Estimating predation pressure in ecological studies: controlling bias imposed by using sentinel plasticine prey

Sentinel plasticine prey has been increasingly used to estimate predation pressure. The use of plasticine prey may, however, bias the results, as this method was originally designed to account for predation by organisms that can visually recognize the shapes and colors of their prey. To evaluate the limitations of using sentinel plasticine prey, we compared predator attack rates between real prey – dead and live mealworms, Tenebrio molitor L. (Coleoptera: Tenebrionidae) – and plasticine models in a monsoonal tropical rainforest of southeastern China. The attack rates by invertebrates were highest on dead prey followed by live prey and plasticine models, whereas the attack rates by vertebrates were lowest on dead prey, and did not differ between live prey and plasticine models. These results confirm that bias imposed by using the plasticine models is affected by the type of predators. In addition, we tested the validity and generality of the premise that predators can distinguish the shapes of plasticine model prey and preferentially attack a caterpillar-like shape over other shapes. To test this hypothesis, we conducted three independent experiments in China, Papua New Guinea, and Finland. In the two latter localities, predation rates on plasticine caterpillars were higher than on models of other shapes, whereas in China, these differences were not significant. Taken together, our study suggests that plasticine models may underestimate the predation by invertebrates to a greater extent than predation by vertebrates, and the preference of model shape by predators may be locality-specific, presumably due to differences in the composition of the predator community. We propose that predation be estimated on both live and plasticine prey in future studies to measure the potential bias imposed by using plasticine models and its variation among various habitats and predator groups.     

Subsidy from the detrital food web, but not microhabitat complexity, affects the role of generalist predators in an aboveground herbivore food web

The activity and density of generalist predators, such as carabid beetles, rove beetles and spiders, may increase in response to: (1) increased availability of prey from the belowground subsystem and/or (2) enhanced complexity of aboveground vegetation. Organic farming practices support decomposer populations and enhance habitat complexity due to an increased weed density. A response by generalist predators to such below‐ or aboveground changes could affect predation rates on herbivores in the aboveground food web. We tested this hypothesis in a replicated field experiment conducted in a winter wheat field, where increased predator activity could lead to improved control of herbivorous pests. In a crossed design, we increased and lowered densities of decomposer prey, and manipulated vegetation complexity using artificial plants in order to examine the effect of structural complexity in isolation from effects of plant‐attracted additional prey. Isotomid Collembola exhibited lowest activity‐densities (AD) in plots treated with soil insecticide and had gradually increasing AD in untreated plots and plots receiving detrital subsidies. Carabid beetles and cursorial spiders did not respond to increased availability of isotomid prey, and they unexpectedly displayed higher AD in the structurally less‐complex plots. Aphid density mirrored the positive response of isotomids to detrital subsidies, suggesting that aphids benefited from reduced predation due to predators switching to abundant prey in the decomposer subsystem. The absence of a numerical response by surface‐active predators apparently strengthened this indirect effect of isotomids on aphids. Our results suggest that indirect predator‐mediated prey‐prey interactions can reduce beneficial effects of detrital subsidies on pest suppression. We further demonstrated that generalist predators may not per se benefit from structural complexity. Both results document the challenges associated with management practices that support generalist predators, as these measures may not necessarily improve herbivore suppression.     

The carabid <Emphasis Type="Italic">Pterostichus melanarius</Emphasis> uses chemical cues for opportunistic predation and saprophagy but not for finding healthy prey

The sentinel prey method can quantify predation pressure in various habitats. Real prey is assumed to more realistically mimic the predator experience but the predator can rarely be identified. Artificial prey made of plasticine may lack real chemical cues, but provides information about predator identity. However, the relationship between predation pressure registered by artificial versus real prey is not clear. We tested the relative attractiveness of artificial caterpillars, and intact, wounded, or dead larvae of the cabbage moth (Mamestra brassicae) for the carabid predator Pterostichus melanarius Illiger (Coleoptera: Carabidae). P. melanarius adults were attracted to dead caterpillars more than to live or wounded ones. Coating artificial caterpillars with caterpillar haemolymph increased their attractiveness. However, predators were not attracted more to healthy, real caterpillars than to “untreated” artificial ones. We conclude that using artificial caterpillars does not underestimate predation pressure by this carabid on healthy caterpillars.     

Predation of Colorado potato beetle eggs by a polyphagous ladybeetle in the presence of alternate prey: potential impact on resistance evolution

The influence of prey choice on the predation of a target prey item by a polyphagous insect predator was investigated in field plot studies. The target prey consisted of eggs of the Colorado potato beetle (CPB), Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), and the predator was the 12‐spotted ladybeetle, Coleomegilla maculata Lengi (Coleoptera: Coccinellidae). Eggs of the European corn borer (ECB), Ostrinia nubilalis Hübner (Lepidoptera: Pyralidae), and nymphs and adults of the green peach aphid, Myzus persicae Sulzer (Homoptera: Aphididae), comprised the alternative prey choices. The objectives of these studies were to: (1) examine predation in a multiprey scenario likely to occur in an agroecosystem, and (2) use the data to simulate the impact of predator‐induced mortality on the evolution of resistance to Bt‐transgenic plants in the target herbivore. Simulations of the rate of resistance evolution were carried out using a deterministic genetic model. Experiments were performed using potato field plots planted in a manner reflecting a 25% or 50% non‐transgenic refuge. CPB eggs were infested so as to mimic the densities of resistant and susceptible populations that might occur in commercial Bt‐transgenic plantings. Densities of predators and alternate prey species were chosen to represent those that might typically occur in potato crops in the eastern USA. Simulation results indicated that when ECB eggs were present, predation on CPB eggs either became inversely spatially density‐dependent, or increased significantly in a density‐dependent manner. When aphids were present, predation became positively density‐dependent. Model simulations predicted that ECB egg presence is beneficial, in that resistance was delayed by up to 40 pest generations (as compared to the scenario with CPB as the only prey), while aphid presence accelerated resistance evolution by 18 generations. Results suggest that resistance management strategies should take into account the composition of prey species available to generalist predators typically present, so as to best delay pest adaptation to Bt‐toxins.     

Foraging responses of Coccinella septempunctata,Hippodamia variegata and Chrysoperla carnea to changing in density of two aphid species

The successful use of predators in classical biocontrol programmes needs several background laboratory investigations, one of which is the evaluation of predator behavioural responses to changes in the density of their prey. The impact effect of the density of two prey species [Myzus persicae Sulzer and Aphis craccivora Koch (Hemiptera: Aphididae)] on the predation rates of third-instar Chrysoperla carnea Stephens (Chrysopidae: Neuroptera) and fourth-instar Coccinella septempunctata L. and Hippodamia variegata Goeze (Coccinellidae: Coleoptera) larvae was studied. Although prey species, predator species, prey density, and their interactions all had significant effects on the numbers of aphids consumed, the type of functional response did not vary, remaining a type II response in all treatments. However, the type II parameters differed among predator species on the same prey species, and for each predator species on the two prey species. Chrysoperla. carnea on M. persicae and H. variegata on A. craccivora were more voracious than other predators. In the context of functional response and biological control, the release of these predators, that show inverse density-dependent mortality, has to be started in early season to build up their population on low aphid densities and attack later high aphid populations.     

Fresh,frozen or fake: A comparison of predation rates measured by various types of sentinel prey

Arthropod predators and parasitoids support the health and functioning of the world's ecosystems, most notably by supplying biological control services to agricultural landscapes. Quantifying the impact that these organisms have on their prey can be challenging, as direct observation and measurement of arthropod predation is difficult. The use of sentinel prey is one method to measure predator impact; however, despite widespread use, few studies have compared predation on different prey types within a single experiment. This study evaluated the predation rates on four sentinel prey items in grass and wheat fields in south-east Queensland, Australia. Attack rates on live and dead Helicoverpa armigera eggs, and dead H. armigera larvae and artificial plasticine larvae, were compared and the predators that were attracted to each prey type were documented with the use of field cameras. There was no significant difference in predation rates between sentinel eggs, while dead larvae were significantly more attacked than artificial larvae. Prey were attacked by a diverse range of predators, including ants, beetles, various nymph and juvenile insects and small mammals. Different predators were active in grass and crop fields, with predator activity peaking around dawn and dusk. The same trends were observed within and between the two habitats studied, providing a measure of confidence in the sentinel prey method. A range of different sentinel prey types could be suitable for use in most comparative studies; however, each prey type has its own benefits and limitations, and these should be carefully evaluated to determine which is most suitable to address the research questions.     

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.     

Interspecific differences in antipredator strategies determine the strength of non‐consumptive predator effects on stream detritivores

Animal species differ considerably in their response to predation risks. Interspecific variability in prey behaviour and morphology can alter cascading effects of predators on ecosystem structure and functioning. We tested whether species‐specific morphological defenses may affect responses of leaf litter consuming invertebrate prey to sit‐and‐wait predators, the odonate Cordulegaster boltonii larvae, in aquatic food webs. Partly or completely blocking the predator mouthparts (mandibles and/or extensible labium), thus eliminating consumptive (i.e. lethal) predator effects, we created a gradient of predator‐prey interaction intensities (no predator < predator – no attack < predator – non‐lethal attacks < lethal predator). A field experiment was first used to assess both consumptive and non‐consumptive predator effects on leaf litter decomposition and prey abundances. Laboratory microcosms were then used to examine behavioural responses of armored and non‐armored prey to predation risk and their consequences on litter decomposition. Results show that armored and non‐armored prey responded to both acute (predator – non‐lethal attacks) and chronic (predator – no attack) predation risks. Acute predation risk had stronger effects on litter decomposition, prey feeding rate and prey habitat use than predator presence alone (chronic predation risk). Predator presence induced a reduction in feeding activity (i.e. resource consumption) of both prey types but a shift to predator‐free habitat patches in non‐armored detritivores only. Non‐consumptive predator effects on prey subsequently decreased litter decomposition rate. Species‐specific prey morphological defenses and behaviour should thus be considered when studying non‐consumptive predator effects on prey community structure and ecosystem functioning.     

Prey risk assessment depends on conspecific density

In many systems, the number of prey killed by predators increases with prey density. This in turn generates higher levels of the indirect signals that prey use to assess predation risk. A model developed by Peacor (2003) showed that prey that respond to predator cues without accounting for conspecific density will consistently over‐ or under‐estimate risk and therefore invest improperly in anti‐predator defense. We tested this model using Rana temporaria tadpoles as prey and Aeshna cyanea dragonfly larvae as predators. As assumed by the model, prey reduced risky activity with increasing concentrations of predator kairomones and increased activity at high prey density. However, prey did not react to changes in cue or density if the ratio of cue‐to‐density remained constant. Prey therefore monitored their per capita risk, strongly supporting Peacor's model.     

Antipredator Responses by Native Mosquitofish to Non-Native Cichlids: An Examination of the Role of Prey Naiveté

The strong impact of non‐native predators in aquatic systems is thought to relate to the evolutionary naiveté of prey. Due to isolation and limited dispersal, this naiveté may be relatively high in freshwater systems. In this study, we tested this notion by examining the antipredator response of native mosquitofish, Gambusia holbrooki, to two non‐native predators found in the Everglades, the African jewelfish, Hemichromis letourneuxi, and the Mayan cichlid, Cichlasoma urophthalmus. We manipulated prey naiveté by using two mosquitofish populations that varied in their experience with the recent invader, the African jewelfish, but had similar levels of experience with the longer‐established Mayan cichlid. Specifically, we tested these predictions: (1) predator hunting modes differed between the two predators, (2) predation rates would be higher by the novel jewelfish predator, (3) particularly on the naive population living where jewelfish have not invaded yet, (4) antipredator responses would be stronger to Mayan cichlids due to greater experience and weaker and/or ineffective to jewelfish, and (5) especially weakest by the naive population. We assayed prey and predator behavior, and prey mortality in lab aquaria where both predators and prey were free‐ranging. Predator hunting modes and habitat domains differed, with jewelfish being more active search predators that used slightly higher parts of the water column and less of the habitat structure relative to Mayan cichlids. In disagreement with our predictions, predation rates were similar between the two predators, antipredator responses were stronger to African jewelfish (except for predator inspections), and there was no difference in response between jewelfish‐savvy and jewelfish‐naive populations. These results suggest that despite the novelty of introduced predators, prey may be able to respond appropriately if non‐native predator archetypes are similar enough to those of native predators, if prey rely on general antipredator responses or predation cues, and/or show neophobic responses.     

Does aggregation benefit bark beetles by diluting predation? Links between a group‐colonisation strategy and the absence of emergent multiple predator effects

Abstract. 1. Aggregation in bark beetles (Coleoptera: Scolytidae) aids in mate attraction and resource procurement when colonising well‐defended plants; however, some species colonise primarily poorly defended plants, and intraspecific competition increases mortality. The hypothesis that decreased risk of predation was a potential benefit to aggregation in such circumstances was tested, using the pine engraver, Ips pini (Say) and its two major predators Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae). Both single‐ and multiple‐predator effects, across a range of prey densities, were tested. 2. Both male and female colonisation events increased with herbivore density, in an asymptotic fashion. 3. Predators decreased the number of colonisers in a density‐dependent manner, consistent with a type II functional response. 4. The proportional impact of predators decreased with increased herbivore colonisation densities. These findings indicate that predator dilution may be a viable benefit to aggregation. 5. Total emergence of the herbivore also increased with density, although the net replacement rate during one generation was independent of initial arrival density. This was likely due to larval predation, which negates potential relationships between per capita reproductive success and establishment density. 6. Each predator species decreased I. pini's net replacement rate by approximately 42%, and their combined effect was approximately 70%. 7. Overall, these predators modified their prey's establishment and adult mortality relationships in additive manners. This is somewhat surprising, given the potential for emergent effects due to interactions between multiple predators foraging within a common habitat. The persistence of additivity, rather than risk reduction or enhancement to the prey, may increase the predator‐swamping benefit to aggregation for this herbivore. 8. The effects of these predators are substitutable, and likely exert equivalent selective pressures to mask signals at the whole‐plant level.     

Additive effects of predator diversity on pest control caused by few interactions among predator species

1. Studies of the impact of predator diversity on biological pest control have shown idiosyncratic results. This is often assumed to be as a result of differences among systems in the importance of predator–predator interactions such as facilitation and intraguild predation. The frequency of such interactions may be altered by prey availability and structural complexity. A direct assessment of interactions among predators is needed for a better understanding of the mechanisms affecting prey abundance by complex predator communities. 2. In a field cage experiment, the effect of increased predator diversity (single species vs. three‐species assemblage) and the presence of weeds (providing structural complexity) on the biological control of cereal aphids were tested and the mechanisms involved were investigated using molecular gut content analysis. 3. The impact of the three‐predator species assemblages of aphid populations was found to be similar to those of the single‐predator species treatments, and the presence or absence of weeds did not alter the patterns observed. This suggests that both predator facilitation and intraguild predation were absent or weak in this system, or that these interactions had counteracting effects on prey suppression. Molecular gut content analysis of predators provided little evidence for the latter hypothesis: predator facilitation was not detected and intraguild predation occurred at a low frequency. 4. The present study suggests additive effects of predators and, therefore, that predator diversity per se neither strengthens nor weakens the biological control of aphids in this system.     

Interactive effects of productivity and predation on zooplankton diversity

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator‐prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally‐mediated numerical responses by the flight‐capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.     

Potential cotton aphid,Aphis gossypii,population suppression by arthropod predators in upland cotton

The cotton aphid, Aphis gossypii Glover, predation rate of convergent lady beetle, Hippodamia convergens Guerin‐Meneville, was determined by assigning a single predator randomly to each of four prey density treatments in the laboratory. Prey densities included 25, 50, 100, and 200 aphids per Petri dish arena. Predation response was recorded at 1, 4, 8, 16, 24, and 48 h after assigning predators to their prey treatments. Rate of consumption increased through time, with all 25 aphids consumed during the first 4 h of the experiment. At the highest density, adult lady beetle consumed on average 49, 99, 131, 163, 183, and 200 aphids within 1, 4, 8, 16, 24 and 48 h, respectively. Predators showed a curvilinear feeding response in relation to total available time, indicating that convergent lady beetles have the potential to suppress larger populations of aphids through continuous feeding by regulating their predation efficiency during feeding. The analysis of age‐specific mortality in absence of prey revealed that lady beetles could survive for an extended period of time (more than 2 weeks) without prey. The ability of a predator to survive without prey delays or prevents the rebound of pest populations that is a significant factor in natural biological control. A two‐year field sampling of 10 cotton arthropod predator species showed that spiders (27%) were the most dominant foliage dwelling predators in the Texas High Plains cotton followed by convergent lady beetles (23.5%), hooded beetles (13.5%), minute pirate bugs (11%), green lacewings (9.5%), bigeyed bugs (7.5%), scymnus beetles (3%), soft‐winged flower beetles (2%), damsel bugs (1.5%), and assassin bugs (1.5%). A field cage study showed that one H. convergens adult per plant released at prey density of one aphid per leaf kept the aphid population below economic threshold for the entire growing season.     

Habitat and food specificity in aphidophagous predators

Current knowledge of the processes underlying prey location and choice by aphidophagous predators is reviewed by considering the succession of behavioural mechanisms required for the predator to obtain prey. The predator may locate areas where prey are likely to be found by responding to physical aspects of the habitat, or to semiochemicals produced by the host plant. The predator may then respond to visual or olfactory cues to locate the aphid prey. The predator's readiness to attack and consume aphids is influenced by any behavioural or chemical defence strategies, and by the palatability or nutrient value of the aphids. Toxic allelochemicals ingested by aphids from their host plant may have a detrimental effect on predators.     

Manipulating field margins to increase predation intensity in fields of winter wheat (Triticum aestivum)

The effectiveness of natural enemies to control pests can be enhanced through habitat manipulation. However, due to the differences in their ecology, generalist and specialist species may respond differently to the same manipulation. Moreover, interactions among natural enemies (i.e. cannibalism, intraguild predation, hyperparasitism) may complicate the assumption that a higher density of natural enemies would increase the level of biological control. We investigated the natural enemy guild composition and the predation rate along flower vs. grass margins at the edge of winter wheat (Triticum aestivum) fields in Denmark. Natural enemies were sampled by pitfall trapping and by suction sampling; predation intensity was measured using two different sentinel prey methods: artificial caterpillars made of plasticine, and sentinel aphid colonies. Specialist and generalist species responded differently to the two margin types: specialists (mostly parasitic wasps) were attracted by the flower margins, while generalists (ground beetles, rove beetles and spiders) were more active in grass margins. The number of artificial caterpillars attacked was significantly greater in grass margins (mean = 48.9%, SD = 24.3) than in flower margins (mean = 30.7%, SD = 17.4). We found a significant positive relationship between the number of artificial caterpillars attacked by chewing insects, and activity density for large (≥15 mm) ground beetles. Predation of sentinel aphids in wheat fields did not vary significantly in relation to margin type. Our results suggest that flowering margins may be beneficial for canopy‐active specialist natural enemies, but grassy margins are more useful for ground‐active generalist predators.     

Variation in foraging success among predators and its implications for population dynamics

The effects of the expected predation rate on population dynamics have been studied intensively, but little is known about the effects of predation rate variability (i.e., predator individuals having variable foraging success) on population dynamics. In this study, variation in foraging success among predators was quantified by observing the predation of the wolf spider Pardosa pseudoannulata on the cricket Gryllus bimaculatus in the laboratory. A population model was then developed, and the effect of foraging variability on predator–prey dynamics was examined by incorporating levels of variation comparable to those quantified in the experiment. The variability in the foraging success among spiders was greater than would be expected by chance (i.e., the random allocation of prey to predators). The foraging variation was density‐dependent; it became higher as the predator density increased. A population model that incorporates foraging variation shows that the variation influences population dynamics by affecting the numerical response of predators. In particular, the variation induces negative density‐dependent effects among predators and stabilizes predator–prey dynamics.     

A review of the sentinel prey method as a way of quantifying invertebrate predation under field conditions

Sentinel prey can provide a direct, quantitative measure of predation under field conditions. Live sentinel prey provides more realistic data but rarely allows the partitioning of the total predation pressure; artificial prey is less natural but traces left by different predators are identifiable, making it suitable for comparative studies. We reviewed the available evidence of the use of both types of invertebrate sentinel prey. Fifty‐seven papers used real prey, usually measuring predation on a focal (often pest) species, with studies overwhelmingly from North America. The median predation was 25.8% d^?1. Artificial sentinel prey (45 papers) were used in both temperate and tropical areas, placed more above ground than at ground level. The most commonly used artificial prey imitated a caterpillar. Up to 14 predator groups were identified, registering a median of 8.8% d^?1 predation; half the studies reported only bird predation. Predation on real prey was higher than on artificial ones, but invertebrate predation was not higher than vertebrate predation. Invertertebrate but not vertebrate predation was negatively related to prey size. Predation near the Equator was not higher than at higher latitudes, nor in cultivated than noncultivated habitats. The use of sentinel prey is not yet standardised in terms of prey size, arrangement, exposure period or data reporting. Due to the simplicity and ease of use of the method, such standardisation may increase the usefulness of comparative studies, contributing to the understanding of the importance and level of predation in various habitats worldwide.     

Density-dependent effects of multiple predators sharing a common prey in an endophytic habitat

Multiple predator species feeding on a common prey can lead to higher or lower predation than would be expected by simply combining their individual effects. Such emergent multiple predator effects may be especially prevalent if predators share feeding habitat. Despite the prevalence of endophagous insects, no studies have examined how multiple predators sharing an endophytic habitat affect prey or predator reproduction. We investigated density-dependent predation of Thanasimus dubius (Coleoptera: Cleridae) and Platysoma cylindrica (Coleoptera: Histeridae) on a bark beetle prey, Ips pini (Coleoptera: Scolytidae), in a laboratory assay. I. pini utilize aggregation pheromones to group-colonize and reproduce within the stems of conifers. T. dubius and P. cylindrica exploit these aggregation pheromones to arrive simultaneously with the herbivore. Adult T. dubius prey exophytically, while P. cylindrica adults enter and prey within the bark beetle galleries. Larvae of both predators prey endophytically. We used a multiple regression analysis, which avoids confounding predator composition with density, to examine the effects of varying predator densities alone and in combination on herbivore establishment, herbivore reproduction, and predator reproduction. Predators reduced colonization success by both sexes, and decreased I. pini reproduction on a per male and per female basis. The combined effects of these predators did not enhance or reduce prey establishment or reproduction in unexpected manners, and these predators were entirely substitutable. The herbivores net replacement rate was never reduced significantly below one at prey and predator densities emulating field conditions. Similar numbers of each predator species emerged from the logs, but predator reproduction suffered from high intraspecific interference. The net replacement rate of P. cylindrica was not affected by conspecifics or T. dubius. In contrast, the net replacement rate of T. dubius decreased with the presence of conspecifics or P. cylindrica. Combinations of both predators led to an emergent effect, a slightly increased net replacement rate of T. dubius. This may have been due to predation by larval T. dubius on pupal P. cylindrica, as P. cylindrica develops more rapidly than T. dubius within this shared habitat.     

Mask of wax: Secretions of wax conceal aphids from detection by spider's eyes

Abstract

We investigated how insects use wax as a defence against visual predators, using a New Zealand salticid species, Marpissa marina, as the predator and Eriosoma lanigerum, an aphid that covers itself with wax, as the prey. For live‐prey testing, the predator was presented with two aphids, one with its wax covering intact and one with its wax removed. The predator ate more of the waxless than wax‐covered aphids. The predators were presented with two lures at a time: (1) one that was fully covered with wax (hid the aphid's head) compared with one that was without wax (waxless) or (2) one that was fully covered with wax compared with one that was only partially covered with wax (the head of the prey exposed), or (3) one that was waxless compared with one that was partially covered with wax. The predators stalked waxless prey more often than they stalked prey that was fully or partially covered with wax. When wax only partially covered the prey (i.e., when the prey's head was left exposed), the predator more often stalked than when the insect was fully covered. These findings suggest that the aphid's wax covering functions in part to hide prey‐identification cues from vision‐guided predators.