm diameter). Involucres covered with corn plant residue underwent less predation by rodents from November to February than uncovered involucres, but residue cover had no effect on seed predation by invertebrates. In a laboratory feeding trial, the carabid Harpalus pensylvanicus preferred seed of smooth pigweed and yellow foxtail to giant ragweed seed, suggesting that giant ragweed seed is an incidental rather than a preferred food source for some carabids. Because giant ragweed exhibits relatively low fecundity and short seed bank persistence, results of this study suggest that postdispersal predation may directly reduce giant ragweed recruitment the next year by reducing new seed bank inputs. However, seed losses from predation alone may be insufficient to maintain giant ragweed populations below economic threshold levels in no-tillage cornfields. Nomenclature: Giant ragweed, Ambrosia trifida L. AMBTR; smooth pigweed, Amaranthus hybridus L. AMACH; yellow foxtail, Setaria glauca L. Beauv. SETLU; corn, Zea mays L. ‘DK 595’.
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Comparisons of catches of beetles in standard pitfall traps and various patterns of barrier pitfall traps were made in open grassy woodland in Victoria, Australia, in late summer. Comparative results are given for abundance and morphospecies richness of total Coleoptera, and for Carabidae as a more limited focal group. For both categories, in three separate trials (1) use of 120cm barriers gave larger catches than standard traps (1360 individual beetles [503 Carabidae] in barrier traps, 363 [57 Carabidae] in standard traps, pool of 129 morphospecies); (2) catch sizes increased almost 5-fold with increasing barrier length (0, 30, 60, 120cm) (61 morphospecies); and (3) use of a five-trap system with barriers gave higher numbers (3344 and 2303 total beetles, 345 and 135 Carabidae) than the same configuration without barriers (92 morphospecies).
Article
We compared the efficacies of two arthropod pitfall trapping techniques: live (dry) trapping and kill trapping with three killing agents (water, ethylene glycol, and the recently developed propylene glycol, whose efficacy has not been previously assessed). Kill pitfall traps caught more species than did live pitfalls. Forty-one species were collected only from kill traps (3 being unique to water, 11 to ethylene glycol, and 8 to propylene glycol), 12 were collected only from live traps, and 32 were collected from both kill and live traps. The same average number of individuals per species was caught for most of those taxa that were collected in both trap types, indicating that better retention of captured arthropods by the killing agent was not responsible for the differences observed in the two pitfall trapping methods. There were no significant differences in captures between propylene and ethylene glycol traps or between water and live traps. Because of species-specific differences in the efficiencies of live and kill pitfall trapping, cross-study entomological comparisons made using kill pitfall trapping and live pitfalling may be confounded.
Article
This review article on carabids in sustainable agro-ecosystems of the temperate Northern hemisphere presents a compilation of the available knowledge on the significance of carabids for natural pest control and the effects of cultivation methods (except pesticides) and landscape structural elements.Field carabids are species rich and abundant in arable sites, but are affected by intensive agricultural cultivation. For sampling, fenced pitfall trapping or pitfall trapping is recommended according to the type of study.Many of the assumed beneficial pest control activities of carabids are still based on laboratory feeding records. In the field, carabids have been demonstrated to reduce cereal and sugar beet aphid populations in their early colonization phase, mainly by foraging on aphids that have fallen from the vegetation. Egg predation on Dipteran eggs, e.g. the cabbage root fly, has been overestimated in earlier literature. Scattered data indicate carabidforaging on certain coleopteran pest larvae. In North America, some evidence has been found for control of pest lepidopterans. Larger carabids, e.g. Abax parallelepipedus, can effectively control slugs in greenhouses. Because of their spermophagous feeding habits, certain species of Harpalus and Amara could have some potential for biological weed control.As a result of their sensitive reaction to anthropogenic changes in habitat quality, carabids are considered of bioindicative value for cultivation impacts. Carabids seem to be negatively affected by deep ploughing and enhanced by reduced tillage systems. No negative effects have been found for mechanical weed control and flaming. Carabid recruitment is enhanced by proper organic fertilization and green manuring. Intensive nitrogen amendment might indirectly affect carabids by altering crop density and microclimate.Field carabid assemblages are not bound to a certain crop type, but shift in dominance according to the crop-specific rhythmicity of cultivation measures and changes in crop phenology and microclimate. Crop rotation effects could also be influenced by field-size dependent recolonization capability of carabids. They are enhanced by crop diversification in terms of monocrop heterogeneity and weediness as well as by intercropping and the presence of field boundaries, although corresponding increases in their pest reduction efficacy have not yet been evidenced.
Article
We examined whether predator interference could prevent effective conservation biological control of Delia spp. flies, important pests of cole crops, by an assemblage of carabid and staphylinid beetles. In laboratory feeding trials we found that the smaller (<1 cm) beetle species common at our site readily ate dipteran eggs, while the most common large carabid species, Pterostichus melanarius, rarely did. However, P. melanarius did eat several of the smaller beetle species. We conducted two field experiments where we manipulated immigration rates of the ground predator guild and then measured predation on fly eggs. Predation rates were consistently higher in cages where predators were added at ambient densities, compared to cages where ground predators were removed. However, in the second field experiment, when we quadrupled predator immigration rates neither beetle activity-density nor predation rate increased. High immigration rate plots had a higher proportion of P. melanarius in the predator community, compared to plots with beetles added at ambient densities, suggesting that P. melanarius was reducing activity-densities of the smaller beetles, perhaps through intraguild predation. Thus, tactics to improve the biological control of Delia spp. by conserving generalist predators, such as providing in- or extra-field refuges, could be thwarted if the primary predators of fly eggs, small carabids and staphylinids, are the targets of intraguild predation by also-conserved larger predators.
Article
Collards were grown at Ithaca, New York, in two experimental habitats: pure stands and single rows that were bounded on each side by diverse, meadow vegetation. The arthropods associated with these plants were sampled on 20 dates over a 3-year period. The status of the herbivore species was measured by their rank in biomass in each sample. The two most prominent species, Phyllotreta cruciferae and Pieris rapae, maintained high status throughout the investigation, but another important species, Brevicoryne brassicae, was absent for an entire season. Pit feeders usually formed the most important herbivore guild. Nevertheless, the guild spectrum, which describes the functional structure of the fauna, varied widely in time and space. The size distributions of species and of individuals were both highly skewed toward the smaller sizes. Herbivore loads, the mean biomass of herbivores per 100 g of consumable foliage, were consistently higher in the pure stands. Moreover, herbivore loads varied significantly with season in each experimental habitat. Both the number of herbivore species and the diversity of the herbivore load were greater in the diverse habitat. Biomass was more heavily concentrated among the prominent herbivores in the pure stands; increased dominance, rather than differences in species richness, appeared to be the major cause for the lower herbivore diversity in this habitat. The diversity of predators and parasitoids was higher in the pure stands. Most of the abundant species found on collards shared a similar narrow range of hosts. As a result the species in this core group of herbivores and parasitoids were regularly associated with each other. Predators and the less abundant herbivores tended to be less specialized and served to link the collard association with the surrounding community. Plant-arthropod associations are representative of component communities, well-integrated systems that form portions of larger compound communities. This distinction facilitates the analysis of community structure. Microclimates and the effectiveness of @'enemies@' did not appear to differ sufficiently in the two experimental habitats to account for the observed differences in the herbivore load. The results suggest a new proposition, the resource concentration hypothesis, which states that herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands; that the most specialized species frequently attain higher relative densities in simple environments; and that, as a result, biomass tends to become concentrated in a few species, causing a decrease in the diversity of herbivores in pure stands.
Article
The ground beetles from the speciose beetle family Carabidae and, since their emergence in the Tertiary, have populated all habitats except deserts. Our knowledge about carabids is biased toward species living in north-temperate regions. Most carabids are predatory, consume a wide range of food types, and experience food shortages in the field. Feeding on both plant and animal material and scavenging are probably more significant than currently acknowledged. The most important mortality sources are abiotic factors and predators; pathogens and parasites can be important for some developmental stages. Although competition among larvae and adults does occur, the importance of competition as a community organization is not proven. Carabids are abundant in agricultural fields all over the world and may be important natural enemies of agricultural pests.
Dustin Keys, and Nick Teraberry for collecting, sorting and pinning ground beetles. The authors also thank Greg Harris, Emilie Haguewood, and R. D. Offutt, for providing potato seed and field sites in Boardman. The authors recognize the assistance of
  • Special
  • Amanda To
  • Rebekah Smith
  • Adams
acknowledgmentS Special thanks to Amanda Smith, Rebekah Adams, Dustin Keys, and Nick Teraberry for collecting, sorting and pinning ground beetles. The authors also thank Greg Harris, Emilie Haguewood, and R. D. Offutt, for providing potato seed and field sites in Boardman. The authors recognize the assistance of George E. Ball (University of Alberta, Canada), Robert Davidson (Carnegie Museum of Natural History, Pittsburg, Pennsylvania),
An analysis of the carabid beetle fauna of the refugium The Kodiak Island refugium: Its geology, flora, fauna, and history The influence of organic transition systems on beneficial ground-dwelling arthropods and predation of insects and weed seeds
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lindRotH, c. H. 1969. The ground-beetles (Carabidae, Excl. Cicindelinae) of Canada and Alaska. Entomlogiska Sallskapet, Lund, Sweden. lindRotH, c. H., 1969. An analysis of the carabid beetle fauna of the refugium, pp. 195-210 In T. N. V. Karlstrom and G. E. Ball [eds.], The Kodiak Island refugium: Its geology, flora, fauna, and history. Ryerson Press; Toronto 262 p. + i-xii. lundgRen, j. g., SHaw, j. t., zaboRSki, e. R., and eaStman, c. e. 2006. The influence of organic transition systems on beneficial ground-dwelling arthropods and predation of insects and weed seeds. Renew. Agric. Food Syst. 21: 227-237.
Dustin Keys, and Nick Teraberry for collecting, sorting and pinning ground beetles. The authors also thank
  • Amanda Thanks
  • Rebekah Smith
  • Adams
thanks to Amanda Smith, Rebekah Adams, Dustin Keys, and Nick Teraberry for collecting, sorting and pinning ground beetles. The authors also thank
Museum of the North) for taxonomical assistance. Thanks also to Arnold Appleby (OSU Crop and Soil Science) and Andrew Jensen (Washington Potato Commission)
  • Greg Harris
  • Emilie Haguewood
  • R D Offutt
  • Robert Davidson
Greg Harris, Emilie Haguewood, and R. D. Offutt, for providing potato seed and field sites in Boardman. The authors recognize the assistance of George E. Ball (University of Alberta, Canada), Robert Davidson (Carnegie Museum of Natural History, Pittsburg, Pennsylvania), Christopher J. Marshall, (Oregon State Arthropod Collection, Corvallis, Oregon), and Derek Sikes (University of Alaska, Museum of the North) for taxonomical assistance. Thanks also to Arnold Appleby (OSU Crop and Soil Science) and Andrew Jensen (Washington Potato Commission) for reviewing early versions of the manuscript. RefeRenceS cited
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