Predator biodiversity increases the survivorship of juvenile predators

When predator biodiversity strengthens herbivore suppression, the pattern generally is attributed to interspecific complementarity. However, the relaxation of intraspecific interference within diverse communities has received less attention as an underlying factor, and most experiments to date span much less than one predator generation. Here, working with a community of aphid predators, we compared the survivorship of juvenile predators embedded within diverse versus single-species communities of adult predators. We found that greater predator diversity improved juvenile survivorship for three of four predator taxa (the lady beetles Hippodamia convergens and Coccinella septempunctata, and the bug Nabis alternatus; but not the small bug Geocoris bullatus), whereas survivorship was relatively low when juveniles foraged among only conspecific adults. When aphid densities differed they were lowest for the diverse treatment, and so resource availability could not explain differences in juvenile survivorship. Instead, feeding trials indicated that cannibalism generally posed a greater risk to juveniles than did intraguild predation (with Geocoris again the exception). Our results suggest that the dilution of intraspecific interference may play an important, and perhaps underappreciated, role in shaping predator diversity effects. Furthermore, relatively strong cannibalism but weak intraguild predation has the potential to project diversity effects forward into subsequent generations.     

Intraguild predation and native lady beetle decline

Coccinellid communities across North America have experienced significant changes in recent decades, with declines in several native species reported. One potential mechanism for these declines is interference competition via intraguild predation; specifically, increased predation of native coccinellid eggs and larvae following the introduction of exotic coccinellids. Our previous studies have shown that agricultural fields in Michigan support a higher diversity and abundance of exotic coccinellids than similar fields in Iowa, and that the landscape surrounding agricultural fields across the north central U.S. influences the abundance and activity of coccinellid species. The goal of this study was to quantify the amount of egg predation experienced by a native coccinellid within Michigan and Iowa soybean fields and explore the influence of local and large-scale landscape structure. Using the native lady beetle Coleomegilla maculata as a model, we found that sentinel egg masses were subject to intense predation within both Michigan and Iowa soybean fields, with 60.7% of egg masses attacked and 43.0% of available eggs consumed within 48 h. In Michigan, the exotic coccinellids Coccinella septempunctata and Harmonia axyridis were the most abundant predators found in soybean fields whereas in Iowa, native species including C. maculata, Hippodamia parenthesis and the soft-winged flower beetle Collops nigriceps dominated the predator community. Predator abundance was greater in soybean fields within diverse landscapes, yet variation in predator numbers did not influence the intensity of egg predation observed. In contrast, the strongest predictor of native coccinellid egg predation was the composition of edge habitats bordering specific fields. Field sites surrounded by semi-natural habitats including forests, restored prairies, old fields, and pasturelands experienced greater egg predation than fields surrounded by other croplands. This study shows that intraguild predation by both native and exotic predators may contribute to native coccinellid decline, and that landscape structure interacts with local predator communities to shape the specific outcomes of predator-predator interactions.     

Biodiversity Loss following the Introduction of Exotic Competitors: Does Intraguild Predation Explain the Decline of Native Lady Beetles?

Exotic species are widely accepted as a leading cause of biodiversity decline. Lady beetles (Coccinellidae) provide an important model to study how competitor introductions impact native communities since several native coccinellids have experienced declines that coincide with the establishment and spread of exotic coccinellids. This study tested the central hypothesis that intraguild predation by exotic species has caused these declines. Using sentinel egg experiments, we quantified the extent of predation on previously-common (Hippodamia convergens) and common (Coleomegilla maculata) native coccinellid eggs versus exotic coccinellid (Harmonia axyridis) eggs in three habitats: semi-natural grassland, alfalfa, and soybean. Following the experiments quantifying egg predation, we used video surveillance to determine the composition of the predator community attacking the eggs. The extent of predation varied across habitats, and egg species. Native coccinellids often sustained greater egg predation than H. axyridis. We found no evidence that exotic coccinellids consumed coccinellid eggs in the field. Harvestmen and slugs were responsible for the greatest proportion of attacks. This research challenges the widely-accepted hypothesis that intraguild predation by exotic competitors explains the loss of native coccinellids. Although exotic coccinellids may not be a direct competitor, reduced egg predation could indirectly confer a competitive advantage to these species. A lower proportion of H. axyridis eggs removed by predators may have aided its expansion and population increase and could indirectly affect native species via exploitative or apparent competition. These results do not support the intraguild predation hypothesis for native coccinellid decline, but do bring to light the existence of complex interactions between coccinellids and the guild of generalist predators in coccinellid foraging habitats.     

Predator interference limits fly egg biological control by a guild of ground-active beetles

We examined whether predator interference could prevent effective conservation biological control of Delia spp. flies, important pests of cole crops, by an assemblage of carabid and staphylinid beetles. In laboratory feeding trials we found that the smaller (<1 cm) beetle species common at our site readily ate dipteran eggs, while the most common large carabid species, Pterostichus melanarius, rarely did. However, P. melanarius did eat several of the smaller beetle species. We conducted two field experiments where we manipulated immigration rates of the ground predator guild and then measured predation on fly eggs. Predation rates were consistently higher in cages where predators were added at ambient densities, compared to cages where ground predators were removed. However, in the second field experiment, when we quadrupled predator immigration rates neither beetle activity-density nor predation rate increased. High immigration rate plots had a higher proportion of P. melanarius in the predator community, compared to plots with beetles added at ambient densities, suggesting that P. melanarius was reducing activity-densities of the smaller beetles, perhaps through intraguild predation. Thus, tactics to improve the biological control of Delia spp. by conserving generalist predators, such as providing in- or extra-field refuges, could be thwarted if the primary predators of fly eggs, small carabids and staphylinids, are the targets of intraguild predation by also-conserved larger predators.     

Egg Predation by the Introduced Lady Beetle,Coccinella septempunctata (Coleoptera: Coccinellidae), Lowers Mortality but Raises Relative Risk for the Native Lady Beetle,Coccinella novemnotata

Populations of the native ninespotted lady beetle, Coccinella novemnotata Herbst, have undergone precipitous declines in North America following the establishment of the exotic sevenspotted lady beetle, Coccinella septempunctata L. Recent volunteer efforts have made it possible to establish colonies of the now-rare C. novemnotata and test mechanisms contributing to its decline. We evaluated the relative frequencies of intraguild predation and cannibalism of eggs between these two species. A single C. novemnotata or C. septempunctata adult was exposed to conspecific and heterospecific eggs in either the presence or absence of pea aphids. The study revealed two expected results: 1) eggs of C. novemnotata were consumed more frequently than eggs of C. septempunctata by both species, and 2) egg consumption was higher when aphids were absent, independent of predator and egg species. There were also two unexpected results from the study: 1) the asymmetry between egg predation rates was higher when aphids were present, and 2) higher predation rates on C. novemnotata eggs in the absence of alternate prey was due to a relatively higher rate of intraspecific cannibalism. This implies that C. novemnotata would have suffered higher egg mortality rates before the invasion of C. septempunctata, but even though the aggregate rate of egg predation on C. novemnotata eggs is lower post-invasion, it is still significantly higher than the aggregate rate of predation of C. septempunctata eggs. This differential pattern of asymmetric predation could contribute to habitat compression and the overall decline of C. novemnotata.     

Role of surface chemicals in egg cannibalism and intraguild predation by neonates of two aphidophagous ladybirds, Propylea dissecta and Coccinella transversalis

Abstract:  The role of surface chemicals in egg cannibalism and intraguild predation by neonates of two aphidophagous ladybirds, Propylea dissecta and Coccinella transversalis were examined. Neonates of both species prefer to eat non-sibling conspecific eggs than heterospecific eggs, with higher preference in P. dissecta . Surface chemicals appear to play a major role for the preference of conspecific eggs, as the ladybird behaviour was reversed when these chemicals were interchanged. The surface chemicals present on the eggs possibly act as feeding stimulants to conspecific neonates but not for heterospecific neonates. Egg clustering enhances the effect of surface chemical and stimulate non-sibling egg cannibalism and appears to reduce intraguild predation. Egg clustering is advantageous to ladybirds, as it can stimulate non-sibling cannibalism by neonates. It appears that aggregation of chemicals present on the conspecific egg surface possibly attracts the hungry conspecific neonate, providing the first meal for its survival, while protecting against heterospecific predators. Neonates of both ladybirds were reluctant to eat heterospecific eggs.     

Intraguild predation and cannibalism between the predatory mites Neoseiulus neobaraki and N. paspalivorus, natural enemies of the coconut mite Aceria guerreronis

Neoseiulus neobaraki and N. paspalivorus are amongst the most common phytoseiid predators of coconut mite, Aceria guerreronis, found in the spatial niche beneath coconut fruit bracts. Both predators may occur on the same coconut palms in Benin and Tanzania and are therefore likely to interact with each other. Here, we assessed cannibalism and intraguild predation (IGP) of the two predators in the absence and presence of their primary prey A. guerreronis. In the absence of the shared extraguild prey, A. guerreronis, N. neobaraki killed 19 larvae of N. paspalivorus per day and produced 0.36?eggs/female/day, while the latter species killed only 7 larvae of the former and produced 0.35?eggs/female/day. Presence of A. guerreronis only slightly decreased IGP by N. neobaraki but strongly decreased IGP by N. paspalivorus, which consumed 4-7 times less IG prey than N. neobaraki. Resulting predator offspring to IG prey ratios were, however, 4-5 times higher in N. paspalivorus than N. neobaraki. Overall, provision of A. guerreronis increased oviposition in both species. In the cannibalism tests, in the absence of A. guerreronis, N. neobaraki and N. paspalivorus consumed 1.8 and 1.2 conspecific larvae and produced almost no eggs. In the presence of abundant herbivorous prey, cannibalism dramatically decreased but oviposition increased in both N. neobaraki and N. paspalivorus. In summary, we conclude that (1) N. neobaraki is a much stronger intraguild predator than N. paspalivorus, (2) cannibalism is very limited in both species, and (3) both IGP and cannibalism are reduced in the presence of the common herbivorous prey with the exception of IGP by N. neobaraki, which remained at high levels despite presence of herbivorous prey. We discuss the implications of cannibalism and IGP on the population dynamics of A. guerreronis and the predators in view of their geographic and within-palm distribution patterns.     

A non-trophic interaction chain links predators in different spatial niches

Non-trophic interactions, driven by one species changing the behavior but not density of another species, appear to be as pervasive as those involving consumption. However, ecologists have only begun to explore non-trophic interactions in species-rich communities. We investigated interactions within a community including two predator–prey linkages separated in space: ground-active predatory beetles and their fly egg prey on the ground, and lady beetles and their aphid prey in plant foliage. In field and greenhouse experiments we found that ground-active predators preyed heavily on fly eggs except when both aphids and lady beetles were present. The aphids drop from the foliage to escape foraging lady beetles, and once on the ground apparently triggered ground-active predators to switch from attacking fly eggs to attacking aphids. This suggests that the first non-trophic interaction in the foliage, mediated by aphid antipredator behavior, in turn initiated and accentuated a second non-trophic interaction on the ground, mediated by prey-switching behavior by ground predators. Our results demonstrate that successive non-trophic interactions can be propagated along chains of more than three species, and can serve to link species that are otherwise spatially isolated.     

Laboratory and field tests of predation on the cereal leaf beetle,Oulema melanopus (Coleoptera: Chrysomelidae)

The cereal leaf beetle (CLB), Oulema melanopus (L.) (Coleoptera: Chrysomelidae), is an invasive pest in North America recently reported in the Canadian Prairies. We performed a series of laboratory assays to identify potential predators and a field study to quantify predation of CLB eggs. In no-choice Petri dish assays, ground beetles (Carabidae), rove beetles (Staphylinidae), and several common lady beetle species (Coccinellidae) were the most consistent predators of eggs and larvae. Nabis spp. (Hemiptera: Nabidae) and wolf spiders (Araneae: Lycosidae) consumed many larvae, but did not consume eggs. Hippodamia spp., Coccinella septempunctata (L.) (Coleoptera: Coccinellidae), and Pterostichus melanarius (Illiger) (Coleoptera: Carabidae) also fed on CLB eggs on potted plants when an alternative food source was available, Sitobion avenae (Fabricius) (Hemiptera: Aphididae). In our field study, we found an average of 24.5% of sentinel eggs disappeared over a 24?h period, likely due to predation. Our results suggest that generalist predators can play an important role in the biological control of CLB, and warrant further study.     

Maternally induced intraclutch cannibalism: an adaptive response to predation risk?

Theory on condition‐dependent risk‐taking indicates that when prey are in poor condition, their anti‐predator responses should be weak. However, variation in responses resulting from differences in condition is generally considered an incidental by‐product of organisms living in a heterogeneous environment. Using Leptinotarsa decemlineata beetles and stinkbug (Podisus maculiventris) predators, we hypothesised that in response to predation risk, parents improve larval nutritional condition and expression of anti‐predator responses by promoting intraclutch cannibalism. We showed that mothers experiencing predation risk increase production of unviable trophic eggs, which assures provisioning of an egg meal to the newly hatched offspring. Next, we experimentally demonstrated that egg cannibalism reduces L. decemlineata vulnerability to predation by improving larval nutritional condition and expression of anti‐predator responses. Intraclutch cannibalism in herbivorous insects might be a ubiquitous strategy, aimed to overcome the dual challenge of feeding on protein‐limited diets while living under constant predation threat.     

Intraguild predation of the aphid parasitoid Aphelinus certus by Coccinella septempunctata and Harmonia axyridis

Coincidental intraguild predation is expected to be less disruptive to biological control than omnivorous intraguild predation, and strong intraguild predation is not expected to occur in natural systems. Coincidental intraguild predation in a foodweb involving introduced pest and natural enemy species was examined to determine whether intraguild predation would be disruptive of biological control services in soybean agroecosystems. Introduced natural enemies are important regulators of soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), populations in North America. Seven-spotted lady beetles, Coccinella septempunctata L., and multicolored Asian lady beetles, Harmonia axyridis Pallas (Coleoptera: Coccinellidae), are key predators of soybean aphid in North America while the chalcidoid wasp, Aphelinus certus Yasnosh (Hymenoptera: Aphelinidae), is the most common parasitoid of soybean aphid in Ontario, Canada. Predation of parasitized soybean aphids at two stages (newly parasitized aphids and mummified aphids) by adults and third instar larvae of both C. septempunctata and H. axyridis was examined in laboratory experiments. In choice experiments, all stages of lady beetles preferred non-parasitized aphids over mummified aphids. In cage experiments, third instar larvae and male and female adults of both lady beetles did not discriminate between newly parasitized and non-parasitized aphids. The influence of coincidental intraguild predation on the efficacy of parasitoids as biological control agents, and implications for soybean aphid management decisions based on natural enemies, are discussed.     

Preference for Cannibalism and Ontogenetic Constraints in Competitive Ability of Piscivorous Top Predators

Occurrence of cannibalism and inferior competitive ability of predators compared to their prey have been suggested to promote coexistence in size-structured intraguild predation (IGP) systems. The intrinsic size-structure of fish provides the necessary prerequisites to test whether the above mechanisms are general features of species interactions in fish communities where IGP is common. We first experimentally tested whether Arctic char (Salvelinus alpinus) were more efficient as a cannibal than as an interspecific predator on the prey fish ninespine stickleback (Pungitius pungitius) and whether ninespine stickleback were a more efficient competitor on the shared zooplankton prey than its predator, Arctic char. Secondly, we performed a literature survey to evaluate if piscivores in general are more efficient as cannibals than as interspecific predators and whether piscivores are inferior competitors on shared resources compared to their prey fish species. Both controlled pool experiments and outdoor pond experiments showed that char imposed a higher mortality on YOY char than on ninespine sticklebacks, suggesting that piscivorous char is a more efficient cannibal than interspecific predator. Estimates of size dependent attack rates on zooplankton further showed a consistently higher attack rate of ninespine sticklebacks compared to similar sized char on zooplankton, suggesting that ninespine stickleback is a more efficient competitor than char on zooplankton resources. The literature survey showed that piscivorous top consumers generally selected conspecifics over interspecific prey, and that prey species are competitively superior compared to juvenile piscivorous species in the zooplankton niche. We suggest that the observed selectivity for cannibal prey over interspecific prey and the competitive advantage of prey species over juvenile piscivores are common features in fish communities and that the observed selectivity for cannibalism over interspecific prey has the potential to mediate coexistence in size structured intraguild predation systems.     

Spatial refuge from intraguild predation: implications for prey suppression and trophic cascades

The ability of predators to elicit a trophic cascade with positive impacts on primary productivity may depend on the complexity of the habitat where the players interact. In structurally-simple habitats, trophic interactions among predators, such as intraguild predation, can diminish the cascading effects of a predator community on herbivore suppression and plant biomass. However, complex habitats may provide a spatial refuge for predators from intraguild predation, enhance the collective ability of multiple predator species to limit herbivore populations, and thus increase the overall strength of a trophic cascade on plant productivity. Using the community of terrestrial arthropods inhabiting Atlantic coastal salt marshes, this study examined the impact of predation by an assemblage of predators containing Pardosa wolf spiders, Grammonota web-building spiders, and Tytthus mirid bugs on herbivore populations (Prokelisia planthoppers) and on the biomass of Spartina cordgrass in simple (thatch-free) and complex (thatch-rich) vegetation. We found that complex-structured habitats enhanced planthopper suppression by the predator assemblage because habitats with thatch provided a refuge for predators from intraguild predation including cannibalism. The ultimate result of reduced antagonistic interactions among predator species and increased prey suppression was enhanced conductance of predator effects through the food web to positively impact primary producers. Behavioral observations in the laboratory confirmed that intraguild predation occurred in the simple, thatch-free habitat, and that the encounter and capture rates of intraguild prey by intraguild predators was diminished in the presence of thatch. On the other hand, there was no effect of thatch on the encounter and capture rates of herbivores by predators. The differential impact of thatch on the susceptibility of intraguild and herbivorous prey resulted in enhanced top-down effects in the thatch-rich habitat. Therefore, changes in habitat complexity can enhance trophic cascades by predator communities and positively impact productivity by moderating negative interactions among predators.     

Predation by adult and larval lady beetles (Coleoptera: Coccinellidae) on initial contact with lady beetle eggs

Na?ve adults and larvae of the native lady beetles Coleomegilla maculata (DeGeer), Cycloneda munda (Say), Hippodamia convergens Guérin-Méneville, Olla v-nigrum (Mulsant), and the exotic lady beetle Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) were tested for their initial response to eggs of these five lady beetle species and for egg consumption on first contact and after 3 h. Additionally, field-collected O. v-nigrum and H. axyridis adults were tested. C. maculata, H. axyridis, and O. v-nigrum adults responded similarly to all egg species on first contact. Higher numbers of C. munda adults did not eat C. maculata, H. convergens, and O. v-nigrum eggs on first contact compared with numbers that did eat C. munda and H. axyridis eggs. H. convergens adults always ate C. munda eggs but hardly ate H. axyridis eggs on first contact. Results showed that over the 3-h interval, egg predation by those predators feeding on first contact was always higher, except for adults and larvae of C. maculata, than for those that did not feed on first contact. Thus, acceptance of eggs on initial contact does impact egg survival. It is likely that eggs of all native species tested (i.e., C. maculata, C. munda, H. convergens, and O. v-nigrum), but not exotic H. axyridis eggs, are suitable food for C. maculata, H. convergens, and O. v-nigrum, whereas only C. munda eggs serve as suitable food for C. munda. In direct contrast, all egg species tested would likely serve as suitable food for the exotic H. axyridis.     

Sum of fears among intraguild predators drives the survival of green sea turtle (Chelonia mydas) eggs

Ecologists have long theorized that apex predators stabilize trophic systems by exerting a net protective effect on the basal resource of a food web. Although experimental and observational studies have borne this out, it is not always clear what behavioural mechanisms among the trophically connected species are responsible for this stability. Fear of intraguild predation is commonly identified as one such mechanism in models and mesocosm studies, but empirical evidence in natural systems remains limited, as the complexity of many trophic systems renders detailed behavioural studies of species interactions challenging. Here, we combine long-term field observations of a trophic system in nature with experimental behavioural studies of how all the species in this system interact, in both pairs and groups. The results demonstrate how an abundant, sessile and palatable prey item (sea turtle eggs, Chelonia mydas) survives when faced by three potential predators that all readily eat eggs: an apex predator (the stink ratsnake, Elaphe carinata) and two mesopredators (the brown rat, Rattus norvegicus, and kukri snake, Oligodon formosanus). Our results detail how fear of intraguild predation, conspecific cannibalism, habitat structure and territorial behaviour among these species interact in a complex fashion that results in high egg survival.     

Fish predation can induce mesohabitat-specific differences in food web structures in small stream ecosystems

Diverse benthic communities in streams include a wide variety of predators with different habitat preferences, e.g. for pools or riffles. We hypothesised that these preferences result in mesohabitat-specific predator community structures with quantitative differences concerning predation intensity by vertebrate and invertebrate predators, importance of intraguild predation, or top–down pressure. This hypothesis was evaluated for a small submontane stream by means of mesohabitat-specific quantification of prey consumption by two benthivorous fish species (Gobio gobio and Barbatula barbatula) and several invertebrate predators. The estimation was based on daily food rations and diet composition of predators and mesohabitat-specific predator biomass. We found clear differences between the two mesohabitat types. Predator food webs were less complex in pools than in riffles. Fish predation was more important than invertebrate predation in pools, and intraguild predation had a higher relative importance in these mesohabitats. These differences were probably caused by the mesohabitat use of G. gobio, the largest top predator, which preferred pools. Consequently, the predator food webs were more similar between the mesohabitats when fish were absent. Top–down pressure on primary consumers by all predators together was lowest in pools without fish, but the effect was not significant. Omnivory (including cannibalism) was intense, but its potentially destabilising effects were probably counterbalanced by mesohabitat connectivity. From the results of our experimental study, we conclude that even in small stream ecosystems, food web structures and predation pathways can differ between mesohabitats and that a mesohabitat-specific consideration will help to explain the variety of top–down effects on benthic communities.     

Development of an immunological technique for identifying multiple predator–prey interactions in a complex arthropod assemblage

A simplified but highly effective approach for the post‐mortem evaluation of predation on several targeted members of an arthropod assemblage that does not require the development of pest‐specific enzyme‐linked immunosorbent assay (ELISA) (e.g. pest‐specific monoclonal antibodies) or PCR assays (DNA primers) is described. Laboratory feeding studies were conducted to determine if predation events could be detected from predators that consumed prey marked with foreign protein. I determined that large and small rabbit immunoglobulin G (IgG)‐marked prey can be detected by a rabbit‐IgG‐specific ELISA in the guts of chewing and piercing–sucking type predators. I then conducted multifaceted inclusion and exclusion field cage studies to qualify the degree of interguild and intraguild predation occurring among a complex arthropod assemblage during four separate light phase treatments. The field cages contained an arthropod assemblage consisting of 11 or 12 species of predaceous arthropods and three pest species. The three pests introduced into the cages included third instar Trichoplusia ni marked with rabbit IgG, third instar Lygus hesperus marked with chicken IgG and Pectinophora gossypiella sentinel egg masses. The inclusion cages allowed foraging fire ants, Solenopis xyloni, to freely enter the cages while the exclusion cages contained barriers that prevented ant entry. The results obtained using the conventional inclusion/exclusion field cage methodology revealed that there was substantial interguild and intraguild predation occurring on the majority of the arthropods in the assemblage, particularly in those cages that included ants. I then precisely identified which predators in the assemblage were feeding on the three targeted pests by conducting three post‐mortem gut content analyses on each individual predator (1503 individuals) in the assemblage. Specifically, P. gossypiella egg predation events were detected using an established P. gossypiella‐egg‐specific ELISA, and third instar T. ni and L. hesperus predation events were detected using rabbit‐IgG‐specific and chicken‐IgG‐specific ELISAs, respectively. Generally, the gut ELISAs revealed that Collops vittatus, Spanagonicus albofasciatus and Geocoris punctipes readily preyed on P. gossypiella eggs; Nabis alternatus, Zelus renardii and spiders (primarily Misumenops celer) readily preyed on marked L. hesperus nymphs, and spiders, S. albofasciatus and N. alternatus readily preyed on T. ni larvae. Furthermore, the cage methods and the post‐mortem predator gut ELISAs revealed very few distinctive patterns of predation with regard to the light cycle the assemblage was exposed to.     

Attack, escape and predation rates of larvae of two aphidophagous ladybirds during conspecific and heterospecific interactions

The attack, escape and predation rates for larvae of aphidophagous ladybird Propylea dissecta (Mulsant) and Coccinella transversalis Fabricius were quantified as a potential mechanism leading to the differences in the incidence of cannibalism and intraguild predation. These rates were compared at four larval instars within and between the species. The attack rates of larvae of C. transversalis were significantly higher than those of P. dissecta towards conspecific and heterospecific victims. For both species, third instars exhibited maximum tendency to attack. Escape rates in C. transversalis were higher than P. dissecta. In P. dissecta, the second instars made a greater number of escapes than other conspecific instars after being attacked by same stage cannibal or heterospecific predator. In P. dissecta, first instars suffered maximum mortality due to cannibalism and intraguild predation by conspecifics and heterospecifics of the same and older developmental stage. No larvae of C. transversalis were eaten by P. dissecta of the same stage. These results suggest that the larvae of P. dissecta were more often potential cannibals than intraguild predators, while the reverse was the case in C. transversalis. Based on this finding, it could be predicted that in patchy prey habitats, high rates of larval cannibalism in P. dissecta would occur with a high risk of cannibalism of first instars. Larvae of C. transversalis would respond as intraguild predators, while those of P. dissecta as intraguild prey. The greater size and walking activity of C. transversalis could be possible reason for this tendency.     

Essential versus alternative foods of insect predators: benefits of a mixed diet

Although many predatory insects appear to be opportunistic generalists in their selection of prey, only a subset of prey species may in fact serve as “essential foods” capable of supporting immature growth and adult reproduction. It has been suggested that other, “alternative foods” serve only to maintain the predator when essential foods are not available, but little research has evaluated the significance of a mixed diet of essential and alternative foods for predator growth or reproduction. Here we test the general hypothesis that although alternative prey may be inadequate to support reproduction when consumed alone by adult predators, consumption of such prey may enhance the predator's reproductive output when the predator also has access to essential prey. We compared egg production by two aphidophagous lady beetles, Coccinella septempunctata and C. transversoguttata, provided with diets of aphids (essential prey) and weevils (alternative prey). As predicted, female predators produced greater numbers of eggs when a diet of pea aphids in limited number was supplemented by alfalfa weevil larvae. The predators laid no eggs when provided only with weevils or only with sugar. But once aphids were added to the diet, females of C. transversoguttata (but not C. septempunctata) laid eggs in greater numbers when they had fed previously on weevils than on sugar. Females of both species also produced eggs in modest numbers when provided with both weevils in excess and sugar, but this diet supported a lower rate of egg production than did a diet of weevils in excess plus a limited number of aphids. Although C. septempunctata has a longer history of association with the alfalfa weevil than does C. transversoguttata, the former species was not more effective in exploiting this alternative prey in support of reproduction. The tendency of generalist predators such as adult lady beetles to consume alternative as well as essential prey probably enhances considerably their ability to capitalize on short-lived and scattered opportunities as they seek out suitable sites in which to reproduce. Received: 11 May 1998 / Accepted: 1 May 1999     

Oviposition Behavior of Interacting Predatory Mites: Response to the Presence of Con- and Heterospecific Eggs

Oviposition behavior may be affected by the presence of potential future competitors, mates, or predators of offspring. We examined patch choice, oviposition site preference and egg production in the predaceous mites Phytoseiulus persimilis and Neoseiulus californicus (Acari: Phytoseiidae) when given a choice between paired spider mite patches with and without conspecific eggs, with and without heterospecific eggs, and with conspecific or heterospecific eggs. Neoseiulus californicus females had no patch preference and distributed their eggs randomly in all choice situations. This was also the case with P. persimilis females given a choice between patches with and without conspecific eggs and between patches with either con- or heterospecific eggs. Phytoseiulus persimilis females confronted with patches with and without heterospecific eggs preferentially stayed and oviposited in the predator free patches. We discuss the oviposition strategies of P. persimilis and N. californicus with respect to food competition, cannibalism and intraguild predation.