Influence of native flowering plant strips on natural enemies and herbivores in adjacent blueberry fields

Conservation plantings of native wildflowers were established adjacent to highbush blueberry (Vaccinium corymbosum L.) fields to test the hypothesis that provision of resources for natural enemies increases their abundance in adjacent crop fields without increasing the abundance of pest insects. For two growing seasons, natural enemies and herbivorous insects were sampled in fields with flowering borders and in control fields where growers maintained standard mown grass perimeters. Insects were categorized according to their trophic level and their potential pest status, and their abundance was compared between years and between treatments. Syrphid flies (Diptera: Syrphidae) were significantly more abundant in fields with conservation strips, as were plant bugs (Hemiptera: Miridae), thrips (Thysanoptera: Thripidae), and hoppers (Hemiptera: Auchenorrhyncha). Aphids (Hemiptera: Aphididae), thrips, fruit flies (Diptera: Tephritidae), and pirate bugs (Hemiptera: Anthocoridae) decreased significantly in abundance from 2007 to 2008. Beneficial insect abundance in crop fields increased in the latter half of the season in both years and this increase was more pronounced in fields adjacent to conservation plantings. We discuss the implications of these findings for pest management and conservation of biodiversity in farmland.     

Landscape-scale forest cover increases the abundance of Drosophila suzukii and parasitoid wasps

Agricultural landscapes rich in natural and semi-natural habitats promote biodiversity and important ecosystem services for crops such as pest control. However, semi-natural habitats may fail to deliver these services if agricultural pests are disconnected from the available pool of natural enemies, as may be the case with invasive species. This study aimed to provide insights into the relationship between landscape complexity and the abundance of the recently established invasive pest species Drosophila suzukii and a group of natural enemies (parasitoid wasps), which contain species that parasitize D. suzukii in native and invaded ecosystems. The importance of landscape complexity was examined at two spatial scales. At the field scale, the response to introduction of wildflower strips was analysed, while the relationship with forest cover was assessed at the landscape scale. Half of the surveys were done next to blueberry crops (Vaccinium corymbosum), the other half was done in landscapes without fruit crops to examine effects of D. suzukii host presence. As expected, the number of observed parasitoid wasps increased with amount of forest surrounding the blueberry fields, but the number of D. suzukii individuals likewise increased with forest cover. Establishment of wildflower strips did not significantly affect the abundance of D. suzukii or parasitoid wasps and insect phenology was similar in landscapes with and without blueberry crops. This suggests that D. suzukii is enhanced by landscape complexity and is largely unlinked from the species group that, in its native range, hosts key natural enemies. Although management practices that rely on enhancing natural enemies through habitat manipulations can contribute to the long-term stability of agroecosystems and to control agricultural pests, other control measures may still be necessary in the short term to counteract the benefits obtained by D. suzukii from natural habitats.     

The effect of within-crop habitat manipulations on the conservation biological control of aphids in field-grown lettuce

Within-crop habitat manipulations have the potential to increase the biological control of pests in horticultural field crops. Wildflower strips have been shown to increase the abundance of natural enemies, but there is little evidence to date of an impact on pest populations. The aim of this study was to determine whether within-crop wildflower strips can increase the natural regulation of pests in horticultural field crops. Aphid numbers in plots of lettuce grown adjacent to wildflower strips were compared with those in plots grown in the absence of wildflowers. The presence of wildflower strips led to a decrease in aphid numbers on adjacent lettuce plants during June and July, but had less impact in August and September. The decrease in aphid numbers was greatest close to the wildflower strips and, the decrease in aphid numbers declined with increasing distance from the wildflower strips, with little effect at a distance of ten metres. The main natural enemies found in the crop were those that dispersed aerially, which is consistent with data from previous studies on cereal crops. Analysis and interpretation of natural enemy numbers was difficult due to low recovery of natural enemies, and the numbers appeared to follow changes in aphid abundance rather than being directly linked to the presence of wildflower strips. Cutting the wildflower strips, to remove floral resources, had no impact on the reduction in aphid numbers achieved during June and July, but decreased the effect of the wildflower strips during August and September. The results suggest that wildflower strips can lead to increased natural regulation of pest aphids in outdoor lettuce crops, but more research is required to determine how this is mediated by natural enemies and how the impact of wildflower strips on natural pest regulation changes during the growing season.     

Effect of flower traits and hosts on the abundance of parasitoids in perennial multiple species wildflower strips sown within oilseed rape (<Emphasis Type="Italic">Brassica napus</Emphasis>) crops

Reducing the use of insecticides is an important issue for agriculture today. Sowing wildflower strips along field margins or within crops represents a promising tool to support natural enemy populations in agricultural landscapes and, thus, enhance conservation biological control. However, it is important to sow appropriate flower species that attract natural enemies efficiently. The presence of prey and hosts may also guide natural enemies to wildflower strips, potentially preventing them from migrating into adjacent crops. Here, we assessed how seven flower traits, along with the abundance of pollen beetles (Meligethes spp., Coleoptera: Nitidulidae) and true weevils (Ceutorhynchus spp., Coleoptera: Curculionidae), affect the density of parasitoids of these two coleopterans in wildflower strips sown in an oilseed rape field in Gembloux (Belgium). Only flower traits, not host (i.e. pollen beetles and true weevils) abundance, significantly affected the density of parasitoids. Flower colour, ultraviolet reflectance and nectar availability were the main drivers affecting parasitoids. These results demonstrate how parasitoids of oilseed rape pests react to flower cues under field conditions. Similar analyses on the pests and natural enemies of other crops are expected to help to develop perennial flower mixtures able to enhance biological control throughout a rotation system.     

Influence of the margin vegetation on the conservation of aphid biological control in apple orchards

The influence of three margin strip treatments (wildflower strips, grass strips and spontaneous vegetation) adjacent to apple orchards on the biological control of Dysaphis plantaginea Passerini (Hemiptera: Aphididae) was compared during two consecutive years. The wildflower strips provided the highest amount of floral resources. Within the margin strips, hoverflies responded positively to higher resource provisioning whereas ladybird abundance did not differ between strip treatments. Within the orchards, the presence of parasitoids, hoverflies, and ladybirds in aphid colonies and the predation of sentinel aphids were not significantly affected by the adjacent strip treatments. The number of natural enemies observed in aphid colonies was mainly driven by aphid number. Aphid numbers were higher close to the margin strips suggesting that aphid colonization from orchard edges may counteract the positive effect of wildflower strips on natural enemy abundance and on a reduction of aphid infestation. The results confirm the positive influence of floral resource provisioning by wildflower strips on the conservation of aphid natural enemies, but also suggest that effects of wildflower strips on aphid regulation inside orchards are not very strong compared with spontaneous vegetation naturally occurring in the margins.     

Wild bees and natural enemies prefer similar flower species and respond to similar plant traits

Designing wildflower habitats to support beneficial insects providing pollination and pest control services is important for supporting sustainable crop production. It is often desirable to support both groups of insects, making the selection of resource plants for insect conservation programs more challenging. Moreover, the process of selecting resource plants is complicated by the array of possible options in each region, and the need to provide resources over the entire growing season. Identifying traits shared by resource plants that are attractive to both bees and natural enemies can reduce the need to evaluate new plants in each region, by providing a guide for the types of plants expected to be rewarding to these insects. Using insect visitation data collected from replicated common garden plantings of native wildflower and shrub species from the Great Lakes region of the United States, we found a high degree of correlation between the abundance of bees and natural enemies visiting native plant species. These results were used to identify a set of 15 plant species that can provide resources for these insects throughout the summer. Across all tested species, pollen quantity per flower and the week of bloom were positively correlated with some, but not all, taxonomic groupings of beneficial insects. In contrast, floral area was consistently positively associated with visitation of both natural enemies and wild bees. This trait is easy to document and can allow for efficient local testing of potential resource plants, providing a faster path to implementing insect conservation in working landscapes.     

Larger wildflower plantings increase natural enemy density,diversity, and biological control of sentinel prey,without increasing herbivore density

1. An important means of conserving beneficial insects in resource‐limited habitats is to meet their ecological requirements, which may be achieved by providing areas containing flowering plants that bloom throughout the season, but little is known about the importance of wildflower plot size for supporting natural enemies or the biological control they provide. 2. Wildflowers were established in plots of sizes ranging from 1 to 100 m², and found that natural enemy density, group richness, and diversity of natural enemy groups increased with plot size. 3. The density of insect herbivores was lower in all flower plots than in the control samples, whereas the diversity of herbivore groups was significantly higher in flower plots. 4. Comparing population growth of sentinel soybean aphids (Aphis glycines Matsumura) among plot sizes, aphid colonies were smaller as plot size increased. 5. Providing beneficial insects with flowering resources resulted in significantly more natural enemies and greater pest control than in smaller flower plots or mown grass areas. 6. These results indicate that the density, diversity, and function of natural enemies are sensitive to the size of wildflower plantings, even at relatively small scales. Therefore, larger wildflower plots are more suitable for the conservation of beneficial insects and their provision of natural pest control.     

Attractiveness of sown wildflower strips to flower-visiting insects depends on seed mixture and establishment success

Establishing wildflower strips has been suggested as an effective measure to promote pollination services, pest control or general insect biodiversity, but little is known about the integration of these different objectives when selecting flower seed mixtures. In ten agricultural landscapes in the Netherlands, we established a wildflower strip (0.4 – 4.9 ha) with half of each strip sown with a mixture targeting longer-tongued pollinators and the other half sown with a mixture targeting shorter-tongued pollinators and natural enemies. We determined establishment success of sown wildflowers and evaluated the attractiveness of the established flower communities to multiple functional groups of flower visitors: bumblebees (long-tongued pollinators), hoverflies (short-tongued pollinators and natural enemies), and butterflies and total flower-visitor richness (indicators of wider biodiversity values). Bumblebees clearly preferred the pollinator-targeted seed mixture and were positively associated with cover of Fabaceae and negatively with Apiaceae. Hoverflies consistently preferred the natural enemy mixture and were positively associated with Apiaceae. The other target groups displayed no clear responses to seed mixture type but instead were associated with local flower richness within strips. Across sites, responses of flower-visitors to sown mixture types did not depend on wildflower strip size, proportion of surrounding semi-natural habitat, or flower variables. However, all flower-visitors except butterflies increased with increasing established cover or richness of (sown) flower species across sites. Our results suggest that, although species-rich wildflower strips may benefit several species groups, maximising different objectives involves trade-offs between functional groups that prefer short- or long-corolla flowers. Furthermore, our study suggests that sowing a wildflower mixture does not necessarily result in a vegetation with the same composition as the seed mixture as species may establish poorly or not at all. Selection of flower species for seed mixtures should therefore, in addition to insect target group, take the establishment characteristics of plant species into account.     

Larger patches of diverse floral resources increase insect pollinator density,diversity, and their pollination of native wildflowers

Native wildflower plantings can be used to provide nutritional resources to support pollinating insects, yet the effects of planting size and bloom richness on the density, diversity, and function of these insects are not well understood. We established stands of twelve native flowering perennial plant species in replicated plots ranging in size from 1 to 100 m². These plots were sampled for insect pollinators, bloom richness, and seed production by three wildflower species. Honeybees, wild bees, and hoverflies all responded positively to increasing flower richness, whereas particular insect pollinator groups responded differently to the size of the flowering plant area. The density of honeybees and hoverflies was not affected by increasing flowering patch size, whereas in general, wild bees were observed at higher density and diversity in the 30 and 100 m² patches. Increasing wildflower patch size, and thus wild bee density, resulted in greater seed set in the sampled wildflowers. These results indicate that wild bees are sensitive to the area and richness of floral resources in patches, even at relatively small scales. Therefore, larger wildflower plantings with more diverse flower species mixes are more suitable for the conservation of wild pollinators and reproduction of sown species.     

Establishment and management of wildflower areas for insect pollinators in commercial orchards

Sown wildflower areas are increasingly recommended as an agri-environmental intervention measure, but evidence for their success is limited to particular insect groups or hampered by the challenges of establishing seed mixes and maintaining flower abundance over time. We conducted a replicated experiment to establish wildflower areas to support insect pollinators in apple orchards. Over three years, and across 23 commercial UK orchards with and without sown wildflowers, we conducted 828 transect surveys across various non-crop habitats. We found that the abundance of flower-visiting solitary bees, bumblebees, honeybees, and beetles was increased in sown wildflower areas, compared with existing non-crop habitats in control orchards, from the second year following floral establishment. Abundance of hoverflies and other non-syrphid flies was increased in wildflower areas from the first year. Beyond the effect of wildflower areas, solitary bee abundance was also positively related to levels of floral cover in other local habitats within orchards, but neither local nor wider landscape-scale context affected abundance of other studied insect taxa within study orchards. There was a change in plant community composition on the sown wildflower areas between years, and in patterns of flowering within and between years, showing a succession from unsown weedy species towards a dominance of sown species over time. We discuss how the successful establishment of sown wildflower areas and delivery of benefits for different insect taxa relies on appropriate and reactive management practices as a key component of any such agri-environment scheme.     

A multi-site experiment to test biocontrol effects of wildflower strips in different French climate zones

Sowing of wildflower strips has been integrated in agri-environment schemes of several European countries. Their beneficial effects on natural enemies of pest insects are well documented but (1) the desired spill-over into crop fields has not always been demonstrated, and (2) the need to adapt sown mixtures to regional climatic differences has been rarely addressed.We set up a multi-site experiment in different French climatic regions to compare effects of a wildflower strip with a grass mixture and spontaneous vegetation. The design included five regions, three to five fields per region and the three strip treatments being repeated in each field. We tested strip treatment effects on vegetation (plant species richness, plant and flower cover) and on natural enemies (hoverflies, ladybirds, aphid predation). In a subset, we further analysed the spill-over into winter wheat fields including natural enemies and pest insects (cereal aphids, leaf beetles).The wildflower strip mixture developed well in all regions and increased plant species richness and flower cover compared with grass strips and spontaneous vegetation. We found a corresponding higher hoverfly abundance and aphid predation in wildflower strips that were consistent in all regions, whereas ladybird abundance was not affected. A significantly higher hoverfly abundance, aphid predation and aphid parasitism in wheat fields close to wildflower strips indicated a spill-over. No corresponding margin treatment effects were observed for aphid and leaf beetle abundance in the field. A multivariate analysis comparing the influence of climate and vegetation parameters showed that floral cover better explained variation in natural enemy abundance and predation than climate. Our results demonstrated that similar mixtures of native plants can be used over large climatic gradients to improve biocontrol. Further research is needed to improve spill-over into crop fields and to obtain consistently strong effects in different climate zones.     

Wildflower plantings promote blue orchard bee,Osmia lignaria (Hymenoptera: Megachilidae), reproduction in California almond orchards

Concerns over the availability of honeybees (Apis mellifera L.) to meet pollination demands have elicited interest in alternative pollinators to mitigate pressures on the commercial beekeeping industry. The blue orchard bee, Osmia lignaria (Say), is a commercially available native bee that can be employed as a copollinator with, or alternative pollinator to, honeybees in orchards. To date, their successful implementation in agriculture has been limited by poor recovery of bee progeny for use during the next spring. This lack of reproductive success may be tied to an inadequate diversity and abundance of alternative floral resources during the foraging period. Managed, supplementary wildflower plantings may promote O. lignaria reproduction in California almond orchards. Three wildflower plantings were installed and maintained along orchard edges to supplement bee forage. Plantings were seeded with native wildflower species that overlapped with and extended beyond almond bloom. We measured bee visitation to planted wildflowers, bee reproduction, and progeny outcomes across orchard blocks at variable distances from wildflower plantings during 2015 and 2016. Pollen provision composition was also determined to confirm O. lignaria wildflower pollen use. Osmia lignaria were frequently observed visiting wildflower plantings during, and after, almond bloom. Most O. lignaria nesting occurred at orchard edges. The greatest recovery of progeny occurred along the orchard edges having the closest proximity (80 m) to managed wildflower plantings versus edges farther away. After almond bloom, O. lignaria nesting closest to the wildflower plantings collected 72% of their pollen from Phacelia spp., which supplied 96% of the managed floral area. Phacelia spp. pollen collection declined with distance from the plantings, but still reached 17% 800 m into the orchard. This study highlights the importance of landscape context and proximity to supplementary floral resources in promoting the propagation of solitary bees as alternative managed pollinators in commercial agriculture.     

Sowing of margin strips rich in floral resources improves herbivore control in adjacent crop fields

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Natural enemy responses and pest control: Importance of local vegetation

Natural invertebrate enemies on farmland may respond to local vegetation adjacent to the farmland. This can encourage individual landholders to maintain vegetation even when there are costs in terms of a reduction in crop area as well as maintenance of the vegetation. Here we investigated abundance of natural enemies in 61 vineyards from south eastern Australia, with either adjacent woody vegetation consisting of remnant native forests or wooded margins planted after establishment of the crop, or lacking in adjacent vegetation. Invertebrates were sampled five times at monthly intervals using canopy sticky traps. There was an increase in abundance of different groups of natural enemies when woody vegetation was present although these increases depended on size. Small natural enemies tended to be influenced by local features, whereas larger potentially more mobile groups were not affected. Predation of eggs of a vineyard insect pest, Epiphyas postvittana, was relatively higher in vineyards with either adjacent remnant forest or planted woody vegetation. Various types of local vegetation may therefore encourage a range of predators and parasitoids, increasing predation and control of a common vineyard pest.     

Plant traits and landscape simplification drive intraspecific trait diversity of Bombus terrestris in wildflower plantings

Wildflower plantings are an important mitigation tool within agri-environmental schemes to counter insect decline in resource-scarce agricultural landscapes. Effectiveness of wildflower plantings for insect conservation is typically studied at the community or species level. It is the individual, however, that is subject to changing abiotic and biotic conditions, not the species per se. Accordingly, functional traits of individuals, i.e., the intraspecific functional diversity within species, likely mediate responses to wildflower resources and landscape context. Here we focused on the ecologically and economically important wild insect pollinator Bombus terrestris to study its intraspecific functional diversity and plant-pollinator individual interactions in wildflower plantings. We found considerable trait variation among flower-visiting B. terrestris workers. Locally, this variation could be attributed to flowering plant traits, with larger workers visiting larger inflorescences and individuals with longer tongues preferentially feeding on zygomorphic but not radially symmetrical flowers. In addition, wildflower plantings with high floral abundance attracted individuals with larger pollen baskets. At the landscape scale, increasing proportion of arable land resulted in smaller B. terrestris individuals in wildflower plantings, and a decrease in the overall size diversity of workers. These findings highlight the so far little considered role of intraspecific variation in functional traits of wild pollinators, which can mediate the trait-matching between plants and pollinator individuals. Landscape simplification from agriculture threatens intraspecific pollinator diversity, with potential harmful effects for pollinator fitness and plant reproduction. Tailored wildflower plantings can thus serve as an important tool to increase intraspecific variation in simplified landscapes. When designing seed mixtures for these plantings, high complementarity in plant traits is key for promoting high intraspecific trait diversity of bumblebees and potentially of other associated insect species.     

Wildflower plantings on fruit farms provide pollen resources and increase nesting by stem nesting bees

1. Wildflower plantings on farms have been shown to attract foraging wild bees, however, whether these added floral resources increase nesting densities of bees remains largely untested.
2. We placed nest boxes containing natural reeds at 20 fruit farms in Michigan. We then compared nesting densities between farms with and without wildflower plantings and analysed nest provisions to evaluate use of wildflower plantings for brood provisioning.
3. We found significantly greater nesting at farms with wildflower plantings, with only one out of 236 completed nests at a farm without a planting. The majority of nests were completed by Megachile pugnata, with a portion of nests completed by Osmia caerulescens.
4. We found that nesting bees collected pollen from only a subset of the available flowers in the wildflower plantings, with a strong preference for Centaurea maculosa, and Rudbeckia type pollens. While these species were found growing in the plantings, only Rudbeckia type species were seeded in the plantings.
5. This study provides evidence that wildflower plantings (though not only seeded species) are filling a critical resource gap for stem-nesting bees in agricultural landscapes and likely support local populations.

     

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.     

Cover crop mulches influence biological control of the imported cabbageworm (Pieris rapae L., Lepidoptera: Pieridae) in cabbage

Increasing structural complexity within crop fields can provide a way to manipulate pest abundance and biological control in agroecosystems. Here, we examine the effect of cover crop mulches in cabbage on the structure and function of an insect food web, investigating the role of cover crop species, structure, and volatile cues on important interactions. We focused on the imported cabbageworm (Pieris rapae L., Lepidoptera: Pieridae), and three of its natural enemies, the spined soldier bug (Podisus maculiventris (Say), Hemiptera: Pentatomidae), the convergent lady beetle (Hippodamia convergens (Guerin), Coleoptera: Coccinellidae), and the parasitoid, Cotesia rubecula Marshall (Hymenoptera: Braconidae). We measured the abundance of these insects in a field experiment and conducted a natural enemy exclusion cage study to determine the level of biological control of the imported cabbageworm in the field. Our field experiments indicated that cover crop species, but not structure, influenced insect abundance, with significantly more imported cabbageworm and C. rubecula in rye cover crop mulch plots compared to vetch mulch or bare soil plots. In the Y-tube assays we found some evidence that the increased parasitoid abundance did not result in increased parasitism because of interference of the mulch with short-range host finding odor cues. The natural abundance of the two predators was not different among our field plots with different cover crop treatments. Mortality and parasitism of sentinel imported cabbageworm larvae was not different in field cages among the different cover crop mulch treatments, but there was a significant difference among cage types indicating that small natural enemies play an important role in the biological control of this cabbage pest.     

New tools to boost butterfly habitat quality in existing grass buffer strips

There are approximately 29,000 ha of grass buffer strips in the UK under Agri-Environment Schemes; however, typically they are floristically poor and as such are of limited biodiversity value. Introducing a sown wildflower component has the potential to increase dramatically the value of these buffer strips for a suite of native species, including butterflies. This study investigates management practices aiming to promote the establishment and maintenance of wildflowers in existing buffer strips. The effectiveness of two methods used to increase the establishment of wildflowers for the benefit of native butterfly species were tested, both individually and in combination. The management practices were: (1) the application of a selective graminicide (fluazifop-P-butyl) which reduces the dominance of competitive grasses; and (2) scarification of the soil which creates germination niches for sown wildflower seeds. A wildflower seed mix consisting of nine species was sown in conjunction with the scarification treatment. Responses of wildflowers and butterflies were monitored for two years after establishment. Results indicate that the combined scarification and graminicide treatment produced the greatest cover and species richness of sown wildflowers. Butterfly abundance, species richness and diversity were positively correlated with sown wildflower species richness, with the highest values in the combined scarification and graminicide treatment. These findings have confirmed the importance of both scarification as a means of introducing wildflower seed into existing buffer strips, and subsequent management using graminicides, for the benefit of butterflies. Application of this approach could provide tools to help butterfly conservation on farmland in the future.     

Effects of position within wheat field and adjacent habitats on the density and diversity of cereal aphids and their natural enemies

The spatial structure of agricultural landscapes can have a strong impact on the distribution and diversity of insects. Here we studied the effects of within-field position (edge or center) as well as adjacent habitats on the community structure of the natural enemies of cereal aphids. Twelve agricultural sites were included in the study, with two spring wheat fields selected for each site (one adjacent to an alfalfa field, the other adjacent to a corn field). We sampled two rows per field (1 and 20 m from the edge) using pitfall trapping for ground-dwelling predators, sweep netting for leaf-dwelling predators and hand collecting of aphid mummies for parasitoids. Adjacent alfalfa areas, as opposed to corn fields, can significantly increase the abundance and diversity of leaf-dwelling predators and parasitoids near the field edges. Abundance and diversity were found significantly higher near the edges than in the centers of fields adjacent to alfalfa areas. In contrast, no significant differences were found between edges and centers of fields adjacent to corn fields. Of the fifteen most abundant species, Aphidius avenae (Haliday), A. gifuensis (Ashmead), Hippodamia variegata (Goeze) and Chrysopa sinica (Tjeder) were significantly more abundant near the edge than in the center. Being adjacent to alfalfa habitats could enhance parasitism and predator/prey ratios of leaf-dwelling predators at the edges, but has no effects on ground-dwelling predators. In conclusion, the effect of within-field position and adjacent habitats on natural enemies of agricultural pests was species specific. This should be considered for designing efficient plans of biological control.