The Role of Natural Enemy Foraging Guilds in Controlling Cereal Aphids in Michigan Wheat

Insect natural enemies (predators and parasitoids) provide important ecosystem services by suppressing populations of insect pests in many agricultural crops. However, the role of natural enemies against cereal aphids in Michigan winter wheat (Triticum aestivum L.) is largely unknown. The objectives of this research were to characterize the natural enemy community in wheat fields and evaluate the role of different natural enemy foraging guilds (foliar-foraging versus ground-dwelling predators) in regulating cereal aphid population growth. We investigated these objectives during the spring and summer of 2012 and 2013 in four winter wheat fields on the Michigan State University campus farm in East Lansing, Michigan. We monitored and measured the impact of natural enemies by experimentally excluding or allowing their access to wheat plants infested with Rhopalosiphum padi (L.) and Sitobion avenae (F.) (Hemiptera: Aphidae). Our results indicate that the natural enemy community in the wheat fields consisted mostly of foliar-foraging and ground-dwelling predators with relatively few parasitoids. In combination, these natural enemy groups were very effective at reducing cereal aphid populations. We also investigated the role of each natural enemy foraging guild (foliar-foraging versus ground-dwelling predators) independently. Overall, our results suggest that, in combination, natural enemies can almost completely halt early-season aphid population increase. Independently, ground-dwelling predators were more effective at suppressing cereal aphid populations than foliar-foraging predators under the conditions we studied. Our results differ from studies in Europe and the US Great Plains where foliar foraging predators and parasitoids are frequently more important cereal aphid natural enemies.     

Adjacent Habitat Influence on Stink Bug (Hemiptera: Pentatomidae) Densities and the Associated Damage at Field Corn and Soybean Edges

The local dispersal of polyphagous, mobile insects within agricultural systems impacts pest management. In the mid-Atlantic region of the United States, stink bugs, especially the invasive Halyomorpha halys (Stål 1855), contribute to economic losses across a range of cropping systems. Here, we characterized the density of stink bugs along the field edges of field corn and soybean at different study sites. Specifically, we examined the influence of adjacent managed and natural habitats on the density of stink bugs in corn and soybean fields at different distances along transects from the field edge. We also quantified damage to corn grain, and to soybean pods and seeds, and measured yield in relation to the observed stink bug densities at different distances from field edge. Highest density of stink bugs was limited to the edge of both corn and soybean fields. Fields adjacent to wooded, crop and building habitats harbored higher densities of stink bugs than those adjacent to open habitats. Damage to corn kernels and to soybean pods and seeds increased with stink bug density in plots and was highest at the field edges. Stink bug density was also negatively associated with yield per plant in soybean. The spatial pattern of stink bugs in both corn and soybeans, with significant edge effects, suggests the use of pest management strategies for crop placement in the landscape, as well as spatially targeted pest suppression within fields.     

Hibernation of predatory arthropods in semi-natural habitats

Non-crop habitats provide important resources for natural enemies. Many natural enemies hibernate in non-crop habitats, from which they may colonise arable fields in the spring. Spring colonisation ensures annual repopulation of the crop with natural enemies, allowing them to keep pace with the development of pest populations. The availability of non-crop habitats can, therefore, be crucial to successful conservation biological control. We quantified the density of overwintering natural enemies near organic Brussels sprout crops in five different non-crop habitats (short grassy field margin, herbaceous field margin, herbaceous field margin under a tree line, ditch and forest). Soil and litter samples of non-crop habitats were taken at two sites. One site was located in an open agricultural landscape, the other in a landscape dominated by mixed forest. Insects belonging to Staphylinidae, Araneae, Carabidae, Coccinellidae and Dermaptera were the most abundant. Mean densities of predatory arthropods were higher in the open agricultural landscape (290 predators m⁻²) than in the forested landscape (137 predators m⁻²). Herbaceous habitat types supported the highest densities of overwintering predators (up to 400 predators m⁻²), whereas densities in the forest were lowest (10 predators m⁻²). These results indicate that herbaceous non-crop habitats are important refugia for predators and may play a vital role in conservation biological control. Handling Editor: Arne Janssen.     

Grass abundance maintains positive plant–arthropod diversity relationships in maize fields and margins in South Africa

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Relative importance of predators and parasitoids for cereal aphid control

Field experiments with manipulations of natural enemies of plant-feeding insects may show how a diverse enemy group ensures an important ecosystem function such as naturally occurring biological pest control. We studied cereal aphid populations in winter wheat under experimentally reduced densities of: (i) ground-dwelling generalist predators (mostly spiders, carabid and staphylinid beetles); (ii) flying predators (coccinellid beetles, syrphid flies, gall midges, etc.) and parasitoids (aphidiid wasps), and a combination of (i) and (ii), compared with open controls. Aphid populations were 18% higher at reduced densities of ground-dwelling predators, 70% higher when flying predators and parasitoids were removed, and 172% higher on the removal of both enemy groups. Parasitoid wasps probably had the strongest effect, as flying predators occurred only in negligible densities. The great importance of parasitism is a new finding for aphid control in cereal fields. In conclusion, a more detailed knowledge of the mechanisms of natural pest control would help to develop environmentally sound crop management with reduced pesticide applications.     

Records of Natural Enemies of Bemisia tabaci (Genn.) (Hemiptera: Aleyrodidae) Biotype B in Brazil

Collections of natural enemies of Bemisia tabaci biotype B (Genn.) (Hemiptera: Aleyrodidae) were made in Lavras, state of Minas Gerais, Brazil. In the greenhouse, 6,495 predators and 16,628 parasitoids belonging to three families were collected. In the field, 267 predators and 344 parasitoids belonging to five families were found. For the first time in Brazil, five species of predators associated with this whitefly were reported. Because of the diversity of natural enemies of B. tabaci biotype B recorded, this study points out the importance of these data for studies on integrated pest management.     

An entomophage park to promote natural enemy diversity

Undisturbed habitats of natural vegetation near agricultural areas protect and enhance specific natural enemies and provide them with resources such as nectar, pollen, physical refuge, alternative prey, alternative hosts and mating sites. In order to reduce the pesticide-induced mortality of natural enemies and to improve natural enemy fitness and effectiveness, one such area (termed an ‘entomophage park’) was established at the Sher-e-Kashmir University of Agricultural Sciences and Technology of Jammu, at Chatha, India in 2007. Naturally occurring plants, weeds, cultivated crops and flowers were monitored regularly for natural enemies. Seven sampling methods were employed to compare the abundance of natural enemies in the entomophage park and adjoining crop fields. Both entomophage diversity and abundance in the park were much higher than in the adjacent agricultural fields of vegetables and cereals. A total of 61 species of natural enemies were recovered from the entomophage park, as compared to 22 and 20 species in cereal and vegetable fields, respectively. The abundance of parasitoids (ichneumonids, braconids, scelionids and chalcidoids) was significantly higher in the park as compared to surveyed agricultural fields, as was egg parasitism by scelionids (Telenomus spp.) and trichogrammatids, and parasitisation by the larval parasitoid Campoletis chlorideae on Helicoverpa armigera. The entomophage park also significantly enhanced the fecundity and survival of the ichneumonid C. chlorideae, when compared to individuals collected from vegetable and cereal fields. Seventeen species of plants were recorded as ‘insectary plants' (one providing substantial floral resources) in the entomophage park. Such parks may play an important role in maintaining the biodiversity of natural enemies and enhancing natural pest control.     

Interactive effects of local and landscape factors on farmland carabids

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Predation rates on semi-natural grasslands depend on adjacent habitat type

Spillover in ecological systems, that is the dispersal or foraging of organisms across habitat borders, can affect ecosystem functioning and food web interactions of local communities. While spillover of organisms from perennial habitats into agricultural fields received some attention in the context of ecosystem service provisioning, the spillover into semi-natural habitats has rarely been addressed, although spillover of generalist predators or competition for pollinators can have consequences for nature conservation. We studied predation rates of ground-dwelling predators on 20 calcareous grasslands, with either coniferous forest or a crop field as adjacent habitat. As prey items we exposed 32,000 ladybird eggs on the grasslands. Within two study periods (June to September) predation rates were higher at warm compared to cool days, but did not depend on the study period itself or the distance from the edge where prey items were placed. In each study period we found higher predation rates when coniferous forest was the adjacent habitat, however, only on cool days. On warm days, prey items were consumed to very high extents (often 100%), which did not allow the detection of possible differences between adjacent habitat types. The higher predation rates on grasslands adjacent to forests can be explained by predator spillover from forests to grasslands. We conclude that semi-natural habitats provide not only ecosystem services in adjacent human dominated habitats, but are also exposed to antagonistic spillover effects. Such antagonistic spillover should be considered in conservation strategies for semi-natural habitats.     

Wildflower plantings enhance the abundance of natural enemies and their services in adjacent blueberry fields

Wildflower plantings can support local abundance of natural enemies, but their influence on biological control of pests in adjacent crop fields is less well documented. To test whether biological control is enhanced by these plantings, we established native, perennial wildflowers in areas adjacent to highbush blueberry fields. Once wildflowers were established we found greater abundance of natural enemies in the fields adjacent to wildflower plantings compared with those adjacent to unenhanced control field perimeters. Predaceous arthropods, including spiders, hoverflies, and lady beetles, were among the most common natural enemies observed and collected in the blueberry fields. Using corn earworm eggs, Helicoverpa zea (Lepidoptera: Noctuidae), as sentinel prey, we found a similar pattern of biological control, with higher biological control services index values in fields adjacent to the wildflower plantings than in the unenhanced control fields. Our results provide evidence for the ability of wildflower plantings to support natural enemy populations in agricultural landscapes, and to potentially provide local enhancement of biological control in adjacent crops.     

Plant-insect communities and predator-prey ratios in field margin strips,adjacent crop fields,and fallows

The management of field margin strips for the enhancement of biodiversity of plant-insect communities and natural-enemy populations was studied on experimental farms near Göttingen (Germany). Young and old, sown and naturally developed field margin strips were compared and differences to large fallows established. The five types of field margin strips (around cereal fields) were: (1, 2) 1- or 6-year-old naturally developed strips, (3) strips sown with a Phacelia mixture, (4) strips sown with a mixture of 19 wild flower species, and (5) strips sown with winter wheat or oat as a control. The naturally developed vegetation of the field margin strips was dominated by aggressive weeds, presumably due to the intensive farming practices and the fertile soils. Cirsium arvense populations decreased, while Elymus repens populations increased with age of habitat. Sowings were suitable to suppress these aggressive weeds. Potted plants of mugwort (Artemisia vulgaris) and red clover (Trifolium pratense) were exposed in the field margin strips to study arthropod colonization of these experimentally standardized plant patches. Arthropod species richness did not differ between field margin types, reflecting the overall similarity in floristic diversity, but sprayed and strip-free edges of cereal fields had a reduced diversity. Dispersal of insect populations of red clover into the cereal fields decreased with increasing distance, but benefited from adjacent field margin strips. Populations of predators (mainly spiders) as well as predator-prey ratios were significantly larger in 6-year-old than in 1-year-old strips emphasizing the importance of habitat age for natural enemies and possible biological control. Predator-prey ratios were also higher on old than young fallows. Large fallows had greater predator-prey ratios than small field margin strips emphasizing the trophic-level hypothesis of island biogeography in that the relative importance of natural enemies increased with habitat area. Insect species richness was only marginally influenced by area and not by age. As species richness of predators did not increase with area and age, species diversity and the possible biological-control function did not covary.     

Development of the herbivore Pieris rapae and its endoparasitoid Cotesia rubecula on crucifers of field edges

Abstract:  Several studies have reported that flowering herbs, which grow naturally or are sown adjacent to agricultural fields, may be an important source of nutrients for natural enemies. Many parasitoids readily feed on plant exudates such as floral nectar, which contain different types of sugars that enable the insects to optimize their longevity, mobility and reproductive success. However, leaf tissues of plants grown in the margins of agricultural fields may also provide food for immature stages of insect herbivores, such as caterpillars, that are in turn attacked by parasitoids. Herbivores and their parasitoids may later disperse into the crop, so the nutritional quality of surrounding plants, as this affects herbivore and parasitoid fitness, may also influence the success of biological control programmes, especially later in the season. Here, we compare the suitability of three species of cruciferous plants (Brassicaceae) on the development of Pieris rapae L. (Lep., Pieridae) and its solitary endoparasitoid, Cotesia rubecula Marshall (Hym., Bracondiae). Insects were reared on a feral population of cabbage, Brassica oleracea , on radish Raphanus sativus , which is widely sown in agricultural margins, and on hedge mustard, Sisymbrium officinale , a wild crucifer which often grows in medium to large stands along road verges and field edges. Development time in both the herbivore and parasitoid were extended on R. sativus , compared with the other two species, whereas C. rubecula completed its development most rapidly on B. oleracea . Moreover, adult butterflies and parasitoids were significantly smaller when reared on R. sativus plants. Our results reveal that differences in the quality of plants growing adjacent to agricultural fields can affect the development of key herbivores and their parasitoids. This should be borne in mind when establishing criteria for the selection of floral biodiversity.     

Bidirectional movement of aphid parasitoids (Braconidae: Aphidiinae) between crops and non-crop plants in agroecosystems of central Argentina

The movement of predators and parasitoids between natural and cultivated habitats is a common process in agroecosystems, which may be affected by different biotic and abiotic factors, mostly related to the availability of resources. Here, through a broad approach, we aimed to obtain an overview of factors affecting the bidirectional movement of aphid parasitoids (Braconidae: Aphidiinae) across cultivated habitats and their natural vegetated borders. Using bidirectional flight traps, we measured the number of parasitoids moving from borders to crops and vice versa, in fields of three common crop species (alfalfa, oat and wheat) in the Pampean region, Santa Fe, Argentina. The effects of the abundance of aphid prey, abundance and richness of flowers in both habitats, as well as temperature and wind speed on parasitoid movement, were assessed through generalized mixed models, considering sampling date and field as random factors. The relationship between parasitism percentages and parasitoid movement from the borders to the crops was explored separately for three pest aphid species: Aphis craccivora Koch, Rhopalosiphum padi (L.) and Schizaphis graminum (Rondani). Overall, we found a prevalence of parasitoids moving in the border-crop direction, mainly in wheat and alfalfa crops. Aphid abundance in the arrival habitat affected parasitoid movement in both directions. A link between parasitoid movement and parasitism percentages was observed for the aphid species S. graminum in wheat, suggesting a beneficial role of natural vegetation in pest control.     

Predation pressure in Ugandan cotton fields measured by a sentinel prey method

Pest suppression by natural enemies is an important ecosystem service, which is a valuable resource to poor smallholders in developing countries. Diverse natural enemy assemblages of arthropod predators and parasitoids are documented in various regions in Africa, but our knowledge of their impact on herbivores in agroecosystems remains limited. We conducted experiments in cotton, Gossypium hirsutum L. (Malvaceae), under typical local agronomic practices in Uganda to assess levels of predation pressure ascribed to natural enemies. We measured predation rates on artificial caterpillars made of plasticine glued to cotton plants. Predation pressure on cotton fields varied between 1.96 and 4.1% per day, but was not significantly influenced by cotton treatments (insecticide/no insecticide, monocropping/intercropping with Phaseolus spp.). Predation pressure in non‐crop habitats adjacent to cotton fields was up to 12× higher than in the fields. Marks left on the artificial caterpillars revealed that arthropods and birds were largely accountable for predation in cotton fields, whereas arthropods and small mammals were dominant in non‐cultivated habitats.     

麦蚜和寄生蜂对农业景观格局的响应及其关键景观因子分析

大量研究表明多样性的农业景观格局能够影响蚜虫及寄生蜂的分布。本文利用并设计了两种尺度的麦田农业景观格局,依据麦蚜种群发生特点,分为迁入期、增长期与高峰期三个时期,论述了不同尺度下农业景观元素对麦蚜及寄生蜂系统的影响,通过逐步回归筛选了不同时期麦蚜及寄生蜂分布的关键景观元素,最后通过CANOCO软件模拟了麦蚜及寄生蜂的分布排序格局。结果表明不同景观因子对麦蚜及寄生蜂种群影响不同,迁入期两种尺度下裸地最有利于两种有翅蚜的迁飞入田,塑料大棚对有翅蚜种群的入田有抑制作用。增长期草地与林地生境对麦蚜种群增长率促进最大,塑料大棚同样抑制了麦蚜的种群增长率;但塑料大棚、草地与林地对蚜茧蜂的种群增长率有促进作用,非麦类作物生境对蚜茧蜂种群增长率抑制作用最大;草地与林地有利于重寄生蜂的种群增长率;高峰期裸地比例大的农业景观下麦蚜的最大种群密度较大,草地与林地对蚜茧蜂与重寄生蜂的最大种群密度均有促进作用。两种尺度下的研究结果一致。不同麦物种的对不同景观元素反应与形态学与生活史特征有关,而且景观结构中特定的植物种类、非作物植物的密度与物候期都可能影响寄生蜂群落的多样性与功能。     

植被边界带对相邻麦田地表步甲和蜘蛛分布及蚜虫发生的影响

通过管理半自然生境提高农田中天敌多样性及农田害虫的生物控制效率是当前生态农业研究的基本问题之一。为探讨不同类型半自然植被边界带对相邻麦田地表天敌的发生以及对小麦蚜虫生物控制潜力的影响,在北京顺义区赵全营镇采用陷阱法调查了不同类型植被边界带（人工林地、人工种植草带、自然演替草带）相邻的小麦田中,距离边界0,15 m和30 m处步甲和蜘蛛两类重要地表天敌的多度分布及小麦田蚜虫发生情况,同时分析了边界带植被群落结构对步甲和蜘蛛群落分布的影响。结果表明：人工林地相邻的小麦田中维持了显著较高的步甲、蜘蛛和蚜虫发生密度;不同类型植被边界带相邻小麦田中步甲和蜘蛛群落组成显著不同,植被边界带宽度以及乔木层盖度是影响相邻麦田步甲和蜘蛛群落分布的主要因子;距离植被边界带不同取样界面处,不同迁移方向的步甲和蜘蛛的活动密度无显著性差异;与林地相邻的麦田中,蜘蛛活动密度从农田边界向农田内部处依次降低,与人工种植草带以及自然演替草带相邻的麦田中,农田边界处蜘蛛的活动密度显著高于农田内部。研究显示,半自然植被边界带与麦田交界处维持了较高的蜘蛛活动密度,并具有向农田内部扩散的趋势,然而,并非高天敌密度就意味着低害虫发生率,深入研究天敌群落与害虫发生关系及其影响因素是未来天敌保护和害虫生物防治的重要方面。同时,因为不同类型植被边界带维持的天敌群落具有高度异质性,未来的研究需要充分考虑不同类型植被边界带对天敌维持的作用,以及不同类型生境界面地表天敌的活动规律。     

Taxonomic and functional characteristics of field edge weed communities along a gradient of crop management intensity

The widespread loss of weed diversity and associated ecosystem functions is raising important concerns. Field edges could play a major role in the maintenance of weed functional diversity in arable landscapes as these habitats still harbour high weed diversity, owing to either a reduced farming management intensity and/or to a spillover of species from adjacent perennial field margins. Here, we investigated the taxonomic and functional characteristics of weed species recorded in surveys of field edges and their associated field cores over six consecutive years in 60 arable fields farmed with five crop management strategies. We found that field edges were richer, with species more functionally diverse and composition more stable over years than field core surveys. The distribution of individual functional traits differed between field edges and field cores, with higher values for seed mass and nitrophily (Ellenberg.N), and a wider distribution of specific leaf area values in field edges. The bimodal distribution of plant height and germination period observed in field edges became unimodal in field cores. Field edges harboured species with ecological strategies associated with field cores (ruderal species) plus a conservative strategy which could be explained by a spillover from the adjacent perennial field margins. Crop management strategies impacted field edge flora, though to a lesser extent than the field core flora whereas the functional differences between the field edge and the field core flora were less marked when crop management intensity was lower. These results indicate that field edges harbour a unique assemblage of species and highly contribute to the maintenance of weed diversity in arable landscapes. Future studies should thus focus on the importance of these specific functional traits to the agroecosystem functioning.     

Temporal variability of aphid biological control in contrasting landscape contexts

Landscape complexity may provide ecosystem services to agriculture through the provision of natural enemies of agricultural pests. Strong positive effect of adjacent semi-natural habitats on natural enemies in croplands has been evidenced, but the resulting impact on biological control remains unclear. Taking into account the temporal dynamics of pest and natural enemies in agricultural landscapes provides better resolution to the studies and better understanding of the biological control service.In this study, the population dynamics of aphids and two groups of predators (coccinellid and carabid beetles) were examined. Insects were sampled in 20 wheat fields, surrounded by structurally simple and complex landscapes in Chilean central valley. Considering the whole sampling period, the diversity of aphids and natural enemies were similar in wheat crops surrounded by both types of landscapes, and the abundance of ladybirds was higher in crops in the complex landscapes. The dynamics of predators was more advanced in complex landscapes than in the simple ones, whereas the dynamics of aphids were similar in both types of landscape. Negative correlation between abundance of predators and aphid population growth rate in both landscape contexts were observed suggesting a control of the pest population by the predators. Different temporal patterns were observed in these correlations in the two landscape contexts, which suggests differences in the biological control related to the landscape composition.The present study shows that colonization of crops by natural enemies occurs sooner in structurally complex landscapes and suggests that this early colonization may facilitate an early and efficient control of aphid populations, nevertheless the biological control efficiency seems to be higher in structurally simple landscapes later in the season.     

Edge contrast does not modulate edge effect on plants and pollinators

Edge contrast, is one of the main determinants of edge effects. This study examines the response of plant and pollinator diversity (bees and butterflies) to forest edge contrast, i.e. the difference between forests and adjacent open habitats with different disturbance regimes. We also investigated a potential cascading effect from plants to pollinators and whether edge structure and landscape composition mediate the relationship between edge contrast and beta diversity of pollinators. We sampled 51 low-contrast edges where forests were adjacent to habitats showing low levels of disturbance (i.e. grey dunes, mowed fire-breaks, orchards, grasslands) and 29 high-contrast edges where forests were adjacent to more intensively disturbed habitats (i.e. tilled firebreaks, oilseed rape) in three regions of France. We showed that plant diversities were higher in edges than in adjacent open habitat, whatever the edge contrast. However, plant beta diversity did not differ significantly between low and high-contrast edges. While we observed higher pollinator diversities in adjacent habitats than in low-contrast edges, there were no significant differences in pollinator beta diversity depending on edge contrast. We did not observe a cascading effect from plants to pollinators. Plant and bee beta diversities were mainly explained by local factors (edge structure and flower cover) while butterfly beta diversity was explained by surrounding landscape characteristics (proportion of land cover in grassland).     

Biodiversity analysis of natural arthropods enemies in vineyard agroecosystems in La Rioja,Spain

Major knowledge gaps exist regarding effects of landscape-level agroecosystem composition on the presence of natural enemies of agricultural pests. The objective of this study was to evaluate the influence of landscape mosaic characteristics on the diversity of predators and parasitoids of a viticultural landscape in La Rioja, Spain. Five habitats were evaluated: Mediterranean forest, Mediterranean scrub, olive groves, natural grassland, and vineyards. In all, we collected 28,640 arthropods, representing 10 orders: Araneae, Coleoptera, Dermaptera, Diptera, Hemiptera, Hymenoptera, Mantodea, Neuroptera, Solifugae, and Thysanoptera. The grassland habitat presented the highest arthropod abundances. Carabid beetles showed preferences for Mediterranean forest and scrub, whereas reduviid bugs showed preferences for natural grassland. Landscape heterogeneity and connectivity with natural elements in the study area turn out to be effective in conservation of diversity of natural enemies of viticulture.