An order-specific monoclonal antibody to Diptera reveals the impact of alternative prey on spider feeding behavior in a complex food web

Generalist predators have the capacity to restrict pest population growth, especially early in the season before densities increase. However, their polyphagous feeding habits sometimes translate into reduced pest consumption when they target alternative prey. An order-specific monoclonal antibody was developed to examine the strength of trophic connections between Diptera, a major category of non-pest prey, and linyphiid spiders in alfalfa. We report the development and characterization of a monoclonal antibody with order-level specificity to Diptera. This antibody elicited strong absorbance to 22 Diptera from 13 families, no false-positive reactivity to non-dipteran invertebrates, and antigen detection periods following prey consumption that were comparable between spiders. Over 900 field-collected females of the linyphiid spiders Erigone autumnalis and Bathyphantes pallidus were screened for Diptera antigen. Significantly more B. pallidus screened positive for Diptera (40%) compared to E. autumnalis (16%), indicating differential reliance on these prey. In parallel with the collection of spiders for gut-content analysis, prey availability was estimated at web sites. The two spiders exhibited different feeding responses to prey availability. Consumption of Diptera by B. pallidus was strongly correlated with Diptera abundance whilst the availability of other potential prey did not influence predation rates. Conversely, E. autumnalis did not prey upon Diptera in proportion to availability, but increased Collembola activity-density reduced dipteran consumption. Integration of molecular gut-content analysis with precise sampling of prey demonstrated how two closely related linyphiid spiders exhibit different feeding responses to the availability of prey under natural field conditions. Elucidating the feeding preferences of natural enemies is critical to effective incorporation of biological control by generalist predators in the management of agricultural pests.     

Pollen interception by linyphiid spiders in a corn agroecosystem: implications for dietary diversification and risk-assessment

Dietary diversification, including consumption of plant tissues such as pollen, can enhance the fecundity of generalist predators, resulting in improved control of pest prey. Supplemental pollen feeding has been observed in many natural enemies, including sheet-web spiders (Araneae: Linyphiidae), which represent a major component of food webs in agroecosystems. Their horizontal, ground-based webs have the potential to intercept pollen grains during anthesis of crop plants, providing the opportunity for consumption of pollen to occur. In laboratory feeding trials, Frontinella communis and Tennesseellum formicum (Araneae: Linyphiidae) readily fed on pollen grains dusted on their webs, with 82 and 92% of spiders consuming pollen within the 210 min trial. These results revealed a strong potential for dietary supplementation with pollen in ground-based sheet-web spiders, indicating that pollen feeding may be an important component of the feeding biology of linyphiids. To measure pollen and prey interception in simulated linyphiid webs, a 20 m × 20 m grid of miniature sticky traps was established within and downwind of a corn agroecosystem. Traps were exposed for 24 h, all intercepted material was transferred to the laboratory for subsequent identification, and replaced with additional traps for 28 consecutive days in July and August 2008, to encompass periods before, during and after anthesis. Over 150,000 corn pollen grains and 5,000 prey items (dominated by Collembola and Hemiptera) were intercepted at simulated web-sites. Dates of peak anthesis resulted in pollen counts as high as 4,000 grains per web-site in the interior of the cornfield. Spatial Analysis by Distance Indices (SADIE) indicated significant temporal and spatial variability in pollen interception within and outside the corn field, but interestingly there was no significant spatial association between pollen and prey. Furthermore, transgenic Bacillus thuringiensis corn expresses insecticidal proteins in pollen, posing an exposure risk to non-target arthropods. Consumption of corn pollen may be a route to transgenic protein exposure in this important taxon of generalist predators.     

Selectivity underlies the dissociation between seasonal prey availability and prey consumption in a generalist predator

Generalist predators are capable of selective foraging, but are predicted to feed in close proportion to prey availability to maximize energetic intake especially when overall prey availability is low. By extension, they are also expected to feed in a more frequency‐dependent manner during winter compared to the more favourable foraging conditions during spring, summer and fall seasons. For 18 months, we observed the foraging patterns of forest‐dwelling wolf spiders from the genus Schizocosa (Araneae: Lycosidae) using PCR‐based gut‐content analysis and simultaneously monitored the activity densities of two common prey: springtails (Collembola) and flies (Diptera). Rates of prey detection within spider guts relative to rates of prey collected in traps were estimated using Roualdes’ c_st model and compared using various linear contrasts to make inferences pertaining to seasonal prey selectivity. Results indicated spiders foraged selectively over the course of the study, contrary to predictions derived from optimal foraging theory. Even during winter, with overall low prey densities, the relative rates of predation compared to available prey differed significantly over time and by prey group. Moreover, these spiders appeared to diversify their diets; the least abundant prey group was consistently overrepresented in the diet within a given season. We suggest that foraging in generalist predators is not necessarily restricted to frequency dependency during winter. In fact, foraging motives other than energy maximization, such as a more nutrient‐focused strategy, may also be optimal for generalist predators during prey‐scarce winters.     

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.     

Consumption of aphids by spiders and the effect of additional prey: evidence from microcosm experiments

Spiders are common generalist predators in agroecosystems and have been suggested to lower herbivore abundance in crops. It is not clear, however, if spiders can effectively suppress pest populations, and if so, by what mechanisms. In a microcosm experiment, we examined the consumption of the bird cherry-oat aphid, Rhopalosiphum padi L. (Homoptera: Aphididae), a pest species in wheat fields, by three spider species that differ in their hunting methods. We then tested the effect of additional prey type on the ability of erigonid spiders to reduce aphids. In a 48-h experiment Mermessus denticulatus (Banks) (Araneae: Linyphiidae; Erigoninae) consumed more aphids than did Enoplognatha gemina Bosmans and Van Keer (Araneae: Theridiidae) and Bathyphantes cf. extricatus (O·P.-Cambridge) (Araneae: Linyphiidae; Linyphiinae). This difference may be due to the ability of erigonids to forage actively on the vegetation in addition to using their webs to catch prey. In a 7-week experiment, we provided springtails (Collembola) in high and low densities as additional prey to mated erigonids, prior to aphid introduction. Spiders in the low-density springtail treatment built more webs on the vegetation, and caused a 50% reduction in aphid populations. There were significantly fewer aphids in the low-density springtail treatment, but not in the high-density treatment, in comparison to the control (high-density springtails without spiders). The results suggest that additional prey density affects predatory interactions between M. denticulatus and R. padi and that erigonids, which occur in high densities in wheat fields in the Negev desert, may be involved in aphid suppression in these agroecosystems.

Costs and benefits of freezing behaviour in the harvestman Eumesosoma roeweri (Arachnida, Opiliones)

Animals present an enormous variety of behavioural defensive mechanisms, which increase their survival, but often at a cost. Several animal taxa reduce their chances of being detected and/or recognized as prey items by freezing (remaining completely motionless) in the presence of a predator. We studied costs and benefits of freezing in immature Eumesosoma roeweri (Opiliones, Sclerosomatidae). Preliminary observations showed that these individuals often freeze in the presence of the syntopic predatory spider Schizocosa ocreata (Araneae, Lycosidae). We verified that harvestmen paired with predators spent more time freezing than when alone or when paired with a conspecific. Then, we determined that predator chemical cues alone did not elicit freezing behaviour. Next, we examined predator behaviour towards moving/non-moving prey and found that spiders attacked moving prey significantly more, suggesting an advantage of freezing in the presence of a predator. Finally, as measure of the foraging costs of freezing, we found that individuals paired with a predator for 2 h gained significantly less weight than individuals paired with a conspecific or left alone. Taken together, our results suggest that freezing may protect E. roeweri harvestmen from predatory attacks by wolf spiders, but at the cost of reduced food and/or water intake.     

Conspicuous colouration attracts prey to a stationary predator

Abstract 1. Conspicuous body colouration is counter‐intuitive in stationary predators because sit‐and‐wait tactics frequently rely on concealed traps to capture prey. Consequently, bright colours and contrasting patterns should be rare in predators using traps as they may alert potential prey. Yet, some orb‐weaving spiders are brightly coloured and contrastingly patterned. How can conspicuousness of trap‐building sit‐and‐wait predators be favoured by natural selection? 2. Observations of spiny spiders Gasteracantha fornicata in north‐eastern Australia showed that the size of spiders relative to their orb webs correlated positively with relative prey numbers already captured in their webs. A possible explanation is that the relatively larger appearance of the yellow–black striped dorsal surface of this spider attracts more visually oriented prey items. Prey attracted to webs may get trapped, thereby increasing the spiders' foraging success. 3. To test this hypothesis for the function of conspicuous body colouration, a field experiment was conducted that documented the prey capture rates of spiny spiders after manipulating or sham‐manipulating their appearance. 4. As predicted, spiders that were dyed black on their striped dorsal surface caught relatively fewer prey items than did control spiders. Thus, conspicuous dorsal body colouration may be adaptive in spiny spiders because it increases foraging success and, presumably, survival rates and reproductive outputs. Overall, these data support the colour‐as‐prey‐attractant hypothesis in a stationary, trap‐building predator.     

Signalling conflict between prey and predator attraction

Predators may utilize signals to exploit the sensory biases of their prey or their predators. The inclusion of conspicuous silk structures called decorations or stabilimenta in the webs of some orb‐web spiders (Araneae: Araneidae, Tetragnathidae, Uloboridae) appears to be an example of a sensory exploitation system. The function of these structures is controversial but they may signal to attract prey and/or deter predators. Here, we test these predictions, using a combination of field manipulations and laboratory experiments. In the field, decorations influenced the foraging success of adult female St. Andrew’s Cross spiders, Argiope keyserlingi: inclusion of decorations increased prey capture rates as the available prey also increased. In contrast, when decorations were removed, prey capture rates were low and unrelated to the amount of available prey. Laboratory choice experiments showed that significantly more flies (Chrysomya varipes; Diptera: Calliphoridae) were attracted to decorated webs. However, decorations also attracted predators (adult and juvenile praying mantids, Archimantis latistylus; Mantodea: Mantidae) to the web. St. Andrew’s Cross spiders apparently resolve the conflicting nature of a prey‐ and predator‐attracting signal by varying their decorating behaviour according to the risk of predation: spiders spun fewer decorations if their webs were located in dense vegetation where predators had greater access, than if the webs were located in sparse vegetation.     

Effects of Foraging Experiences on Residence Time of the Predatory Mite <Emphasis Type="BoldItalic">Neoseiulus womersleyi</Emphasis> in a Prey Patch

We investigated the effects of foraging experiences on the residence time of Neoseiulus womersleyi in a currently inhabited prey (Tetranychus urticae) patch. Satiated predators that had experienced starvation stayed longer in a current patch than those that had not experienced starvation. Satiated predators that had experienced a prey-rich patch showed approximately the same residence time in the current patch irrespective of the number of prey therein. By contrast, satiated predators that had experienced a prey-poor patch stayed longer in a current patch of high prey density than in one of low prey density. N. womersleyi appears to determine residence time in the current patch based on foraging experiences together with the quantity of prey in the current patch.     

Refuge from cannibalism in complex-structured habitats: implications for the accumulation of invertebrate predators

Abstract.  1. Like many invertebrate predators, the wolf spider Pardosa littoralis Banks (Araneae: Lycosidae) accumulates in complex-structured habitats replete with leaf litter (thatch). Here we test the hypothesis that P. littoralis accumulates in complex habitats to gain refuge from cannibalism.
2. A laboratory experiment examined the effects of habitat complexity (thatch present or absent) and size-class pairing of conspecific spiders (large vs. small, small vs. small, and large vs. large) on the incidence of cannibalism. Spider survival was significantly higher (22%) in complex-structured habitats with thatch than in simple-structured habitats lacking thatch. Furthermore, cannibalism occurred more frequently in P. littoralis when the size of conspecifics was asymmetric (large vs. small spiders) than when spiders were of equal size. There was no interactive effect of habitat complexity and size-class pairing on spider survival.
3. A field experiment examined the effects of habitat complexity, conspecific density, and access to alternative prey on the prevalence of cannibalism in P. littoralis . Access to alternative prey significantly increased the number of spiders recovered from field enclosures, as did the presence of leaf litter thatch. That fewer spiders were recovered when thatch and alternative prey were absent suggests that cannibalism was most prevalent under these conditions.
4. Overall, results suggest that habitat complexity reduces agonistic interactions and cannibalism among wolf spiders, providing encouragement to pest managers that the structure of agricultural habitats can be managed to maximise densities of generalist predators for enhanced pest suppression.     

Prey selection by linyphiid spiders: molecular tracking of the effects of alternative prey on rates of aphid consumption in the field

A molecular approach, using aphid-specific monoclonal antibodies, was used to test the hypothesis that alternative prey can affect predation on aphids by linyphiid spiders. These spiders locate their webs in cereal crops within microsites where prey density is high. Previous work demonstrated that of two subfamilies of Linyphiidae, one, the Linyphiinae, is web-dependent and makes its webs at sites where they were more likely to intercept flying insects plus those (principally aphids) falling from the crop above. The other, the Erigoninae, is less web-dependent, making its webs at ground level at sites with higher densities of ground-living detritivores, especially Collembola. The guts of the spiders were analysed to detect aphid proteins using enzyme-linked immunosorbent assay (ELISA). Female spiders were consuming more aphid than males of both subfamilies and female Linyphiinae were, as predicted, eating more aphid than female Erigoninae. Rates of predation on aphids by Linyphiinae were related to aphid density and were not affected by the availability of alternative prey. However, predation by the Erigoninae on aphids was significantly affected by Collembola density. Itinerant Linyphiinae, caught away from their webs, contained the same concentration of aphid in their guts as web-owners. However, nonweb-owning Erigoninae, living away from Collembola aggregations at web-sites, contained significantly higher concentrations of aphid. For both subfamilies there was evidence of a disproportionate increase in predation on aphids once Collembola populations had declined. It was concluded that nonaphid prey, by helping to maintain spiders in the field, can significantly affect predation on aphids.     

Temperature and prey capture: opposite relationships in two predator taxa

Abstract 1. All other things equal, predator capture rates are expected to depend on encounter rate with prey, prey escape capability (including prey defences), and on predator agility. Ectotherm predators and their prey both respond to increasing temperature by increased activity, i.e. predators increase their search area and prey may enhance their escape capability. This means that, as temperature changes, the ability of a predator to catch prey will decrease, increase, or remain unchanged depending on the relative effect of temperature on predator and prey. Their responses may further be differentially moulded by light conditions depending on whether the predator is diurnally or nocturnally active. It was hypothesised that flying Diptera are vulnerable to carabid beetles only at low temperatures and over the full temperature range for spiders because carabids, in contrast to spiders, are not built to catch swiftly moving prey. 2. The first experiment examined the spontaneous locomotor activity of the predators and of fruit flies at different temperatures (5, 10, 15, 20, 25, and 30 °C) and light conditions (light, dark). A second experiment examined the effect of temperature and light on the predation rate of two carabid beetles (Pterostichus versicolor and Calathus fuscipes) and two spiders (Clubiona phragmitis and Pardosa prativaga) using fruit flies (Drosophila melanogaster) as prey. 3. All four predators and the fruit fly increased their locomotory activity at higher temperatures. Activity of the carabid beetles peaked at intermediate temperatures; spiders and fruit flies were most active at the highest temperatures. Predation rate of the spiders increased with temperature whereas the beetles caught flies only at low temperatures (5 and 10 °C). 4. Diurnal variation in temperature may bring different prey groups within the set of potential prey at different times of the day or at different seasons. The ability of many carabid beetles to forage at low temperatures may have nutritional benefits and increases the diversity of interactions in terrestrial food webs.     

A 3-year field-scale monitoring of foliage-dwelling spiders (Araneae) in transgenic Bt maize fields and adjacent field margins

Concerns have been raised that genetically modified Bt maize may harm non-target organisms, and there is a general call and need for a risk assessment of Bt maize. Spiders are important pest predators in agroecosystems and in maize, and can be exposed to the Bt toxin by herbivorous or pollen-collecting prey, by active Bt maize pollen feeding, and by ingesting their pollen-dusted webs. The foliage-dwelling spider fauna of Bt maize fields and adjacent margins was monitored and compared to non-transgenic maize fields. The study took place during the vegetation seasons of 2001–2003 in Bavaria, South Germany. Maize fields and adjacent nettle field margins were colonized by a typical spider assemblage, dominated by space-web spiders (Theridiidae and Linyphiidae). Abundance and species richness of spiders was higher in nettle margins than in maize fields. The proportion of hunting spiders tended to be higher in nettle margins, whereas space-web spiders tended to be more frequent in maize fields. Bt maize showed no consistent effect on individual numbers, species richness and guild structure of spiders in maize fields and adjacent nettle field margin strips. The spider abundance was higher in Bt treatments in 2003, whereas in 2001 and 2002 no significant differences were found. The results provide an important contribution for the implementation of case-specific and general surveillance of transgenic plants to be employed due to the regulations of the European Community.     

Augmentation of beneficial arthropods by strip-management. 3. Artificial introduction of a spider species which preys on wheat pest insects

During a 3-year-experiment on strip-management a population of the spiderDictyna arundinacea (L.) was released in a winter wheat field.D. arundinacea built its webs with high preference at the ears of the wheat and 26–28% of the released spiders were rediscovered at the marked first web-sites some weeks later. Enclosure studies and prey samples from several sub-populations showed thatD. arundinacea caught almost exclusively wheat pest insects, withOscinella frit (L.), cereal aphids [Sitobion avenae (F.) andRhopalosiphum padi (L.); mainly winged specimens] and thysanopterans as main prey groups.D. arundinacea moved from the release site into adjacent successional strips, where it survived the harvest and successfully overwintered. From here, a repopulation of the wheat-strips in the next year occurred but the total abundance was so low that after this two-fold habitat change no high population density was reached. The ability of spiders, as unspecialized predators, to prey on wheat pest insects and the suitability of successional strips to preserve high spider densities by habitat change during harvest or other critical events are discussed.      

The effect of wounds on desiccation of prey: implications for a predator with extra-oral digestion

Predators that inject prey with proteolytic enzymes, thereby breaking down their tissues for subsequent ingestion, run the risk that desiccation will hinder eventual retrieval of resources from these prey. Wounds made in capture might exacerbate this problem. However, desiccation rates of small syrphid flies Toxomerusmarginatus (Diptera: Syrphidae) killed by juvenile crab spiders Misumena vatia (Araneae: Thomisidae) and intact dead syrphid flies did not differ over the normal period of feeding, though desiccation rates in shade and sun differed several-fold. Neither the size of the spider (and presumably the size of the wounds it inflicted) nor the location of the wounds on the flies' bodies affected desiccation rates. Thus, this tactic of prey handling does not exact an added processing cost on Misumena. Received: 6 October 1997 / Accepted 19 December 1997     

Diel and seasonal patterns of prey available to epigeal predators: Evidence for food limitation in a linyphiid spider community

Linyphiid spiders, as part of the community of natural enemies that frequent agroecosystems, can exert significant pressure on prey populations. Many aspects of linyphiid feeding ecology remain understudied, including temporal and seasonal patterns of prey utilization. To quantify the diversity, quantity, and spatial pattern of availability over diel and seasonal gradients, we monitored prey in an alfalfa crop, using web-site specific sticky traps. Although there were no differences in total number of prey between the day and night, significantly more Collembola were intercepted nocturnally rather than diurnally. Conversely, Diptera and Aphididae were significantly more abundant diurnally. Overall there were more prey at web-sites than in the general crop environment, indicating that spiders were selectively constructing webs where prey was relatively more abundant. However, spiders still may have experienced a high degree of prey limitation because the abundance of Collembola, an important detrital prey, was low over much of the season and below levels calculated as being required to sustain growth and reproductive output in linyphiid spiders. In conclusion, our data suggests that linyphiid spiders, in order to avoid food limitation, may benefit by foraging throughout the diel cycle, consuming herbivorous prey in addition to their core diet of detrital prey. This link between above and below ground food webs indicates the importance of understanding the trophic interactions between epigean spiders and their prey if they are to be integrated into future biological control programs.     

Spider predation on a mirid pest in Japanese rice fields

Spiders are common generalist predators, and understanding their potential in biological control is important for the development of integrated pest management programs. In this study, predation by three groups of spiders on the mirid bug Stenotus rubrovittatus (Hemiptera: Miridae) in rice paddies was investigated using DNA-based gut-content analysis. A laboratory feeding study revealed that the detection half-lives of bug DNA in the spider gut at 25 °C was 3.4 days for Lycosidae and 1.5 days for Tetragnathidae. Individual spider predation on the mirid bug was investigated by detecting DNA of prey in field-collected spiders. In total, 1199 spiders were assayed from three spider groups: Pirata subpiraticus (Lycosidae), Tetragnatha spp. (Tetra-gnathidae), and Pachygnatha clercki (Tetra-gnathidae), which each differ in their preferred microhabitat as well as their predatory habits. Detection rates of prey DNA in spiders increased significantly with the density of prey across all spider groups. P. subpiraticus and Tetragnatha spp. predation showed a better fit to a saturated response curve to increasing prey density, while P. clercki showed a simple linear relationship with prey density. Densities of alternative prey species did not affect the detection rates of mirids. These results suggest that predation on pests by generalist predators in an agroecosystem is affected not only by prey abundance but also by predator preference for specific prey. Predator preference is therefore an important factor to consider when estimating the role of natural enemies as biological control agents.     

Aggregation of polyphagous predators in response to multiple prey: ladybirds (Coleoptera: Coccinellidae) foraging in alfalfa

The spatial distribution of polyphagous predators may often reflect the integration of aggregative responses to local densities of multiple species of prey, and as such may have consequences for the indirect linkages among the prey sharing these predators. In a factorial field experiment in which we manipulated local prey densities within a field of alfalfa in Utah (USA), we tested whether aphidophagous ladybirds would aggregate not only in response to their primary aphid prey, but also in response to an abundant alternative prey, the alfalfa weevil (Hypera postica [Gyllenhal]). Native North American ladybirds (primarily Hippodamia convergens Guerin and H. quinquesignata quinquesignata [Kirby]) responded only to spatial variation in aphid density. In contrast, the introduced ladybird, Coccinella septempunctata L., aggregated also at local concentrations of the weevil late in the experiment when weevil density was high and aphid density was relatively low throughout all experimental plots. The results support the hypothesis that C. septempunctata is more responsive than are native ladybirds to the availability of alternative prey in alfalfa, which may account in part for the displacement of native ladybirds from alfalfa by the introduced species as aphid numbers have declined. The differing responses of the native and introduced ladybirds to spatial patterns of the alternative prey underscore the importance of extending the study of predator aggregation to understand better how polyphagous predators distribute themselves in response to spatial patterns of multiple species of potential prey.     

Contributions of detrital subsidies to aboveground spiders during secondary succession, revealed by radiocarbon and stable isotope signatures

Prey subsidies originating from detritus add nutrients and energy to arboreal communities. Measurement of this subsidy is required in the understanding of how food web dynamics respond to changes in surrounding environments. Shrub spiders are one of the key predators involved in food web coupling. We evaluate the effects of potential changes in prey availabilities during secondary succession on the contribution of subsidy from detrital food webs to shrub spiders and how different spider feeding guilds used the subsidy of prey from detrital food webs. We measured the relative importance of the subsidy for the spider feeding guilds, using the ratios of stable isotopes of C (δ¹³C), and N (δ¹⁵N) and C isotope discrimination (Δ¹⁴C). Diet age was calculated from Δ¹⁴C values, because old diet ages of spiders indicate that the spiders consume prey from detrital food sources. Dominant aerial prey (Diptera) had a distinctively old diet age compared with arboreal prey, which indicates that aerial prey were subsidized from detrital food webs. Sit-and-wait spiders tended to have an older diet age than active hunting spiders, which indicates that sit-and-wait spiders depended more on subsidies. Diet age varied only slightly for spiders in stands of different ages, indicating that rates at which spiders use grazing and detrital prey are probably determined more by foraging strategies and not by stand age. A dominance of sit-and-wait predators will lead to higher detrital subsidy inputs in shrub habitats. This study highlights the effect of shrub spider community structure (feeding guild composition) on the volume of the subsidy received from the detrital food web.     

Invertebrate biodiversity affects predator fitness and hence potential to control pests in crops

Natural enemies that control pests usually allow farmers to avoid, or reduce, the use of pesticides. However, modern farming practices, that maximize yields, are resulting in loss of biodiversity, particularly prey diversity. Does this matter? Pests continue to thrive, and without alternative prey the predators should, perforce, concentrate their attentions upon the pests.We showed that a diverse diet significantly enhances predator fecundity and survival. Experiments were conducted using common generalist predators found in arable fields in Europe, the carabid beetle Pterostichus melanarius (Coleoptera: Carabidae) and the linyphiid spider Erigone atra (Araneae: Linyphiidae). We tested the hypothesis that mixed species diets were optimal, compared with restricted diets, with respect to parameters such as predator weights, egg weights, numbers of eggs laid, egg development times, egg hatching rates and predator survival. In carabids, an exclusive earthworm diet was as good as mixed diets containing earthworms for egg production and hatching, but less good than such mixed diets for increase in beetle mass and sustained egg laying. For spiders, aphids alone (Sitobion avenae) or with the Collembola Folsomia candida, drastically reduced survival. Aphids plus the Collembola Isotoma anglicana improved survival but only aphids with a mixed Collembola diet maximized numbers of hatching eggs.Predators offered only pests (slugs or aphids) had lowest growth rates and fecundity. We therefore demonstrated that conservation of a diversity of prey species within farmland, allowing predators to exploit a diverse diet, is essential if predators are to continue to thrive in crops and regulate agricultural pests.