Breeding synchrony and predator specialization: A test of the predator swamping hypothesis in seabirds

Reproductive synchrony is a widespread phenomenon that is predicted to be adaptive for prey with specialist predators but not for those with generalist ones. I tested this prediction in three polar seabird species characterized by different levels of predator specialization. In the Antarctic petrel, for which the only predator was highly specialized, hatching dates were highly synchronous and chicks that hatched close to the mean hatching date had a higher survival. In black‐legged kittiwakes and Brünnich's guillemots, whose predators were generalists, breeding was less synchronous and there was no fitness advantage in hatching close to the mean. This study emphasizes the potential importance of the relative timing of reproduction for individual fitness and supports the hypothesis that the adaptive value of breeding synchrony depends on the predator functional response.     

Factors determining tadpole vulnerability to predators: can prior experience compensate for a suboptimal shape?

We investigated the role of constitutive morphology and previous experience in predator avoidance in two anuran species associated with different larval habitats. In Rana temporaria, deeper tails and larger body size conferred selective advantage against dragonfly predation. Previous experience with predators had a positive influence on the survival of R. temporaria tadpoles equivalent to predator selection. By contrast, survival in Bufo bufo seems unrelated to tail shape or experience. This suggests that B. bufo lacks constitutive morphological defenses against insect predators, and that morphological and behavioral defenses could result more effective than chemical deterrents for these insect predators. A key novelty of this study is the observation that Rana tadpoles having prior experience with predators have an enhanced success in further encounters, and this occurs before the morphological induced defense has been established. This induced modification for R. temporaria, and its lack of for B. bufo, may be an important determinant of larval survival.     

Spatial synchrony of recruitment in mountain-dwelling woodland caribou

Spatial synchrony in population dynamics is a ubiquitous feature across a range of taxa. Understanding factors influencing this synchrony may shed light on important drivers of population dynamics. Three mechanisms influence the degree of spatial synchrony between populations: dispersal, shared predators, and spatial environmental covariance (the Moran effect). We assessed demographic spatial synchrony in recruitment (calf:cow ratio) of 10 northern mountain caribou herds in the Yukon Territory, Canada (1982–2008). Shared predators and dispersal were ruled out as causal mechanisms of spatial recruitment synchrony in these herds and therefore any spatial synchrony should be due to the Moran effect. We also assessed the degree of spatial synchrony in April snow depth to represent environmental variability. The regional average spatial synchrony in detrended residuals of April snow depth was 0.46 (95% CI 0.37 to 0.55). Spatial synchrony in caribou recruitment was weak at 0.13 (95% CI −0.06 to 0.32). The spatial scale of synchrony in April snow depth and caribou recruitment was 330.2 km (95% CI 236.3 to 370.0 km) and 170.0 km (95% CI 69.5 to 282.8 km), respectively. We also investigated how the similarity in terrain features between herds influenced the degree of spatial synchrony using exponential decay models. Only the difference in elevation variability between herds during calving was supported by the data. Herds with more similar elevation variability may track snowmelt ablation patterns in a more similar fashion, which would subsequently result in more synchronized predation rates on calves and/or nutritional effects impacting juvenile survival. Interspecific interactions with predators and alternate prey may also influence spatial synchrony of recruitment in these herds.     

The effect of density and mutual interference by a Predator: A laboratory study of predation by the Nudibranch Coryphella rufibranchialis on the hydroid Tubularia larynx

The nudibranch Coryphella rufibranchialis (JOHNSTON) feeds on a variety of hydroids, including Tubularia larynx Ellis & Solander. Experiments in which density of prey and predators were altered showed that more prey were eaten as prey density increased. However, more prey were consumed at low predator densities, presumably because of mutual interference among nudibranchs at the higher predator densities. The number of prey consumed per nudibranch was maximal with low predator densities and a ratio of 25–50 polyps per predator. Coryphella seems to show an opportunistic feeding strategy involving solitary predators rapidly depleting hydroid colonies and moving on to new colonies.     

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.     

The significance of breeding synchrony in the wildebeest

Unlike most ungulates, whose offspring remain concealed for some time after birth, the offspring of the wildebeest and other alcelaphine antelopes (except harte-beest) accompany the mother from the very first hour. Most other ungulates that have follower-young either protect them effectively through a maternal or group defence (e.g. cattle, muskox, equids, and rhinoceroses), or are able to flee to sanc-tuaries (e.g. goats, sheep and goat-antelopes). In the Alcelaphini, primary reliance is placed on flight and on concealment among conspecifics; there is no effective defence against predators and no sanctuary. The spotted hyena is the primary predator on wildebeest calves. Over 80% of all wildebeest calves are born within an annual 2–3 week calving peak. During three different calving seasons (1 963-65) in Ngorongoro Crater, Tanzania, mortality in small, dispersed herds was found to be far higher than in large aggregations. The evidence suggested that this antelope's unusual reproductive strategy is adapted to the mobile-aggregated distribution pattern characteristic of migratory populations, rather than to the sedentary-dispersed pattern seen in resident populations. Further study on this specific question was undertaken during the 1973 calving season, with particular attention to reproductive success in small herds. Calf survival in small herds averaged just over 50% at the end of the 1973 birth peak, compared to 84% in aggregations. Daily sampling showed that calving peaked at different times in different parts of the Crater; this resulted in an extended season, although the peak in each population unit was of approximately equal duration. In some areas, calving was virtually finished before the peak began in the aggregations. Mortality ranged from very high among early calves to low in herds whose peak coincided with that of a nearby aggregation. The lack of synchrony between Ngorongoro subpopulations apparently reflects microclimatic differences, there being a pronounced rainfall and vegetation gradient within the caldera despite its small (265 km2) floor area. Low calf density combined with a lack of synchrony between small herds in different localities makes wildebeest living in the sedentary-dispersed pattern subject different localities makes wildebeest living in the sedentary-dispersed pattern subject to high calf mortality by the spotted hyena; in effect, resident subpopulations that calved ahead of the main population were providing a steady protein windfall for Ngorongoro's 470 hyenas. Tn the large aggregations, there were enough older calves to provide ‘cover’ for neonates during the first crucial day or two, making it much harder for hyenas to single out easily caught individuals. Therefore, lack of synchrony between aggregations did not apparently increase calf mortality to the same extent as it did in small herds. However, the highest survival rate was found among wilde-beest that left the Crater altogether and calved in an area with few hyenas. An evolutionary model of the wildebeest's reproductive system is presented, in An evolutionary model of the wildebeest's reproductive system is presented, which it is postulated that the system developed as part of the migratory habit of species. Given a feeding specialization for short, green grass and large, necessarily mobile aggregations, several synergistic selection pressures against concealment in favour of a follower strategy can be readily imagined. Breeding synchrony large aggregations are seen as the keys to the success of this strategy and an alternative to group defence, whereby predator populations are glutted without being sustained and the most vulnerable neonates are protected through the confusion effect created by many calves. Such a strategy is much less adaptive in a sedentary-dispersed population–which is the distribution pattern common to most antelopes-suggesting resident wildebeest populations represent a secondary adaptation to stable environments. Yet calf survival even in resident populations may equal the survival rates hidden young of sympatric antelopes, and be adaptive to the more fixed resources available in such situations.     

Is group living an antipredator defense in a facultatively communal webspinner (Embiidina: Clothodidae)?

I tested whether predator attack rate is a function of expanse of silk for colonies ofAntipaluria urichi, a facultatively communal webspinner. The avoidance effect hypothesis predicts that the probability of a predator detecting a group does not increase proportionately with an increase in expanse of silk, and therefore, larger groups are relatively less likely to be attacked. I counted the number of holes inflicted on silk over 3 weeks, an indirect measure of predation, for 47 colonies. Supporting the avoidance effect, holes per perimeter of silk accumulated at a rate lower than would be predicted by size alone. Further analysis using predation risk, computed as holes per individual per silk perimeter, revealed that risk was extremely variable for colonies with little silk (<200 cm) but, on average, did not vary as silk expanse increased. Overall, predators of webspinner colonies appear to be influenced by the avoidance effect, but whether the occupants of the silk are safer is unresolved.     

内蒙古典型草原区狭颅田鼠集群数量与被捕食风险的关系

2005年9月15日—30日,在内蒙古锡林郭勒盟西乌珠穆沁旗的典型草原上研究了狭颅田鼠(Microtus gregalis)冬季集群与来自艾虎(Mustela eversmanni)的捕食风险。采用鼠洞口数量作为狭颅田鼠集群大小的指标,分析了艾虎对不同大小狭颅田鼠集群的捕食优先选择偏好。对实验样地内狭颅田鼠的洞群洞口数进行了调查统计(样地内总共涉及102个狭颅田鼠有效洞群),并计数此期间的艾虎粪便堆数以及攻击掘开狭颅田鼠洞群的情况。运用非参数的Mann-Whitney U检验法进行数据分析,结果表明:从艾虎遗留的粪便痕迹来看,没有被艾虎访问过的田鼠洞群与被艾虎访问过的洞群之间差异达到极显著水平;另外,从狭颅田鼠洞口被艾虎掘开的痕迹来分析,没有被艾虎挖掘的洞群与被艾虎挖掘的洞群之间的差异也达到极显著的水平;另外,随着田鼠洞群洞口数量的增加,出现艾虎粪便和掘洞的频次和概率就越高。表明艾虎对狭颅田鼠集群洞口数的选择差异性非常显著,明显倾向于选择在洞口数量多的狭颅田鼠洞群停留,同时也更倾向于掘开洞口数量较高的狭颅田鼠洞群作为捕食对象。     

Role of aphid predator guild in controlling spirea aphid populations on apple in West Virginia,USA

Spirea aphid populations and their predators were studied on apple to identify predators of importance in controlling aphid populations. Methods included random and non-random sampling from apple orchards in West Virginia, USA, sentinel aphid colonies, laboratory feeding studies, and predator exclusion studies. Aphidoletes aphidimyza (Diptera: Cecidomyiidae), chrysopids (Neuroptera: Chrysopidae), Harmonia axyridis (Coleoptera: Coccinellidae), and Orius insidiosus (Hemiptera: Anthocoridae) were the most abundant predators associated with spirea aphid colonies on apple. Parasitoids were all but absent in the study. Abundance of all predators was density dependent with greater responses to aphid populations at the orchard scale than to tree or individual colony scales. A. aphidimyza, O. insidiosus, chrysopids, and syrphids (Diptera) had the greatest degree of density dependence on aphid populations, and spiders showed inverse density dependence. Exclusion of predators with both cages and insecticides produced significantly higher aphid populations. Because of high abundance, good synchrony with aphid populations, and high impact per individual, H. axyridis adults were the most important spirea aphid predator on apple.     

Does Formation of Multicellular Colonies by Choanoflagellates Affect Their Susceptibility to Capture by Passive Protozoan Predators?

Microbial eukaryotes, critical links in aquatic food webs, are unicellular, but some, such as choanoflagellates, form multicellular colonies. Are there consequences to predator avoidance of being unicellular vs. forming larger colonies? Choanoflagellates share a common ancestor with animals and are used as model organisms to study the evolution of multicellularity. Escape in size from protozoan predators is suggested as a selective factor favoring evolution of multicellularity. Heterotrophic protozoans are categorized as suspension feeders, motile raptors, or passive predators that eat swimming prey which bump into them. We focused on passive predation and measured the mechanisms responsible for the susceptibility of unicellular vs. multicellular choanoflagellates, Salpingoeca helianthica, to capture by passive heliozoan predators, Actinosphaerium nucleofilum, which trap prey on axopodia radiating from the cell body. Microvideography showed that unicellular and colonial choanoflagellates entered the predator's capture zone at similar frequencies, but a greater proportion of colonies contacted axopodia. However, more colonies than single cells were lost during transport by axopodia to the cell body. Thus, feeding efficiency (proportion of prey entering the capture zone that were engulfed in phagosomes) was the same for unicellular and multicellular prey, suggesting that colony formation is not an effective defense against such passive predators.     

Mast-seeding in the cycad genus Encephalartos: a test of the predator satiation hypothesis

Mast-seeding behaviour was monitored in 18 populations of eight species of the African cycad genus Encephalartos between 1988 and 1991. The coefficient of variation (V) in annual cone production for each population ranged between 88 and 200, indicating large fluctuations in reproductive effort between years. Data were collected to determine whether mast-seeding reduced levels of predispersal seed predation by satiating seed predators in mast years and whether it resulted in a reproductive advantage over plants which reproduced more frequently. Masting intensity was greatest in those populations in which individual plants suffered the highest levels of predispersal seed predation in years when only a few plants produced seeds. The principal seed predators were two congeneric weevil species, Antliarhinus zamiae and A. signatus, which develop exclusively on cycad seeds. The lowest intensity of mast-seeding was recorded for cycad populations with low levels of seed predation and in which A. zamiae and A. signatus occurred only in low numbers or were entirely absent. Larger seed crops appeared to result in lower levels of seed predation by A. zamiae and A. signatus in four populations of E. altensteinii, and differences in seed crop size accounted for 48–66% of variation in levels of seed predation in populations of five cycad species. In one population of E. altensteinii, lower levels of seed predation in plants reproducing periodically resulted in a reproductive advantage over plants reproducing more frequently. These results are consistent with the predator satiation hypothesis. However, in most cycad populations, numbers of seed predators did not appear to decrease significantly after a period of 2–8 years between reproductive episodes and, in two of three populations examined, periodic reproduction did not increase the number of seeds surviving to dispersal over a 4-year period. These results are interpreted to mean that periodic reproduction has not evolved in response to selection imposed by seed predators, but that selection may favour those plants which experience lower levels of seed predation by coning in synchrony with the majority of plants in the population.     

Can redistribution of breeding colonies on a landscape mitigate changing predation danger?

The reproductive success of colonially breeding species depends in part upon a trade‐off between the benefit of a dilution effect against nestling predation within larger colonies and colony conspicuousness. However, there may be no net survivorship benefit of dilution if smaller colonies are sufficiently inconspicuous. This raises the question about how the size distribution of breeding colonies on a landscape might change as the predation danger for nestlings changes. In southwest British Columbia, Canada, bald eagle Haliaeetus leucocephalus populations have increased exponentially at ～5% per year in recent decades and prey upon nestlings of colonial breeding great blue herons Ardea herodias faninni. Motivated by field data on reproductive success in relation to colony size, modeling is used to ask under which circumstances trading off a dilution benefit against colony conspicuousness can improve population reproductive success. That is, which colonial nesting distribution, dispersed and cryptic versus clumped and conspicuous, best mitigates predation danger on nestlings? When predators are territorial, the modeling predicts a dispersed nesting strategy as attack rate increases, but not as predator numbers increase. When predators are non‐territorial, the modeling predicts a dispersed nesting strategy as predator numbers and/or attack rates increase. When predators are both territorial and non‐territorial, colonial nesting within a predator's territory improves reproductive success when attack rates are low. This suggests nesting in association with territorial predators may offer decreased levels of predation when compared with nesting amongst non‐territorial predators. Thus a change in the colony size distribution of colonially breeding species might be anticipated on a landscape experiencing a change in predation danger.     

Behavioural and life history effects of predator diet cues during ontogeny in damselfly larvae

A central issue in predator–prey interactions is how predator associated chemical cues affect the behaviour and life history of prey. In this study, we investigated how growth and behaviour during ontogeny of a damselfly larva (Coenagrion hastulatum) in high and low food environments was affected by the diet of a predator (Aeshna juncea). We reared larvae in three different predator treatments; no predator, predator feeding on conspecifics and predator feeding on heterospecifics. We found that, independent of food availability, larvae displayed the strongest anti-predator behaviours where predators consumed prey conspecifics. Interestingly, the effect of predator diet on prey activity was only present early in ontogeny, whereas late in ontogeny no difference in prey activity between treatments could be found. In contrast, the significant effect of predator diet on prey spatial distribution was unaffected by time. Larval size was affected by both food availability and predator diet. Larvae reared in the high food treatment grew larger than larvae in the low food treatment. Mean larval size was smallest in the treatment where predators consumed prey conspecifics, intermediate where predators consumed heterospecifics and largest in the treatment without predators. The difference in mean larval size between treatments is probably an effect of reduced larval feeding, due to behavioural responses to chemical cues associated with predator diet. Our study suggests that anti-predator responses can be specific for certain stages in ontogeny. This finding shows the importance of considering where in its ontogeny a study organism is before results are interpreted and generalisations are made. Furthermore, this finding accentuates the importance of long-term studies and may have implications for how results generated by short-term studies can be used.     

Putting prey back together again: integrating predator-induced behavior,morphology, and life history

The last decade has seen an explosion in the number of studies exploring predator-induced plasticity. Recently, there has been a call for more comprehensive approaches that can identify functional relationships between traits, constraints on phenotypic responses, and the cost and benefits of alternative phenotypes. In this study, we exposed Helisoma trivolvis, a freshwater snail, to a factorial combination of three resource levels and five predator environments (no predator, one or two water bugs, and one or two crayfish) and examined ten traits including behavior, morphology, and life history. Each predator induced a unique suite of behavioral and morphological responses. Snails increased near-surface habitat use with crayfish but not with water bugs. Further, crayfish induced narrow and high shells whereas water bugs induced wide shells and wide apertures. In terms of life history, both predators induced delayed reproduction and greater mass at reproduction. However, crayfish induced a greater delay in reproduction that resulted in reduced fecundity whereas water bugs did not induce differences in fecundity. Resource levels impacted the morphology of H. trivolvis; snails reared with greater resource levels produced higher shells, narrower shells, and wider apertures. Resource levels also impacted snail life history; lower resources caused longer times to reproduction and reduced fecundity. Based on an analysis of phenotypic correlations, the morphological responses to each predator most likely represent phenotypic trade-offs. Snails could either produce invasion-resistant shells for defense against water bugs or crush-resistant shells for defense against crayfish, but not both. Our use of a comprehensive approach to examine the responses of H. trivolvis has provided important information regarding the complexity of phenotypic responses to different environments, the patterns of phenotypic integration across environments, and the potential costs and benefits associated with plastic traits.     

Do nomadic avian predators synchronize population fluctuations of small mammals? a field experiment

Three-to-five-year population oscillations of northern small rodents are usually synchronous over hundreds of square kilometers. This regional synchrony could be due to similarity in climatic factors, or due to nomadic predators reducing the patches of high prey density close to the average density of a larger area. We estimated avian predator and small rodent densities in 4–5 predator reduction and 4–5 control areas (c. 3 km² each) during 1989–1992 in western Finland. We studied whether nomadic avian predators concentrate at high prey density areas, and whether this decreases spatial variation in prey density. The yearly mean number of avian predator breeding territories was 0.2–1.0 in reduction areas and 3.0–8.2 in control areas. Hunting birds of prey concentrated in high prey density areas after their breeding season (August), but not necessarily during the breeding season (April to June), when they were constrained to hunt in vicinity of the nest. The experimental reduction of breeding avian predators increased variation in prey density among areas but not within areas. The difference in variation between raptor reduction and control areas was largest in the late breeding season of birds of prey, and decreased rapidly after the breeding season. These results appeared to support the hypothesis that the geographic synchrony of population cycles in small mammals may be driven by nomadic predators concentrating in high prey density areas. Predation and climatic factors apparently are complementary, rather than exclusive, factors in contributing to the synchrony.     

Spatial variability in prey phenology determines predator movement patterns and prey survival

Species have phenological variation among local habitats that are located at relatively small spatial scales. However, less studies have tested how this spatial variability in phenology can mediate intra-/inter-specific interactions. When predators track phenological variation of prey among local habitats, survival of prey within a local habitat strongly influenced by phenological synchrony with their conspecifics in adjacent habitats. Theory predicts that phenological synchrony among local habitats increases prey survival in local habitat within spatially structured environments because the predators have to make a habitat choice for foraging. Consequently, total survival of prey at regional scale should be higher. By using a spatially explicit field experiment, we tested above hypothesis using a prey–predator interaction between tadpole (Rhacophorus arboreus) and newt (Cynops pyrrhogaster). We established enclosures (≈regional scale) consisting of two tanks (≈local habitat scale) with different degree of prey phenological synchrony. We found that phenological synchrony of prey between tanks within each enclosure decreased the mean residence time of the predator in each tank, which resulted in higher survival of prey at a local habitat scale, supporting the theoretical prediction. Furthermore, individual-level variation in predator residence time explained the between-tank variation in prey survival in enclosures with phenological synchrony, implying that movement patterns of the predator can mediate variation in local population dynamics of their prey. However, total survival at each enclosure was not higher under phenological synchrony. These results suggest the importance of relative timing of prey phenology, not absolute timing, among local habitats in determining prey–predator interactions.     

Predators and mutualists influence the exclusion of aphid species from natural communities

We investigated why two species of aphids (Aphis jacobaeae and Brachycaudus cardui) were very rare in a study site despite their abundance in the surrounding area. The study site contained many common species of aphid and we tested the hypothesis that the community of aphid predators in the field excluded the missing species. Colonies of the two aphid species were artificially initiated in the experimental site and allocated to one of four treatments: control; ground predator exclusion; total predator exclusion, and provision of mutualist ants. Two measures of colony performance were analysed: longevity and cumulative aphid density. Colonies decline naturally in late summer but control colonies disappeared very quickly while colonies protected from all predators survived the longest. The performance of colonies protected from just ground predators was intermediate. We failed to persuade ants to tend A. jacobaeae. Colonies of B. cardui attended by ants performed better than controls and those with ground predators excluded, but not as well as those with all predators excluded. We conclude that the absence of the two species of aphid in the study site is influenced by the resident predator community, and by the availability of mutualists. Received: 27 April 1998 / Accepted: 30 November 1998     

Effects of vegetation on nest microclimate and breeding performance of lesser black-backed gulls (Larus fuscus)

For birds that breed in large colonies, the overall area occupied by the colony generally comprises several sub-areas that differ in physical and social features such as vegetation and breeding density. Birds arriving at a breeding colony select their nesting sites through a hierarchical process of selecting a sub-area, then a particular nest site with appropriate biotic and physical attributes. Optimal vegetation cover is one such important attribute. Many ground nesting gulls preferentially select nest sites that provide shelter during reproduction, but this presumably has to be balanced against any costs such as reduced visibility of potential predators. The effects of vegetation height in the sub-areas within a colony, and of the amount of vegetation in the immediate vicinity of the nest on nest microclimate were investigated in lesser black-backed gulls Larus fuscus in a colony in which overall vegetation height differed in different sub-areas and was patchily distributed within these areas. Tall vegetation did have a sheltering effect, and this was positively related with chick growth. However, this vegetation area was associated with lower breeding densities, relatively late laying birds and lower chick survival rate, suggesting that sub-areas with tall vegetation held more lower-quality or young breeders. Within the sub-areas, the birds preferentially selected nest sites with more surrounding vegetation, and this was positively correlated with their hatching success.     

Competitive interactions among two specialist predators and a generalist predator of hemlock woolly adelgid, Adelges tsugae (Hemiptera: Adelgidae) in south-western Virginia

1 Competitive interactions among two specialist predators, Laricobius nigrinus and Sasajiscymnus (Pseudoscymnus) tsugae, and a generalist predator, Harmonia axyridis Pallas, of hemlock woolly adelgid, Adelges tsugae were evaluated in hemlock stands in south‐western Virginia. The two specialist predators are part of a biological control program for A. tsugae, and the potential for competition among these species and previously established generalist predators in the field is unknown. 2 Adult predators were evaluated in branch cages during spring and summer at two field sites infested with A. tsugae. Using females only in 2003 and sexual pairs in 2004, predator survival and net reproduction were examined, as well as their feeding and impact on A. tsugae when present alone and in conspecific and heterospecific groupings. 3 Predator survival was not affected by the presence of additional predators. Total feeding was greater for all species when placed in predator groupings, suggesting that interactions do not significantly interfere with feeding activity. Net reproduction per predator was negatively affected by conspecifics, but unaffected by heterospecifics, indicating that direct or indirect intraspecific interference may occur. In spring, L. nigrinus showed the greatest impact on A. tsugae, and H. axyridis had the greatest impact during summer. 4 These results suggest that it would be beneficial to utilize multiple predator species combinations over single species when implementing biological control for A. tsugae. Low‐density releases are also recommended to reduce intraspecific interference.     

Intraspecific Variation in Chemical Attraction of Rice to Insect Predators

The olfactory response of predators of the brown planthopper,Nilaparvata lugensStål, to different genotypes of rice (14 cultivars and breeding lines ofOryza sativaL. and 1 wild species,Oryza nivaraSharma et Shastry) was measured in an airflow olfactometer. Odor from rice plants attracted more females of the mirid predatorCyrtorhinus lividipennisReuter than plain air (control) on only 6 of the 15 rice genotypes. Orientation ofC. lividipennistoward volatiles of certain rice genotypes was apparent even when the plants were free of the brown planthopper. However, the predator distinguished between prey-infested and uninfested plants and preferred plants with eggs over plants with nymphs. The predator did not distinguish different stages of plant growth (vegetative, booting, or flowering). Plants artificially injured to simulate brown planthopper oviposition wounds were not as attractive to the predator as plants on which the planthopper had oviposited. The preassay preconditioning on the cultivar TN1 did not produce a predator bias for this genotype. This suggests that rearing effects or chemically mediated associative learning reported for some natural enemies did not influenceC. lividipennis'host response. Results with another predator, the coccinellidMicraspis hirashimaiSasaji, produced less consistent behavior. Planthopper-infested plants attracted more females ofM. hirashimaithan unifested plants in only 1 of the 12 rice genotypes evaluated. Implications for augmenting predators by rice cultivar selection and modification are discussed.