Aphid suppression by natural enemies in mulched cereals

Large populations of natural enemies are the basis for natural pest control. Effects of mulch on predator–prey interactions in arable fields are poorly known, despite its potential to enhance ground‐dwelling predators and thereby reduce pest infestations. We studied the densities of predators and parasitoids, and their impact on cereal aphids in the presence and absence of mulch. Released populations of the bird cherry aphid, Rhopalosiphum padi (L.) (Homoptera: Aphididae), and two naturally occurring aphid species, were monitored under experimentally reduced densities of: (i) ground‐dwelling predators, (ii) flying predators and parasitoids, and (iii) with straw mulch. The three treatments were applied in a 2 × 2 × 2 factorial design in a field of spring wheat (Triticum aestivum L.). The exclusion of ground‐dwelling predators increased aphid populations by 55% in June and 40% in July, respectively. Mulched plots had 25% lower aphid densities in June. This was presumably due to enhanced densities of spiders (Araneida) in mulched plots. The exclusion of flying predators and parasitoids led to 94% higher aphid populations in late July (109 vs. 56 individuals per 100 shoots), irrespective of mulch or ground predator manipulation. This was attributed to the larvae of gall midges Aphidoletes cf. aphidimyza (Rondani) (Diptera: Cecidomyiidae) and hoverflies (Diptera: Syrphidae). The results indicate that a scarcity of predators and a bare soil surface renders crops more susceptible to arthropod pests. Farming schemes should aim at enhancing both ground‐dwelling and flying predators for elevated levels of natural pest control.     

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.     

Predator Interference and the Establishment of Generalist Predator Populations for Biocontrol

We conducted a field experiment to determine the extent to which interference among generalist predators limits their effectiveness as biocontrol agents. We manipulated immigration of a guild of actively hunting generalist ground predators, carabid beetles and lycosid spiders, by intercepting them as they attempted to enter fenced 50-m² vegetable gardens. Immigration was blocked, allowed at the mean rate measured at our field site, or doubled. Altered immigration rates were maintained through a spring garden of cabbage, bean, eggplant, and cucumber, followed by a summer garden of squash. We monitored densities of carabids and lycosids to discover if altering their immigration rate changed their densities in the plots. We also measured densities of other predators on the ground and in plant foliage, pest numbers, and vegetable yields. Doubling the immigration rate of carabids and lycosids approximately doubled the densities of carabids inside the plots, but did not increase lycosid densities. Increasing the rate of immigration of carabids and lycosids depressed densities of nonlycosid ground spiders. In the spring gardens, manipulation of carabid and lycosid immigration did not influence numbers of predators or herbivores in the foliage and did not affect vegetable productivity. In contrast, in the summer gardens, foliage-dwelling predators were lower, pest densities were marginally lower, and squash productivity was higher in the carabid and lycosid immigration plots compared to the no-immigration treatment. Doubling carabid and lycosid immigration rate never increased the magnitude of their effects on other predators, pests, or plant productivity. Predator interference limited lycosid establishment, reduced densities of other predator taxa, and apparently prevented a doubling of carabid densities from having an increased impact on pest numbers. Nevertheless, despite widespread effects of predator interference, allowing immigration of lycosids and carabids increased squash productivity.     

Temporal relationships between the generalist predator,Orius insidiosus,and its two major prey in soybean

The generalist predator, Orius insidiosus (Say) is an important early-season predator of the soybean aphid, Aphis glycines Matsumura, a newly invasive pest of major concern in soybean crop management. We conducted a 3 year, multiple field study to characterize the dynamic relationships between the predator, the pest, and alternative prey in soybean. Using field sampling data, we showed that thrips were the only alternative prey to be well-established in fields prior to O. insidiosus arrival and were likely to promote predator colonization of soybean fields prior to the arrival of soybean aphid. The predator displayed a reproductive numerical response to thrips in one of the 3 years and a primarily aggregative response in another year. The predator did not respond numerically to soybean aphid in the majority of fields. Experimental manipulations of thrips populations in field plots temporarily reduced thrips densities but had a minimal effect on O. insidiosus densities, suggesting that the predator is resilient against temporary reductions in a major resource. In the 2 years O. insidiosus populations were well-established in fields prior to soybean aphid arrival, soybean aphid remained at low levels throughout the season. In the year soybean aphid arrived early with respect to the growing season and before O. insidiosus populations were established, soybean aphid reached outbreak levels in all fields. Future research efforts on the factors determining soybean aphid population dynamics need to address the relative importance of early-season soybean aphid colonization and generalist predator population dynamics on the potential for soybean aphid population outbreaks.     

Decreased functional diversity and biological pest control in conventional compared to organic crop fields

Organic farming is one of the most successful agri-environmental schemes, as humans benefit from high quality food, farmers from higher prices for their products and it often successfully protects biodiversity. However there is little knowledge if organic farming also increases ecosystem services like pest control. We assessed 30 triticale fields (15 organic vs. 15 conventional) and recorded vascular plants, pollinators, aphids and their predators. Further, five conventional fields which were treated with insecticides were compared with 10 non-treated conventional fields. Organic fields had five times higher plant species richness and about twenty times higher pollinator species richness compared to conventional fields. Abundance of pollinators was even more than one-hundred times higher on organic fields. In contrast, the abundance of cereal aphids was five times lower in organic fields, while predator abundances were three times higher and predator-prey ratios twenty times higher in organic fields, indicating a significantly higher potential for biological pest control in organic fields. Insecticide treatment in conventional fields had only a short-term effect on aphid densities while later in the season aphid abundances were even higher and predator abundances lower in treated compared to untreated conventional fields. Our data indicate that insecticide treatment kept aphid predators at low abundances throughout the season, thereby significantly reducing top-down control of aphid populations. Plant and pollinator species richness as well as predator abundances and predator-prey ratios were higher at field edges compared to field centres, highlighting the importance of field edges for ecosystem services. In conclusion organic farming increases biodiversity, including important functional groups like plants, pollinators and predators which enhance natural pest control. Preventative insecticide application in conventional fields has only short-term effects on aphid densities but long-term negative effects on biological pest control. Therefore conventional farmers should restrict insecticide applications to situations where thresholds for pest densities are reached.     

Detection of secondary predation by PCR analyses of the gut contents of invertebrate generalist predators

Predation by generalist predators is difficult to study in the field because of the complex effects of positive and negative interactions within and between predator species and guilds. Predation can be monitored by molecular means, through identification of prey DNA within predators. However, polymerase chain reaction (PCR) amplification of prey DNA from predators cannot discriminate between primary and secondary predation (hyperpredation), in which one predator feeds on another that has recently eaten the target prey. Here we quantify, for the first time, the potential error caused by detection of prey DNA following secondary predation, using an aphid-spider-carabid model. First, the aphid Sitobion avenae was fed to the spider Tenuiphantes tenuis and the carabid Pterostichus melanarius, and the postconsumption detection periods, for prey DNA within predators, were calculated. Aphids were then fed to spiders and the spiders to carabids. Aphid DNA was detected in the predators using primers that amplified 245- and 110-bp fragments of the mitochondrial cytochrome oxidase I gene. Fragment size and predator sex had no significant effect on detection periods. Secondary predation could be detected for up to 8 h, when carabids fed on spiders immediately after the latter had consumed aphids. Beetles tested positive up to 4 h after eating spiders that had digested their aphid prey for 4 h. Clearly, the extreme sensitivity of PCR makes detection of secondary predation more likely, and the only reliable answer in future may be to use PCR to identify, in parallel, instances of intraguild predation.     

Control of aphids on wheat by generalist predators: effects of predator density and the presence of alternative prey

There is evidence for both positive and negative effects of generalist predators on pest populations and the various reasons for these contrasting observations are under debate. We studied the influence of a generalist predator, Pardosa lugubris (Walckenaer) (Araneae: Lycosidae), on an aphid pest species, Rhopalosiphum padi (L.) (Hemiptera: Aphididae; low food quality for the spider), and its host plant wheat, Triticum spec. (Poaceae). We focused on the role of spider density and the availability of alternative prey, Drosophila melanogaster Meigen (Diptera: Drosophilidae; high food quality). The presence of spiders significantly affected plant performance and aphid biomass. Alternative prey and spider density strongly interacted in affecting aphids and plants. High spider density significantly improved plant performance but also at low spider density plants benefited from spiders especially in the presence of alternative prey. The results suggest that generalist arthropod predators may successfully reduce plant damage by herbivores. However, their ability to control prey populations varies with predator nutrition, the control of low-quality prey being enhanced if alternative higher-quality prey is available.     

A field experiment on the effectiveness of spiders and carabid beetles as biocontrol agents in soybean

1 Spiders and carabid beetles are abundant generalist predators that prey upon insect pests of soybean. A field experiment was conducted to determine the impact of spiders and carabids on soybean yield. Prior to planting, three 7 × 7 m plots were fenced in order to reduce spider and carabid immigration. Carabids that emerged within the plots were not removed, but spiders that ballooned into these predator‐reduction plots or that entered by climbing the fence were removed by pitfall trapping and searching the vegetation. Three unmanipulated, unfenced plots served as the control treatment. 2 Densities of spiders on soybean vegetation, and activity‐densities of spiders and carabids determined by pitfall trapping, were c. 75% lower in the spider‐carabid reduction treatment than in control plots. Despite clear differences between treatments in numbers and activity of these major generalist predators, the weight of soybeans harvested did not differ between control and spider‐carabid reduction plots. 3 Paralleling the absence of an effect of predator reduction on soybean yield was the absence of any significant difference between treatments in densities of whiteflies (Aleyrodidae), leafhoppers (Cicadellidae), thrips (Thysanoptera), Lepidoptera larvae and herbivorous Coleoptera. 4 Our experiment provides no evidence that spiders and carabid beetles at ambient densities affect soybean yield. Low populations of pest species or low predation pressure on soybean pests by spiders and carabids at the ambient densities of this experiment could be responsible for this result.     

Coccinellid immigration to infested host patches influences suppression of Aphis glycines in soybean

Generalist natural enemies may be well adapted to annual crop systems in which pests and natural enemies re-colonize fields each year. In addition, for patchily-distributed pests, a natural enemy must disperse within a crop field to arrive at infested host patches. As they typically have longer generation times than their prey, theory suggests that generalist natural enemies need high immigration rates to and within fields to effectively suppress pest populations. The soybean aphid, Aphis glycines Matsumura, is a pest of an annual crop and is predominantly controlled by coccinellids. To test if rates of coccinellid arrival at aphid-infested patches are crucial for soybean aphid control, we experimentally varied coccinellid immigration to 1 m² soybean patches using selective barriers and measured effects on A. glycines populations. In a year with low ambient aphid pressure, naturally-occurring levels of coccinellid immigration to host patches were sufficient to suppress aphid populations, while decreasing coccinellid immigration rates resulted in large increases in soybean aphid populations within infested patches. Activity of other predators was low in this year, suggesting that most of the differences in aphid population growth were due to changes in coccinellid immigration. Alternatively, in a year in which alate aphids continually colonized plots, aphid suppression was incomplete and increased activity of other predatory taxa contributed to adult coccinellid predation of A. glycines. Our results suggest that in a system in which natural enemy populations cannot track pest populations through reproduction, immigration of natural enemies to infested patches can compensate and result in pest control.     

Field boundaries of Panicum maximum as a reservoir for predators and a sink for Chilo partellus

Abstract:  The biological-control function of field boundaries of Guinea grass, Panicum maximum Jacq. on the spotted stem borer, Chilo partellus Swinhoe was examined as a reservoir for arthropod predators and as a trap plant for the pest. Field border vegetation and predator density were manipulated to determine the effect of the grass border on the abundance of stem borers and their predators in maize fields, and the effect of predators on the stem borer population. The strip of Guinea grass supported an abundance of earwigs and spiders, the potential predators of stem borer eggs and larvae. Density of C. partellus larvae in the Guinea grass strips was low throughout the season and only young larvae were collected, suggesting the inferiority of the grass stand as a habitat for stem borer larvae. These results indicate that Guinea grass is a good agent of habitat management to selectively enhance arthropod predators of stem borers and act as a sink for the pest. Predator removal resulted in a higher density of C. partellus than control in maize-bordered plots. On the other hand, no difference was found in the stem borer density between predator treatments in grass-bordered plots, probably because of insufficient predator reduction in removal plots. These results suggest that the predator assemblage found in the study site has, if sufficiently abundant, potential to limit the C. partellus population in maize fields. Even though the Guinea grass stand harboured an abundant number of predators, the grass boundaries around maize fields did not enhance predator populations within the crop field. Furthermore, field boundaries of Guinea grass had no measurable effect on the within-field density of C. partellus as a trap crop. Creating a polyculture within the crop and early planting of the grass could further enhance the biological-control function of Guinea grass boundaries.     

Role of host plant alternation in the precocity and intensity of parasitoid activity on of cereal aphids

Integrated pest management programs tend to reduce the chemical input by enhancing the development of biological control. Cereal aphids cause important damages to winter wheat in Europe but are currently under the pressure of several parasitoid species (Braconidae: Aphidiinae). Previous ecological studies have reported the existence of an asynchrony between aphid and parasitoid populations in early spring in cereal cultures. Here, we tested the presence of rose bushes (Rosa rugosa) as a host plant for alternative aphid-host. Aphid and parasitoid densities were recorded for two years using the plant cutting sampling method. The main results were: (i) rose bushes constitute a potential reservoir of alternative aphid hosts species for a number of parasitoid and predator species, (ii) rose aphids appear earlier in spring and were more abundant than the aphids on wheat, (iii) parasitism activity in rose bushes is synchronized with cereal fields, however (iv) rose bushes management did not induce a decrease of cereal aphid population.     

Changes in herbivore control in arable fields by detrital subsidies depend on predator species and vary in space

Prey from the decomposer subsystem may help sustain predator populations in arable fields. Adding organic residues to agricultural systems may therefore enhance pest control. We investigated whether resource addition (maize mulch) strengthens aboveground trophic cascades in winter wheat fields. Evaluating the flux of the maize-borne carbon into the food web after 9 months via stable isotope analysis allowed differentiating between prey in predator diets originating from the above- and belowground subsystems. Furthermore, we recorded aphid populations in predator-reduced and control plots of no-mulch and mulch addition treatments. All analyzed soil dwelling species incorporated maize-borne carbon. In contrast, only 2 out of 13 aboveground predator species incorporated maize carbon, suggesting that these 2 predators forage on prey from the above- and belowground systems. Supporting this conclusion, densities of these two predator species were increased in the mulch addition fields. Nitrogen isotope signatures suggested that these generalist predators in part fed on Collembola thereby benefiting indirectly from detrital resources. Increased density of these two predator species was associated by increased aphid control but the identity of predators responsible for aphid control varied in space. One of the three wheat fields studied even lacked aphid control despite of mulch-mediated increased density of generalist predators. The results suggest that detrital subsidies quickly enter belowground food webs but only a few aboveground predator species include prey out of the decomposer system into their diet. Variation in the identity of predator species benefiting from detrital resources between sites suggest that, depending on locality, different predator species are subsidised by prey out of the decomposer system and that these predators contribute to aphid control. Therefore, by engineering the decomposer subsystem via detrital subsidies, biological control by generalist predators may be strengthened.     

Preceding crop affects soybean aphid abundance and predator–prey dynamics in soybean

Crop rotations alter the soil environment and physiology of the subsequent crop in ways that may affect the abundance of herbivores and their natural enemies. Soybean aphids are a consistent pest of soybean throughout North America, but little work has focused on how preceding crops may affect pest–predator dynamics. In a replicated experiment over three years, we examined how two preceding crops (spring wheat or an oat/pea mixture) affected seasonal soybean aphid pressure and the ratio of aphids to their predator community. Peak aphid populations were reduced by 40% and 75% in years 1 and 2 by planting spring wheat before soybeans (relative to the oat–pea mixture). Aphid densities were unaffected by preceding crop in the third year of study (aphids were at threshold in this year). Predators responded positively to aphid population increases and were unaffected by preceding crops. Additional research on how crop rotations can be used as a tool to manage soybean aphids warrants further attention.     

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.     

Intraguild interference and biocontrol effects of generalist predators in a winter wheat field

Arable land typically harbours communities of polyphagous invertebrate natural enemies, among them numerous soil-surface dwelling predators such as ground beetles (Carabidae) and spiders (Lycosidae, Linyphiidae). Numbers of these predators were experimentally manipulated in a winter wheat field in order to study the predation impact of a generalist predator assemblage on herbivorous insects, the possible interferences among the predators concerned, and subsequent effects on wheat plant parameters. Removing ground beetles doubled numbers of Lycosidae indicative of intraguild interference between these two predator groups. Aphid densities were highest in carabid removal plots implying a substantial predation impact of ground beetles on the pest population. The predation impact of ground beetles was strongest earlier and disappeared later in the season. In mid-season, at intermediate aphid densities, the combined impact of carabid beetles and spiders appeared to be responsible for the reduction in aphid abundance. This result was probably due to a biomass effect rather than to a synergistic effect of the predator community. Thysanoptera decreased when spiders were removed (perhaps because spiders were preying on a predator of thrips), while Cicadellidae and Delphacidae showed no effect at all. The rise of aphid numbers in carabid removal plots corresponded to an increase in protein content of the wheat grains, while other plant parameters such as plant numbers and grain mass were not affected. In conclusion, this study provided field evidence for intraguild interference among generalist ground predators in arable land. Despite this interference the polyphagous predator community was able to depress numbers of aphids in winter wheat, a result cascading down to plant quality parameters.     

Monoclonal antibodies reveal the potential of the tetragnathid spider Pachygnatha degeeri (Araneae: Tetragnathidae) as an aphid predator

The drive towards a more sustainable and integrated approach to pest management has engendered a renewed interest in conservation biological control, the role of natural enemy communities and their interactions with prey. Monoclonal antibodies have provided significant advances in enhancing our knowledge of trophic interactions and can be employed to help quantify predation on target species. The tetragnathid spider Pachygnatha degeeri Sundevall was collected from fields of winter wheat in the UK and assayed by ELISA for aphid proteins. It was demonstrated that this spider did not simply consume greater quantities of aphids because it was bigger. In addition, P. degeeri contained significantly greater concentrations of aphid in their guts than other spiders, showing that aphids comprised a greater proportion of their diet. Although P. degeeri constituted only 6% of the spider population numerically, females and males respectively contained 16% and 37% of total aphid proteins within all spiders screened, significantly more than their density would predict. These spiders also preyed upon aphids at a disproportionately high rate in June, during the aphid establishment phase, theoretically the best time for limiting growth in the aphid population. Although less abundant than other generalist predators, the capability of these hunting spiders to consume large numbers of aphids highlights them as a more significant component of the predator complex than had previously been realized. Limitation of aphid numbers early in the year by generalist predators provides more time for the specialist aphid predators and parasitoids to move in.     

Predation of Rhopalosiphum padi (Homoptera: Aphididae) by polyphagous predatory arthropods during the aphids' pre-peak period in spring barley

Polyphagous predators (e.g. Araneae, Carabidae and Staphylinidae), collected from spring barley fields during 1981-85, were examined by either gut dissection or a R. padi-specific antiserum in enzyme linked immunosorbent assay (ELISA) in order to detect predation of Rhopalosiphum padi during the aphids' establishment and exponential growth phases. Overall 18% of c. 3000 carabids dissected were shown to feed on R. padi during the aphids' pre-peak period. No overall relationship was found between percentage carabids with R. padi in the diet and peak R. padi densities. Relatively high proportions of Bembidion spp. (particularly B. lampros) and Pterostichus cupreus fed on R. padi during the aphids' establishment phase, and proportions of those predator taxa containing R. padi increased with increasing R. padi densities in both high and low aphid density years. P. melanarius and Harpalus rufipes mainly fed on R. padi during the aphids' exponential growth phase. Overall 11% of c. 1350 predators examined in ELISA gave positive reactions to the R. padi antiserum. Relatively high proportions of linyphiid and lycosid spiders were positive throughout the aphid pre-peak period. Several Acari, Opiliones, Trechus spp. (Carabidae), Philonthus spp. (Staphylinidae), Cantharidae and Chilopoda were positive mainly during the aphids' exponential growth phase. Sample sizes were small, however. Very few of the Bembidion spp. tested in ELISA were positive compared with those examined by gut dissection. The maximum period of R. padi protein (antigen) detection in B. lampros was related to temperature, i.e. 8.5 h at 30°C, 20.5 h at 20°C and 34.5 h at 10°C, respectively. It is suggested that the rate of R. padi protein digestion in B. lampros is faster at higher temperatures than the rate of elimination of prey solids from the guts. Several key predators (in this case B. lampros, P. cupreus and linyphiid spiders) which are abundant in spring cereal fields at a time when R. padi emigrants arrive in the crop and which feed on R. padi during the aphids' establishment phase, are identified. The results are compared with those from similar investigations elsewhere with predators of Sitobion avenae and Metopolophium dirhodum.     

Can polyphagous predators control the bird cherry-oat aphid (Rhopalosiphum padi) in spring cereals?: A simulation study

Predation by two carabid species,Bembidion lampros (Herbst.) andPterostichus cupreus L., on the cereal aphid,Rhopalsiphum padi L., was described using a discrete event simulation model. The results were incorporated into an aphid population growth model. The aphid model was run using actual temperatures,R. padi emigration rates, field densities and distributions from three years with distinctly different aphid population levels. Predation was incorporated at different time periods. Predation during the arrival of emigrants and establishment of aphid populations had approximately the same effect as predation over the entire aphid period, whereas predation during the 20 days prior to peak had very little effect on resulting peak populations. We conclude that a) yearly levels ofR. padi infestation are mainly determined by the volume and intensity of emigrants; b) peak levels are sensitive to changes in predator density; and c) in some cases an outbreak ofR. padi may be prevented by the action of spring active polyphagous predators alone.     

Recommendations on minimum experimental plot size and succession of aphidophaga in West Virginia, USA, apple orchards

Five apple orchards in West Virginia, USA, were managed with five different pest management practices ranging from unsprayed to conventional insecticides. Data were collected on aphid, predaceous insect, and spider abundance every 2 weeks from May to July along one or two randomly selected, 15 consecutive tree transects. A total of 892 individual predaceous insects was observed: 32% wereAphidoletes aphidimyza, 21% wereCoccinella septempunctata and the rest were from 13 other taxa. The most important aphid predator seemed to beC. septempunctata. Of the 32 sample transects with predators, 72 % showed an edge effect in which either the standard error or mean of predator abundance differed from the edge towards the center of the orchard. This edge effect extended a mean of 7 trees (30 m) into the orchard. Only 39% of the 33 transects that had spiders showed an edge effect, extending into the orchard a mean of 8 trees (33 m). However, 75% of the transects with more than one spider per tree had an edge effect of 10 trees (43 m). We recommend a minimum border of seven trees before sampling for aphid predators and 10 trees before sampling for spiders. A recommended experimental plot size, for semi-dwarf, free-standing apple orchards, to eliminate edge effects is 1.25 ha for aphidophaga, 1.5 ha for spiders; a minimum plot size of 0.6 ha for aphidophaga and 0.75 ha for spiders would minimize edge effects.     

Cursorial spiders retard initial aphid population growth at low densities in winter wheat

Generalist predators contribute to pest suppression in agroecosystems. Spider communities, which form a substantial fraction of the generalist predator fauna in arable land, are characterized by two functional groups: web-building and cursorial (non-web-building) species. We investigated the relative impact of these two functional groups on a common pest (Sitobion avenae, Aphididae) in wheat by combining a molecular technique that revealed species-specific aphid consumption rates with a factorial field experiment that analyzed the impact, separately and together, of equal densities of these two spider functional groups on aphid population growth. Only cursorial spiders retarded aphid population growth in our cage experiment, but this effect was limited to the initial aphid-population growth period and low-to-intermediate aphid densities. The molecular analysis, which used aphid-specific primers to detect aphid DNA in predator species, detected the highest proportion of aphid-consuming individuals in two cursorial spiders: the foliage-dwelling Xysticus cristatus (Thomisidae) and the ground-active Pardosa palustris (Lycosidae). The results suggest that manipulating the community composition in favour of pest-consuming functional groups may be more important for improving biological control than fostering predator biodiversity per se. Agricultural management practices that specifically foster effective species or functional groups (e.g. mulching for cursorial spiders) should receive more attention in low-pesticide farming systems.