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.     

园蛛总科网型不同的蜘蛛间分子系统发生关系的初步探讨

为初步探讨园蛛总科内网型不同的蜘蛛间分子系统发生关系,测定了9种蜘蛛核18S rRNA基因的部分序列。联合GenBank中的2种蜘蛛18S rRNA基因序列数据,用NJ法、MP法和ML法重建分子系统树,结果表明：（1）织圆网的园蛛科和肖蛸科蜘蛛在园蛛总科中不是姊妹群;（2）不织圆网的球蛛科和皿蛛科也不是单系发生,证实了皿蛛片网和球蛛网独立起源的观点;（3）皿蛛科蜘蛛的分子系统发生地位与肖蛸科较近、而与园蛛科较远,这表明皿蛛科和园蛛科问雄性触肢结构的相似性很可能是类似而不是同源相似。     

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.

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.     

Evidence for competition between carnivorous plants and spiders

Several studies have demonstrated that competition between disparate taxa can be important in determining community structure, yet surprisingly, to our knowledge, no quantitative studies have been conducted on competition between carnivorous plants and animals. To examine potential competition between these taxa, we studied dietary and microhabitat overlap between pink sundews (Drosera capillaris) and wolf spiders (Lycosidae) in the field, and conducted a laboratory experiment examining the effects of wolf spiders on sundew fitness. In the field, we found that sundews and spiders had a high dietary overlap with each other and with the available arthropod prey. Associations between sundews and spiders depended on spatial scale: both sundews and spiders were found more frequently in quadrats with more abundant prey, but within quadrats, spiders constructed larger webs and located them further away from sundews as the total sundew trapping area increased, presumably to reduce competition. Spiders also constructed larger webs when fewer prey were available. In the laboratory, our experiment revealed that spiders can significantly reduce sundew fitness. Our findings suggest that members of the plant and animal kingdoms can and do compete.     

Competitive interactions between a native spider (<Emphasis Type="Italic">Frontinella communis</Emphasis>, Araneae: Linyphiidae) and an invasive spider (<Emphasis Type="Italic">Linyphia triangularis</Emphasis>, Araneae: Linyphiidae)

There are numerous reports of spiders that have become established outside of their native ranges, but few studies examine their impact on native spiders. We examined the effect of the European hammock spider Linyphia triangularis (Araneae, Linyphiidae) on the native bowl-and-doily spider Frontinella communis (Araneae, Linyphiidae) in Acadia National Park, Maine, USA. First, we added L. triangularis to established plots of F. communis. Significantly more F. communis abandoned their webs when L. triangularis were added compared to control plots. Second, we tested whether F. communis were deterred from building webs in areas where L. triangularis was established. Significantly fewer F. communis built webs on plots with L. triangularis than on control plots. In both experiments, L. triangularis sometimes took over webs of F. communis or incorporated F. communis webs into their own webs, but F. communis never took over or incorporated L. triangularis webs. Competition between L. triangularis and F. communis for both webs and web sites may contribute to the decline of F. communis.     

Spider predators of lepidopteran eggs in south Texas field crops

Observations were made of spiders attacking lepidopteran eggs in south Texas field crops (cotton, corn, and soybean) from 2001 to 2004. Twelve species of spider from seven families were observed feeding on the eggs during the 4 years. These spiders were primarily cursorial hunting spiders, and they were observed feeding on eggs most frequently in cotton, representing 26.6% of all observations in cotton over the 4 years. Spider predation on eggs was proportionally less frequent in corn and soybean with 6.3% and 15.4% of observed predation in those crops, respectively. Four species of spider were responsible for 86.1% of the predation by spiders. The anyphaenid Hibana futilis (Banks) was the spider most frequently observed feeding on lepidopteran eggs during the 4 years of this study, constituting 45.1% of all spiders observed. Grammonota texana Banks (Linyphiidae), Hibana arunda Platnick (Anyphaenidae), and Cheiracanthium inclusum (Hentz) (Miturgidae) were the 2nd, 3rd, and 4th most frequently observed spiders constituting 15.6%, 12.8%, and 11.7% of all spiders observed, respectively. Most spiders represented taxa that are known to forage without a web. However, G. texana was observed feeding on eggs independent of a web, which is uncharacteristic of linyphiids. Other cursorial hunting spiders feeding on eggs included members of the Clubionidae, Corrinnidae, and Salticidae. Ninety-eight percent of all observations of egg predation by spiders were nocturnal; only the Salticidae were diurnal. It is likely that previous studies of predation in crops have vastly underestimated the importance of spiders as predators of lepidopteran eggs due to inadequate evaluation of nocturnal predation.     

Diversity of spiders after spring sowing – influence of farming system and habitat type

Spiders contribute considerably to diversity in agroecosystems and are important components of natural pest control. Farming system and adjacent habitats may influence spider diversity. In this study, diversity of the spider families Lycosidae and Linyphiidae was studied after spring sowing until the time when a common pest (Rhopalosiphum padi) colonizes cereal fields. The spiders were collected with pitfall traps at eight organically or conventionally managed farms around Uppsala, Sweden, in three different habitats at each site: field margin, crop field and the edge between the two. The effects of farming system and habitat type on diversity of lycosids and linyphiids were considered using three different measures (activity density, species richness and composition). The most dominant species of each spider family, Pardosa agrestis (Lycosidae) and Oedothorax apicatus (Linyphiidae), had higher activity density at organic sites, and farming systems also contained different species compositions of both lycosid and linyphiid spiders. Also, linyphiid species richness was higher on conventional sites and linyphiid species composition was influenced by habitat type, in contrast with lycosids. Activity density and species richness of lycosid spiders were, on the other hand, more associated with field margins than linyphiid spiders.     

花生田蜘蛛群落的研究

在花生田共采集到蜘蛛41种,隶属于12科、27属。游猎型蜘蛛主要是狼蛛科、跳蛛科、猫蛛科和管巢蛛科的种类,所占比例较高,在春花生田为60.91％～86.21％,秋花生田为50.40％～90.36％。结网蜘蛛主要是皿蛛、球蛛、肖蛸和小型园蛛。在花生的不同生长季节有不同的优势科和优势种。春花生田的优势科是狼蛛科、皿蛛科和猫蛛科,优势种是类水狼蛛、拟环纹豹蛛、脉娲蛛、食虫沟瘤蛛和斜纹猫蛛;狼蛛科、猫蛛科和球蛛科是秋花生田的优势科、脉娲蛛、斜纹猫蛛和八斑鞘蛛为优势种。     

Competition for a limited space in kleptoparasitic Argyrodes spiders revealed by field experiments

Most kleptoparasitic Argyrodes spiders rely exclusively on host spider webs for obtaining their food. Because their densities occasionally reach high levels within a restricted area, competitive interactions may be important for determining the density of these unique spiders. Here I used two Argyrodes species commonly found on webs of the large orb-web spider Nephila clavata to clarify whether inter- and intraspecific competition influences abundance and within-web distribution by using observational data and field experiment. Removing Argyrodes flavescens from the host webs induced a remarkably high immigration of that species while density on control webs remained almost at the same level, which is evidence for strong intraspecific competition. Larger individuals of A. flavescens were located more frequently at the capture area of the host webs where it is easy to access prey ignored by the host spider, and spiders immigrating into webs from which that species had been removed were smaller in body size, suggesting interference competition for space among conspecific kleptoparasites. Argyrodes bonadea increased in number on webs from which A. flavescens had been removed, and the increase was correlated with the number of A. flavescens removed. This finding is evidence for interspecific competition that is rarely reported in spiders. A multiple regression model including numbers of a conspecific parasite as well as web and body sizes of the host spider could not detect competitive interactions between species, suggesting the importance of experimental approaches. Received: May 22, 2000 / Accepted: December 1, 2000     

Conspecific and Heterospecific Attraction: A Mechanism of Web-site Selection Leading to Aggregation Formation by Web-building Spiders

This study investigated whether conspecific and/or heterospecific attraction to silk is a mechanism of web-site selection leading to aggregation formation by two species of web-building spiders, Hypochilus thorelli Marx (Araneae: Hypochilidae) and Achaearanea tepidariorum (C.L. Koch) (Araneac: Theridiidae). We determined that the spatial distribution of these two spiders was clumped and that H. thorelli had a greater tendency to aggregate than did A. tepidariorum. To determine the mechanism responsible for this spatial pattern, we conducted three field experiments. We examined web-site selection by H. thorelli in three contexts: no spiders or webs present (cue removed), vacant webs present, and occupied webs present. In the case where no webs were present, there was no tendency for spiders to choose previously occupied sites as web sites. When vacant webs were present, spiders chose to occupy the vacant webs. When occupied webs were present, spiders either invaded webs and evicted the owners, or settled adjacent to and attached their webs to those of residents. Various microhabitat variables (height, angle, temperature, humidity, and substrate character of the rock surface) of randomly selected unoccupied sites and the web sites chosen by spiders were compared. There were no detectable differences between web sites and unoccupied sites with respect to any of the variables measured. This leads us to conclude that web-site choice by immigrating spiders was based on the presence of silk rather than other features of the site.     

Molecular diagnostics reveal spiders that exploit prey vibrational signals used in sexual communication

Vibrational signalling is a widespread form of animal communication and, in the form of sexual communication, has been generally regarded as inherently short‐range and a private communication channel, free from eavesdropping by generalist predators. A combination of fieldwork and laboratory experiments was used to test the hypothesis that predators can intercept and exploit such signals. First, we developed and characterized PCR primers specific for leafhoppers of the genus Aphrodes and specifically for the species Aphrodes makarovi. Spiders were collected from sites where leafhoppers were present and screened with these primers to establish which spider species were significant predators of this species during the mating period of these leafhoppers. Analysis using PCR of the gut contents of tangle‐web spiders, Enoplognatha ovata (Theridiidae), showed that they consume leafhoppers in the field at a greater rate when signalling adults were present than when nymphs were dominant, suggesting that the spiders were using these vibrations signals to find their prey. Playback and microcosm experiments then showed that E. ovata can use the vibrational signals of male leafhoppers as a cue during foraging and, as a result, killed significantly more male than female A. makarovi. Our results show, for the first time, that arthropod predators can exploit prey vibrational communication to obtain information about prey availability and use this information to locate and capture prey. This may be a widespread mechanism for prey location, one that is likely to be a major unrecognized driver of evolution in both predators and prey.     

Web-site selection strategies of linyphiid spiders in alfalfa: implications for biological control

Site-specific foraging can enhance the ability of generalist predators to provide effective and sustainable levels of pest control in agroecosystems. This can result from increased growth rates, higher population densities, and improved capture frequencies of pests at high prey density microsites. We tested the hypothesis that linyphiid spiders would exhibit microhabitat-specific web-site selection strategies in alfalfa. This was predicted to result in high prey densities at web-sites compared to paired non-web-sites through direct, or indirect, selection of prey-rich habitats. A total of 22,242 potential prey items were collected from mini-sticky traps located at 896 microsites. Web-centered mini-sticky traps on the ground, representative of Erigone autumnalis Emerton (Araneae: Linyphiidae) webs, captured similar numbers of potential prey as paired non-web-centered traps nearby. However, aerial sticky traps at web-sites of Bathyphantes pallidus (Banks) (Araneae: Linyphiidae) contained significantly more Diptera and Empoasca fabae (Harris) (Homoptera: Cicadellidae) than paired non-web-centered sticky-traps. Prey activity-densities also varied between web-sites of E. autumnalis and B. pallidus. Diptera were dominant at aerial microsites of B. pallidus whilst Collembola were abundant on ground-based traps of E. autumnalis. These results suggest that in alfalfa, the pressure for selecting prey-rich web-sites by erigonine spiders is low, but B. pallidus exhibits a selective web-location strategy targeted towards high quality dipteran prey. These sites also captured large numbers of E. fabae, a major pest of alfalfa, thus implicating aerial-based linyphiines as valuable predators in biological control.     

Pollen beetles are consumed by ground‐ and foliage‐dwelling spiders in winter oilseed rape

Pollen beetles, Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae), are major pests in oilseed rape (OSR), Brassica napus L. (Brassicaceae). Among the predator species in the generalist predator complex present in OSR fields, wolf spiders (Araneae: Lycosidae) are found on the ground and cobweb spiders (Araneae: Theridiidae) build webs in the foliage. Here we study the incidence of predation of pollen beetles by these two spider groups using DNA‐based molecular analysis. Wolf spiders of the genus Pardosa and the cobweb spider, Theridion impressum L. Koch, were each collected in three winter OSR fields over a period of about 3 weeks. Pollen beetle densities as well as the occurrence of predators and alternative prey were monitored. In total, 13.8% of the collected Pardosa spp. tested positive for pollen beetle DNA in the PCR analyses, whereas 51.7%T. impressum were positive. The likelihood of detecting pollen beetle DNA in the gut contents of both spider groups was positively related to pollen beetle larval density. The implications of these results for conservation biological control and future studies of food webs in OSR are discussed.     

Foraging ecology and niche partitioning in orb-weaving spiders

Summary Foraging patterns were determined for three orbweaving spiders in several geographical locations varying in percent cover by herbaceous vegetation. Argiope trifasciata was the most common species in early successional habitats, while both Argiope aurantia and Araneus trifolium were more common in wetter, more herbaceous sites. Discriminant analysis revealed that web height selected for webs and body size were the variables that explained most of the variation among populations in foraging patterns. Argiope aurantia forages lowest in vegetation, A. trifasciata at intermediate heights, and A. trifolium near the top of the vegetation. The body size sequence is reversed.Web radius, spider size, and web height appear to explain much of the variation in abundance and size of prey in webs. Species foraging higher in the vegetation take more winged prey, while larger species foraging lower in the vegetation tend to take larger, jumping prey like acridids. Comparison of prey in webs with field estimates of potential prey suggests that orbweavers select large insect prey. Inferential evidence indicates that interspecific competition may be responsible for the divergence in foraging patterns among species reported here. However, field manipulative experiments have not yet indicated that competition among orb-weavers is severe.     

Invasive Canadian goldenrod (<Emphasis Type="Italic">Solidago canadensis</Emphasis> L.) as a preferred foraging habitat for spiders

Invasions of alien species are a great threat to biodiversity and native species communities. There are many examples in the literature on how the invasive plants affect the natural environment. Beside reports on negative effects of these invasions, there are also several studies indicating a positive impact of the invaders. Canadian goldenrod (Solidago canadensis) is one of the most invasive plant species all over Europe. Earlier studies showed that the goldenrod affects natural plant communities and has a negative impact on many animals, both vertebrates and arthropods. However, all the studies were conducted during goldenrod flowering. In contrast, this study has tested the novel hypothesis that Canadian goldenrod has a positive effect on native spider hunting success and increases spider abundance in farmland outside the growing season. Observations were made during spring on 13 meadows: 7 invaded and 6 non-invaded by goldenrod. All tall plants from experimental plots (1 m² each) were examined for the presence of spiders, their nets and prey. Prey items were counted only in spider webs. The results support the hypothesis that S. canadensis is a favourable foraging habitat for spiders: 14.6 spiders/m² on invaded plots versus 2.2 on non-invaded ones. Many spiders of the families Theridiidae and Araneidae were found on goldenrod plants, but on native plants only the Araneidae were found. Moreover, on invaded plots, much more prey items/m² were present in spiders webs than on non-invaded plots (155.3 vs. 13.8). The study is a rare example of a positive influence of invasive plants on the native arthropod community. This is also a novel approach that shows the importance of dry goldenrod stems in invaded ecosystems.     

Interactions between citrus psylla,Trioza erytreae (Hem. Triozidae), and spiders in an unsprayed citrus orchard in the transvaal lowveld

During a two year survey, a total of 3,054 spiders represented by 21 families were sampled in an unsprayed citrus orchard in the Transvaal Lowveld. Numerically the Salticidae was the dominant family (34.4%) followed by the Theridiidae (21.9%), Thomisidae (11.9%), Araneidae (7.9%), Clubionidae (7.0%) and the Tetragnathidae (3.7%). Eighteen species of spiders were observed to prey on citrus psylla,Trioza erytreae (Del Guercio) (Hemiptera: Triozidae), while six species trap nymphs and adults under their retreats and webs. There were significant positive correlations between the weekly psylla populations and the weekly populations of web-building spiders and wandering spiders present one to four weeks later but no significant correlation between the weekly spider populations and the weekly psylla populations present one to five weeks later. This seems to indicate that while spiders are unable to keep citrus psylla populations at acceptable low levels, they may contribute in reducing their numbers.      

Building a better insect trap; An experimental investigation of prey capture in a variety of spider webs

Summary Web-building spiders (Araneae; Theridiidae, Linyphiidae, Araneidae) are catagorized as searchers because they devote a large amount of energy to the construction of the web which constitutes the search phase in the foraging sequence. In this study search energy is equated with the density of threads in a web and the effectiveness of a variety of webs in three broad catagories (tangle webs, sheet webs & orb webs) is tested in the light of current foraging theory. Within each web type there is a distinct thread density at which the number of approaching Drosophila (Diptera; Drosophilidae) that are captured is maximized (Figs. 1, 2, 3). That maximum results from a combination of factors that are a function of the density of threads in the web. The visibility of the web to an approaching Drosophila increases which acts to decrease the number of flies that enter the web (Tables 2, 3, 4). The ability of the web to detain a Drosophila that contacts it (capture efficiency) increases to an asymptote as a function of thread density (Fig. 4). These data support an assumption of many optimal foraging models that with increasing investment in search the predator receives a diminishing return.More Drosophila intercept orb webs than intercept sheet or tangle webs. In addition orb webs detain a greater proportion of the flies that contact them (Fig. 4). Sheet webs are intermediate between orb and tangle webs in their relative abilities to contact and detain Drosophila.     

Mesh Width Influences Prey Retention in Spider Orb Webs

Orb‐weaving spiders depend upon the sticky capture spirals of webs to retain insects long enough to be captured. However, insects often escape from orb webs before the spiders can attack them. Therefore, the architectures of orb webs likely reflect strong selective pressure to increase retention times of insects. We experimentally increased the mesh width of one side of an orb web while maintaining the original mesh width on the other side as a control, and then tested the effect of this manipulation on the retention times of four different taxa of insects. We found evidence that increased mesh width of Argiope aurantia orb webs resulted in a general reduction in the retention times of insects. However, retention times for different taxa of insects were not predicted by any one specific morphological or flight characteristic. The influence of mesh width on the retention times of insects is very complex, but our results suggest that mesh width can act to selectively favor the capture of certain taxa of insect prey over others. This effect may help to explain both species level differences in web‐building behaviors and variation in the architectures of webs spun by individual spiders on different days.     

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.