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.     

Intra-guild predation relaxes natural enemy impacts on herbivore populations

Abstract. 1. To investigate the role of intra-guild predation in mediating the impact of the natural enemy complex on herbivore populations, a manipulative field experiment was conducted using uncaged plots (islets of Spartina cordgrass) on a North American salt marsh. The densities (moderate or low) of two invertebrate predators, the generalist wolf spider Pardosa littoralis and the specialist mirid bug Tytthus vagus , were manipulated in a 2 × 2 factorial design, and the resulting treatment effects on the population growth of their herbivorous prey, Prokelisia planthoppers, were assessed.
2. The abundance of wolf spiders on experimental islets was unaffected by the presence of mirid bugs, however the density of mirid bugs was influenced very negatively by the presence of the wolf spider.
3. The negative effect of the wolf spider on mirid bugs most probably resulted from the intra-guild predation of mirids by spiders because planthopper limitation by the wolf spider alone was significantly greater than when both predators were present.
4. As a result of intra-guild predation, planthopper population growth was positive in the presence of both predators, despite the fact that each predator alone promoted a decrease in planthopper population growth.
5. Notably, the occurrence of intra-guild predation diminished top-down impacts on planthopper populations in a relatively simple food web where strong top-down effects were expected. This result, however, was limited to habitats on the marsh with simply structured vegetation lacking leaf litter.     

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.     

Capture efficiency and trophic adaptations of a specialist and generalist predator: A comparison

Specialist true predators are expected to exhibit higher capture efficiencies for the capture of larger and dangerous prey than generalist predators due to their possession of specialized morphological and behavioral adaptations. We used an araneophagous spider (Lampona murina) and a generalist spider (Drassodes lapidosus) as phylogenetically related model species and investigated their realized and fundamental trophic niches and their efficacy with respect to prey capture and prey handling. The trophic niche of both species confirmed that Lampona had a narrow trophic niche with a predominance of spider prey (including conspecifics), while the niche of Drassodes was wide, without any preference. DNA analysis of the gut contents of Lampona spiders collected in the field revealed that spiders form a significant part of its natural diet. Lampona captured significantly larger prey than itself and the prey captured by Drassodes. As concerns hunting strategy, Lampona grasped the prey with two pairs of legs possessing scopulae, whereas Drassodes immobilized prey with silk. Lampona possess forelegs equipped with scopulae and a thicker cuticle similar to other nonrelated araneophagous spiders. Lampona fed for a longer time and extracted more nutrients than Drassodes. We show that specialized behavioral and morphological adaptations altogether increase the hunting efficiency of specialists when compared to generalists.     

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.     

Resource-mediated impact of spider predation risk on performance in the grasshopper Ageneotettix deorum (Orthoptera: Acrididae)

In response to increased exposure to predators when searching for food, many prey increase the frequency of antipredator behaviors, potentially reducing foraging rate and food intake. Such direct, nonlethal interactions between predators and prey resulting in reduced food intake can indirectly influence lifecycle development through effects on growth, developmental rate, and survival. We investigated the general hypothesis that individual performance of a herbivorous insect can be negatively affected when exposed to nonlethal predation risk, and that the response can be mediated by food quality. This hypothesis was tested using the common rangeland grasshopper Ageneotettix deorum with and without exposure to common wolf spider predators (Lycosidae, Schizocosa spp.) on both untreated natural and fertilized vegetation. All spiders were rendered temporarily incapable of direct feeding by restricting function of the chelicerae with beeswax. Detectable responses by grasshoppers to spiders indicate indirect consequences for lifecycle development. Grasshopper performance was measured as hind femur growth, duration of nymphal lifecycle stages, and survivorship in a caged field experiment conducted over 2 years. Grasshoppers developed faster and grew 3–5% larger when allowed to forage on fertilized vegetation in the absence of risk from a spider predator. Failure-time analysis illustrated enhanced survival probability in response to elevated food quality and the negative effects of grasshopper susceptibility to nonlethal predation risk. Performance on food of relatively low, ambient quality with no predation risk equaled that of grasshoppers caged with high quality vegetation in the presence of a modified spider. Increased resource quality can clearly moderate the negative life history responses caused by the behavioral modification of grasshoppers when exposed to spider predation risk, a compensatory response.     

Macrolophus caliginosus: Functional response to whiteflies and preference and switching capacity between whiteflies and spider mites

Laboratory experiments were performed with adult female Macrolophus caliginosus Wagner (Heteroptera: Miridae) at 22ºC on bean plants to determine the functional response towards whiteflies, as well as the preference and switching capacity between the two prey species: whiteflies and spider mites. Predation of females presented with first instars of Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) was of a Type III functional response. The observed maximum predation was approximately 75 first instars at high prey densities within a 24-h period. The preference of M. caliginosus females between eggs of T. vaporariorum and Tetranychus urticaeKoch (Acarina: Tetranychidae) changed with the ratio of offered prey. The preference for T. vaporariorum eggs increased non-linearly with increasing proportions of this prey type. The average maximum predation of whitefly and spider mite eggs were approximately 166 and 111 eggs per day, respectively, at the highest ratio of the two preys. The proportion of M. caliginosus females found on the test plants at the end of the experiment increased with prey density suggesting that this mirid spends more time in areas with high prey density. Macrolophus caliginosus females are voracious predators of eggs and first instars of T. vaporariorum as well as of spider mite eggs and may thus be a valuable addition to existing methods of biological control of T. vaporariorum and T. urticae.     

稻田穗期主要捕食性天敌对两种盲蝽集团内捕食的初步研究

【目的】集团内捕食是影响农业系统广食性天敌对靶标害虫控制作用的重要因素,全面揭示稻田广食性天敌对稻飞虱重要天敌黑肩绿盲蝽Cyrtorhinus lividipennis Reuter和中华淡翅盲蝽Tytthus chinensis(St?l)(半翅目:盲蝽科)的集团内捕食作用有助于更好地保护利用天敌。【方法】建立以两种盲蝽为猎物对象的种特异性二重定量PCR系统,检测比较不同抗性品种穗期田间主要捕食者对两种盲蝽的集团内捕食作用。【结果】(1)本研究建立的二重定量PCR具有的物种专化性,两种检测靶标灵敏度高且相似,可用于田间捕食者猎物分析;(2)感虫水稻品种上捕食者对盲蝽的捕食强度高于抗性水稻品种;不同种类捕食者对盲蝽的集团内捕食强度有显著差异;捕食者对黑肩绿盲蝽集团内捕食强度显著高于中华淡翅盲蝽。【结论】捕食性盲蝽在稻田生态系统遭遇集团内捕食,其集团内捕食强度与水稻品种抗性、捕食者种类和猎物丰富度有关。     

Multiple spatial scale factors affecting mirid bug abundance and damage level in organic rice paddies

Mirid bugs, major insect pests in rice production, have dramatically expanded their range in Japan, resulting in increased economic losses especially for organic rice farmers. In this study, the within-field and landscape-scale factors affecting the bug and its damage to crops were examined in organic rice farms. Bug densities and crop damage levels showed significant positive relationships with weed densities (Schoenoplectus juncoides and Echinochloa crus-galli) within individual fields. Bug densities were negatively associated with Tetragnatha spp. and Pachygnatha clercki densities, indicating that these spiders act as biological control agents that help to decrease crop damage levels. At the landscape scale, bug densities increased with the proportion of the area that was fallow within a 400-m radius around focal fields. Fallows represent important source habitat for the bug and the spatial extent at which bug abundance was affected indicates the mobility of these insects. These results suggest that the combination of biological control by natural enemies, and weed management within individual paddies and surrounding landscapes inside a buffer zone of less than half a kilometre may be effective in controlling mirid bugs and the damage they cause in organic paddy fields.     

Utilization of prey from the decomposer system by generalist predators of grassland

We investigated the linkage between the detrital subsystem and generalist predators of meadow ecosystems by manipulating prey availability in two different ways: we increased resource availability for the decomposer subsystem and thereby decomposer prey by adding mulch materials (detritus enhancement), and we added fruitflies (Drosophila melanogaster, Diptera; prey enhancement) to fenced plots. Both supplemented materials significantly differed in their ¹³C/¹²C and ¹⁵N/¹⁴N ratios from those of the natural litter. We measured density responses of detritivorous, herbivorous and predaceous arthropods to the increased resource supply. We used ratios of natural stable isotopes of N and C in arthropod tissues to trace the flux from the added resources to consumers and to relate density responses of consumers to changes in resource supply. Effects of resource enhancement propagated through at least two trophic levels, resulting in higher densities of major decomposer and predator taxa. Effects of detritus enhancement were much stronger than those of prey enhancement. Signatures of δ¹³C proved density responses of Collembola taxa to be related to the added mulch materials. Among generalist predators, densities of juvenile wolf spiders (Lycosidae) responded more to detritus-enhancement than to prey-enhancement treatments. In contrast, the density of the web-building linyphiid and the non-web gnaphosid spiders remained unaffected. Each spider taxon, including those which did not respond numerically, was significantly enriched in ¹³C in detritus-enhancement treatments, suggesting that they gain energy from the decomposer system. Numbers of herbivores—cicadellids and aphids—were similar in each of the treatments, indicating that they were unaffected by changes in predator density. Our results indicate that the lack of a numerical response to resource supplementation is not necessarily due to the absence of a trophic linkage, but may be caused by compensatory changes in mortality factors such as cannibalism and intraguild predation.     

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.     

Interspecific predation as a mortality factor among overwintering spiders

Summary Results from field experiments indicate that predation occurs among spruce-living spiders during winter in SW Sweden. Field observations of natural activity showed that Philodromus spp and Pityohyphantes phrygianus together make up 80% of the spiders active on spruce in winter. They are therefore potential predators on other overwintering spiders. Laboratory experiments were performed at +4° C to assess the importance of such predation between spiders. Small spiders (length <2.5 mm) had a mean winter mortality of 58% when kept together with large spiders (2.5 mm) which had a mean mortality of 3% only. Among the small spiders the Erigninae spp seemed to be more vulnerable to predation than other taxonomic groups. Predation also occurred when large P. phrygianus were kept together, but such predation caused mortality of less importance to the spider populations than the mortality among small spiders. Differences in spider density and food availability did not change this pattern. Considerable weight increase occurred in subadult P. phrygianus when fed during winter. This suggests that winter foraging specimens increase their fitness. Interspecific predation among spiders is suggested to be an important mortality factor in natural populations at high spider densities in November and December, when the ambient temperature often is above 0° C and when the density of large spiders is not yet substantially reduced by bird predation.     

A test of the hypothesis that a pathway of intraguild predation limits densities of a wolf spider

Abstract.  1. The high number of potential predatory interactions among the many arthropod generalist predators in terrestrial food webs makes exhaustive testing of interaction strengths by field experiments unfeasible. Thus, correlative patterns and laboratory observations of behaviour often form the basis of inferences about the strength of interaction pathways involving generalist predators (intraguild predation).
2. Previous research has revealed a negative correlation between survival of juvenile wolf spiders of the genus Schizocosa (Lycosidae) and densities of another abundant spider family, the Gnaphosidae.
3. Feeding trials in laboratory microcosms with a leaf-litter substrate revealed that gnaphosids prey on juvenile Schizocosa in a structurally complex habitat.
4. Gnaphosid densities were manipulated in two different field experiments, each conducted in a different year, in order to test directly the hypothesis that intraguild predation by gnaphosids limits densities of juvenile Schizocosa .
5. Reducing numbers of gnaphosids, and doubling their numbers to two times the mean natural density, had no impact on the survival of juvenile Schizocosa in either field experiment. This finding suggests that correlative patterns in nature and feeding trials in the laboratory may at times provide deceptively simple and potentially misleading generalisations about the strengths of interaction pathways in complex networks of generalist predators.     

捕食者对空心莲子草叶甲种群的生物胁迫

广食性捕食者广泛捕食植食性昆虫,常被用于有害生物的生物防治,也因此影响植食性昆虫对杂草的生物效果。空心莲子草叶甲(Agasicles hygrophila)(鞘翅目:叶甲科Chrysomelidae)作为入侵恶性杂草空心莲子草(Alternanthera philoxeroides)(苋科:莲子草属Alternanthera)的专性天敌,从美国的弗罗里达州引入中国,在释放地防治空心莲子草取得了较好的防治效果。虽然空心莲子草叶甲在引入地均已建立田间种群并有一定程度的自然扩散,但丰富的食物资源,并未使空心莲子草叶甲的自然种群数量变得繁荣,因此其未能有效抑制空心莲子草的扩散蔓延。在野外调查时发现空心莲子草生境中存在大量广食性捕食者。这些广食性捕食者是抑制空心莲子草叶甲种群数量扩张的生物胁迫因子吗?为此,选择捕食性昆虫龟纹瓢虫(Propylaea japonica)(鞘翅目:瓢虫科Coccinellidae)、蜘蛛类捕食者拟水狼蛛(Pirata subpiraticus)(蜘蛛目:狼蛛科Lycosidae)与斜纹猫蛛(Oxyopes sertatus)(蜘蛛目:猫蛛科Oxyopidae)为捕食者,分别以空心莲子草叶甲各虫态为猎物,构建简单的捕食者-猎物系统,在室内检测了上述3种捕食者对空心莲子草叶甲各虫态在不同密度下的日捕食量,以期了解捕食者对空心莲子草叶甲的捕食作用,客观评估空心莲子草叶甲的生物防治效能。研究结果表明:捕食者龟纹瓢虫、斜纹猫蛛与拟水狼蛛均捕食空心莲子草叶甲的卵粒及1龄、2龄幼虫,斜纹猫蛛与拟水狼蛛捕食3龄幼虫,捕食者的捕食量均随着猎物密度的升高而增加,寻找效应降低。三者均不捕食成虫。除拟水狼蛛对3龄幼虫的捕食用Holling II模型拟合不呈显著相关关系外,其余捕食反应均拟合Holling II模型并显著相关。通过拟合方程得出捕食者对空心莲子草叶甲卵粒的理论日最大捕食量为:斜纹猫蛛10.9粒,拟水狼蛛为6.2粒,龟纹瓢虫为5.6粒;对1龄幼虫的理论日最大捕食量为:斜纹猫蛛为17.1头;拟水狼蛛为35.8头,龟纹瓢虫为10.4头;对2龄幼虫的理论日最大捕食量为:斜纹猫蛛为6.6头,拟水狼蛛为11.2头,龟纹瓢虫为2.9头;对3龄幼虫的理论日最大捕食量为:斜纹猫蛛捕食12.3头,拟水狼蛛为1.1头。研究结果证实了捕食者可通过捕食作用降低空心莲子草叶甲种群密度,削弱空心莲子草叶甲对空心莲子草的控害效能,是空心莲子草叶甲种群存活的生物胁迫因子。建议在提高空心莲子草叶甲田间种群数量,达到对空心莲子有效的持续控制效果方面开展进一步研究。     

Sex‐Specific Response of Pardosa milvina (Araneae: Lycosidae) to Experience with a Chemotactile Predation Cue

Predators frequently leave behind chemical information (i.e., semiochemicals such as pheromones or kairomones) that can be detected by their prey and used to avoid areas where predators are likely present. Prey that have interacted indirectly with predators via chemical information thus may gain insight into their risk of being consumed that naïve individuals lack. Pardosa milvina (Araneae: Lycosidae) is a chemosensitive wolf spider that shows adaptive responses to chemotactile cues deposited by the larger wolf spider Tigrosa helluo. We raised offspring from P. milvina to examine the effect of experience with a predation cue on activity, foraging, and antipredator behavior. Spiders differed in activity and foraging behavior across ontogeny and between sexes, but there was no effect of experience with a predation cue. However, a sex‐specific effect of experience was found in antipredator behavior. Male spiders, but not females, used experience with a predator cue to increase their survival in the presence of a live predator. Specifically, naïve males were attacked sooner than experienced males, indicating that prior exposure to predator cues can modify Pardosa antipredator behavior. Intersexual differences in how spiders respond to experience with a predation cue likely reflect the risk of predation faced by males and females in nature.     

Generalist predators in organically and conventionally managed grass‐clover fields: implications for conservation biological control

Organically managed agroecosystems rely in part on biological control to prevent pest outbreaks. Generalist predators (Araneae, Carabidae and Staphylinidae) are a major component of the natural enemy community in agroecosystems. We assessed the seasonal dynamics of major generalist predator groups in conventionally and organically managed grass–clover fields that primarily differed by fertilisation strategy. We further established an experiment, manipulating the abundant wolf spider genus Pardosa, to identify the importance of these predators for herbivore suppression in the same system and growth period. Organic management significantly enhanced ground‐active spider numbers early and late in the growing season, with potentially positive effects of plant cover and non‐pest decomposer prey. However, enhancing spider numbers in the field experiment did not improve biological control in organically managed grass–clover fields. Similar to the survey results, reduced densities of Pardosa had no short‐term effect on any prey taxa; however, spider guild structure changed in response to Pardosa manipulation. In the presence of fewer Pardosa, other ground‐running spiders were more abundant; therefore, their impact on herbivore numbers may have been elevated, possibly cancelling increases in herbivore numbers because of reduced predation by Pardosa. Our results indicate positive effects of organic farming on spider activity density; however, our survey data and the predator manipulation experiment failed to find evidence that ground‐running spiders reduced herbivore numbers. We therefore suggest that a positive impact of organic fertilisers on wolf spiders in grass–clover agroecosystems may not necessarily improve biological control when compared with conventional farming.     

Experimental assessment of trophic ecology in a generalist spider predator: Implications for biocontrol in Uruguayan crops

Conservative biological control promotes the use of native natural enemies to limit the size and growth of pest populations. Although spiders constitute one of the most important groups of native predators in several crops, their trophic ecology remains largely unknown, especially for several generalist taxa. In laboratory, we assessed the predatory behaviour of a wandering spider (the wolf spider Lycosa thorelli (Keyserling, 1877) against several arthropods varying in size and trophic positions, all found in South American soybean and rice crops. As prey we used the bug Piezodorus guildinii (Westwood, 1837) as well as larvae and adults of the moth Spodoptera frugiperda (Smith, 1797), both being considered important pests in Uruguayan crops. We also used several non-pest arthropods as prey, sarcophagid flies, carabid beetles and wolf spiders. All prey were attacked in more or less high, although not statistically differing, proportions. However, carabids were not consumed, and bugs were consumed in significantly lower proportions than flies. A negative correlation was found between prey size and acceptance rate. Immobilization times were longer against larvae when compared to moths and flies, while predatory sequences were longer for bugs when compared to flies, moths and spiders. In addition, we found a positive effect of prey size on predatory sequence length and complexity. Our results confirm the ability of spiders to attack and feed upon prey with different morphologies, included well-defended arthropods, and their potential use as natural enemies of several pests in South American crops.     

Positive indirect effect of aquatic insects on terrestrial prey is not offset by increased predator density

1. Changes in one prey species' density can indirectly affect the abundance of another prey species if a shared predator eats both species. Sometimes, indirect effects occur when prey straddle habitats, including when riparian predator populations grow in response to emergent aquatic insects and increase predation on terrestrial prey. However, predators may largely switch to aquatic insects or become satiated, reducing predation on terrestrial prey. 2. To determine the net indirect effect of aquatic insects on terrestrial arthropods via generalist spider predators, a field experiment was conducted mimicking midge influx and a wolf spider numerical response inside enclosures near an Icelandic lake. Lab mesocosms were also used to assess per capita rates of spider predation u nder differing levels of midge abundance. 3. Midges always decreased sentinel prey predation, but this effect increased with predator density. When midges were absent, predation increased 30% at a high spider density, but predation was equal between spider treatments when midges were present. In situ arthropods showed no effect of midge or spider treatments, although non‐significant abundance patterns were observed congruent with sentinel prey results. 4. In lab mesocosms, prey survivorship increased ≥50% where midges were present and rapidly saturated; the addition of 5, 20, 50, and 100 midges equivalently reduced spider predation, supporting predator distraction rather than satiation as the root cause. 5. The present results demonstrate a strong positive indirect effect of midges and broadly support the concept that predator responses to alternative prey are a major influence on the magnitude and direction of predator‐mediated indirect effects.     

Scorpions,spiders and solpugids: predation and competition among distantly related taxa

Summary Scorpions, spiders and solpugids are generalist predators on the same types of arthropod prey. However, these potential competitors also frequently eat one another (=intraguild predation). In a 29 mo. experiment, >6,000 scorpions were removed from 300 (10x10m) quadrats. Significantly more spiders (but not solpugids) occurred in removal versus control quadrats. Two alternate hypotheses potentially explain this result: exploitation competition for jointly exploited prey or intraguild predation. There was no evidence of exploitation competition: no differences existed between removal and control quadrats in prey abundance or spider size and reproductive characteristics. It appears that the release from predation pressure in areas from which scorpions were removed produced the observed increase in spider abundance. Current ecological theory does not fully apply to situations whereby species at the same trophic level interact as both predators and potential competitors.     

Intraspecific variation in responses to aposematic prey in a jumping spider (Phidippus regius)

Aposematic signals often allow chemically defended prey to avoid attack from generalist predators, including jumping spiders. However, not all individual predators in a population behave in the same way. Here, in laboratory trials, we document that most individual Phidippus regius jumping spiders attack and reject chemically defended milkweed bugs (Oncopeltus fasciatus), immediately releasing them unharmed. However, a small number of individuals within the population kill and completely consume these presumably toxic prey items. This phenomenon was infrequent with only 14% of our sample (17/122) consuming the milkweed bugs over the course of the study. Individuals that killed and consumed bugs often did so repeatedly; specifically, individuals that consumed a bug in their first test were more likely to kill a bug in their second test and also tended to consume them again. We explored what might drive some (but not all) individuals to consume these bugs and found that neither sex, sexual maturity, body size, laboratory housing type, nor being wild-caught or being laboratory-reared, predicted milkweed bug consumption. Consuming bugs had no negative effects on spider mass or body condition; contrary to expectations, individuals that consumed milkweed bugs actually gained more body mass and increased in body condition. We discuss potential behavioural and physiological variation between individuals that may drive these rare behaviours and the implications for the evolution of prey defences.