Predation of codling moth Cydia pomonella L. by the Silvereye Zosterops lateralis (Latham)

Predation of cocooned larvae of codling moth Cydia pomonella by silvereyes Zosterops lateralis was studied in an apple orchard in Nelson, New Zealand. Apple logs with known larval densities were made available to the birds for known periods of time, either in cages or exposed in the apple orchard. The numbers of silvereyes and the natural predation of codling moth were recorded in the same orchard. Predation was density dependent. On caged logs with an initial high density of 32 larvae, 1.1 larvae were consumed per bird‐hour; in contrast, one larva was consumed per 34.5 bird‐hours at three larvae per log. A curvilinear relationship was demonstrated between larval density and the bird‐hours required for predation; this relationship was consistent with the known density dependence of silvereye predation of codling moth. A regression of the total annual winter bird predation of larvae in the orchard on bird numbers was significant. However, the density dependence of predation resulted in declining rates of predation over the winter as larval density declined; the first birds to arrive in the orchard benefitted from particularly high predation rates. As a consequence, fluctuations in bird numbers during the winter had only a secondary influence on predation rates. The numbers of silvereyes in the orchard showed no relationship to the density of the codling moth population present. This study confirmed the importance of silvereyes in the predation of codling moth and a functional, not numerical, rseponse of these birds to codling moth density.     

Local outbreaks of Operophtera brumata and Operophtera fagata cannot be explained by low vulnerability to pupal predation

• 1 One of the unresolved questions in studies on population dynamics of forest Lepidoptera is why some populations at times reach outbreak densities, whereas others never do. Resolving this question is especially challenging if populations of the same species in different areas or of closely‐related species in the same area are considered.
• 2 The present study focused on three closely‐related geometrid moth species, autumnal Epirrita autumnata, winter Operophtera brumata and northern winter moths Operophtera fagata, in southern Finland. There, winter and northern winter moth populations can reach outbreak densities, whereas autumnal moth densities stay relatively low.
• 3 We tested the hypothesis that a lower vulnerability to pupal predation may explain the observed differences in population dynamics. The results obtained do not support this hypothesis because pupal predation probabilities were not significantly different between the two genera within or without the Operophtera outbreak area or in years with or without a current Operophtera outbreak.
• 4 Overall, pupal predation was even higher in winter and northern winter moths than in autumnal moths. Differences in larval predation and parasitism, as well as in the reproductive capacities of the species, might be other candidates.

     

Can variation in risk of nest predation explain altitudinal migration in tropical birds?

Migration is among the best studied of animal behaviors, yet few empirical studies have tested hypotheses explaining the ultimate causes of these cyclical annual movements. Fretwell’s (1980) hypothesis predicts that if nest predation explains why many tropical birds migrate uphill to breed, then predation risk must be negatively associated with elevation. Data from 385 artificial nests spanning 2,740 m of elevation on the Atlantic slope of Costa Rica show an overall decline in predation with increasing elevation. However, nest predation risk was highest at intermediate elevations (500–650 m), not at lowest elevations. The proportion of nests depredated by different types of predators differed among elevations. These results imply that over half of the altitudinal migrant bird species in this region migrate to safer breeding areas than their non-breeding areas, suggesting that variation in nest predation risk could be an important benefit of uphill migrations of many species. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reduced parasitism of a leaf-mining moth on trees with high infection frequencies of an endophytic fungus

We investigated relationships between endophytic fungi and a leaf-mining moth, Phyllonorycter sp., along an elevational gradient from 2255 to 2895 m. The fungi and moth larvae inhabit leaves of Quercus gambelii. Fungal frequencies and larval densities varied with elevation. However, larval densities were not associated with the frequencies of infection by endophytic fungi. Survival of larvae was positively associated with the most dominant fungus, Gnomonia cerastis, owing to reduced parasitism of moth larvae on trees with high frequencies of Gnomonia.     

Delayed density-dependent parasitism of eggs and pupae as a contributor to the cyclic population dynamics of the autumnal moth

Many populations of forest Lepidoptera exhibit 10-year cycles in densities, with impressive outbreaks across large regions. Delayed density-dependent interactions with natural enemies are recognized as key factors driving these cyclic population dynamics, but emphasis has typically been on the larval stages. Eggs, pupae and adults also suffer mortality from predators, parasitoids and pathogens, but little is known about possible density relationships between mortality factors and these non-feeding life stages. In a long-term field study, we experimentally deployed autumnal moth (Epirrita autumnata) eggs and pupae to their natural enemies yearly throughout the 10-year population cycle in northern Norway. The abundance of another geometrid, the winter moth (Operophtera brumata), increased in the study area, permitting comparisons between the two moth species in predation and parasitism. Survival of autumnal moth eggs and pupae was related to the moth abundance in an inverse and delayed manner. Egg and pupal parasitoids dominated as density-dependent mortality factors and predicted the subsequent growth rate of the host population size. In contrast, effects of egg and pupal predators were weakly density dependent, and generally predation remained low. Parasitism rates did not differ between the autumnal and winter moth pupae, whereas predators preferred winter moth pupae over those of the autumnal moth. We conclude that parasitism of the autumnal moth by egg and pupal parasitoids can be related to the changes of the moth density in a delayed density-dependent manner. Furthermore, egg and pupal parasitoids cannot be overlooked as causal factors for the population cycles of forest Lepidoptera in general.     

The effect of elevation and habitat cover on nest box occupancy and diet composition of Boreal Owls Aegolius funereus

Capsule: Diet composition of Boreal Owls Aegolius funereus was not affected by habitat cover, but it changed along the elevational gradient.

Aims: To assess the effect of elevation and habitat cover on nest box occupancy and diet composition of a central European population of Boreal Owls.

Methods: A Boreal Owl population was studied in the ?umava Mountains, Czech Republic, at elevations from 500 to 1300?m above sea level (asl), during 1984–2005.

Results: Boreal Owls occupied more frequently nest boxes above 600?m asl, but they did not clearly prefer any elevational band. Habitat cover did not affect the number of nesting attempts. There was also no relationship between habitat cover and diet composition. However, diet composition significantly changed along the elevational gradient. In particular, the proportion of alternative prey of Boreal Owls, i.e. birds and shrews Sorex sp., rose with increasing elevation. The proportion of voles Myodes and mice Apodemus in the diet decreased with increasing elevation. Among bird prey, the proportion of finches Fringillidae positively correlated with elevation.

Conclusions: Central European Boreal Owls did not show a clear preference for any habitat cover or elevational band, but the quality of the owls’ diet significantly decreased with increasing elevation.     

Experimental simulations of climate change induced mismatch in oak and larval development rates impact indicators of fitness in a declining woodland moth

Consequences of climate change-driven shifts in the relative timing of spring activities of interacting species are insufficiently understood, especially for insects. We use a controlled experiment which simulates a trophic mismatch scenario in which lepidopteran larvae predominately feed on older leaves due to foliage developing faster than larvae growth rates. As a case study our experiment uses Orthosia cerasi, which is a widespread but declining woodland moth whose UK declines appear to be driven by warming temperatures. In the control experiment larvae are fed young oak Quercus robur leaves (bud burst stages six and seven), whilst in the treatment newly emerged larvae are fed young leaves but then gradually transition to feed on older leaves (post bud burst stage seven). We assess impacts on duration of the larval stage, pupal size and overwintering duration and survival. Larvae in the phenological mismatch treatment had a longer larval period, and smaller and lighter pupae. Larval diet did not carry over to influence emergence dates as earlier pupation of control larvae was balanced by an equivalent increase in the duration of the pupal stage. Increased time spent as larvae could increase predation rates from avian predators, whilst slowing the seasonal decline in food availability for those bird species. Reduced pupal size and weight are indicators of lower fecundity in emerging adults. Notably, we find that adults emerging from the mismatch treatment exhibited greater rates of abnormal vestigial wing development, which is likely to further reduce fitness. Trophic mismatches in which caterpillars have reduced availability of young leaves may thus contribute to the population declines observed in many woodland moth species due to increased mortality at larval stages, and adverse effects of early life conditions that reduce the reproductive success of emerging adults.     

Plastic collective endothermy in a complex animal society (army ant bivouacs: Eciton burchellii parvispinum)

Endothermic animals do not always have a single adaptive internal temperature; some species exhibit plastic homeostasis, adaptively allowing body temperature to drop when thermoregulatory costs are high. Like large‐bodied endotherms, some animal societies exhibit collective thermal homeostasis. We tested for plasticity of thermoregulation in the self‐assembled temporary nests (bivouacs) of army ants. We measured core bivouac temperatures under a range of environmental conditions and at different colony developmental (larval vs pupal brood) stages. Contrary to previous assertions, bivouacs were not perfect thermoregulators in all developmental stages. Instead, bivouacs functioned as superorganismal facultative endotherms, using a combination of site choice and context‐dependent metabolic heating to adjust core temperatures across an elevational cline in ambient temperature. When ambient temperature was low, the magnitude of metabolic heating was dependent on colony developmental stage: pupal bivouacs were warmer than larval bivouacs. At cooler high elevations, bivouacs functioned like some endothermic animals that intermittently lower their body temperatures to conserve energy. Bivouacs potentially conserved energy by investing less metabolic heating in larval brood because the high costs of impaired worker development may require more stringent thermoregulation of pupae. Our data also suggest that site choice played an important role in bivouac cooling under high ambient temperatures at low elevations. Climate warming may expand upper elevational range limits of Eciton burchellii parvispinum, while reducing the availability of cool and moist bivouac sites at lower elevations, potentially leading to future low‐elevation range contraction.     

Biology ofHyphantria cunea Drury (Lepidoptera: arctiidae) in Japan. V. Preliminary life tables and mortality data in urban areas

Studies on the population dynamics of the fall webworm, Hyphantria cunea have been carried out at three survey stations and along selected roads in the urban area of Tokyo since 1966. Twelve survivorship curves obtained during two years and 8 life tables show that the mortality rate in early developmental stages of the fall webworm is remarkably low as compared with that of other lepidopterous defoliaters and the mortality rate in later developmental stages is compensatory high. The low mortality rate in early stages is considered to be due to the protective role of the nest-web and the lack of egg and larval parasites. All but one parasitic species emerge from prepupae and pupae. Spiders living in the nest-web of the fall webworm play an important role in reducing the number of young larvae. Direct observations and caging experiments showed that relatively high mortality during later larval stages is mainly due to predation by birds (in the first generation) and wasps (in the second generation). The generation mortality in the survey stations always exceeded the level where the population is kept at the steady state, and the outbreak of this moth is considered to be continued by the immigration of adults from large trees growing in gardens on which the larvae can escape from predation pressure.     

Spatial heterogeneity in the relative impacts of foliar quality and predation pressure on red oak, Quercus rubra, arthropod communities

Predation pressure and resource availability often interact in structuring herbivore communities, with their relative influence varying in space and time. The operation of multiple ecological pressures and guild-specific herbivore responses may combine to override simple predictions of how the roles of plant quality and predation pressure vary in space. For 2 years at the Coweeta LTER in the Southern Appalachian Mountains, we conducted a bird exclosure experiment on red oak (Quercus rubra) saplings to investigate the effects of bird predation on red oak arthropod communities. We established bird exclosures at six sites along an elevational gradient and estimated variation in foliar nitrogen and bird predation pressure along this gradient. Foliar nitrogen concentrations increased with elevation while our index of bird predation pressure was variable across sites. Greater arthropod densities were detected inside exclosures; however, this result was mainly driven by the response of phloem feeders which were much more prevalent inside exclosures than on control trees. There was little evidence for an effect of bird predation on the other arthropod guilds. Consequently, there was no evidence of a trophic cascade either in terms of leaf damage or tree growth. Finally, we found more variation in arthropod density among trees within sites than variation in arthropod density among sites, indicating the importance of micro-site variation in structuring arthropod communities.     

Predator protection versus rapid growth in a montane leaf beetle

Summary Adults and larvae of Chrysomela aenicollis (Coleoptera: Chrysomelidae) feed on foliage of Salix species (Salicaceae) between 2,400–3,400 m above sea level in the eastcentral Sierra Nevada mountains of California. We predicted that (1) cold climatic conditions would be a more frequent source of mortality at higher elevations, (2) mildweather agents of mortality such as predation should be more severe at lower elevations, and (3) populations of C. aenicollis would be adapted to the local selective regime at each elevation. We tested these predictions in 1984 and 1985 by transferring over 6,000 eggs and larvae within and between two sites at 2,810 and 3,240 m elevation above sea level. During mild summer weather at both sites, survivorship on Salix branches isolated by a barrier of sticky resin was similar to that on control branches, and we concluded that aerial predators were the primary cause of mortality. At least one major predator, a solitary wasp (Symmorphus sp., Hymenoptera: Eumenidae), was specifically associated with C. aenicollis at the lower site, where beetle mortality was highest. At both sites in 1984 and 1985, larvae originating from the lower site remained in aggregations and survived more frequently than larvae from the upper site, suggesting that they are better defended against predators. During a storm with cold weather late in the 1984 season, larvae and pupae died more frequently at the upper site, and there was a marginally significant trend (P<0.1) for the lower site individuals to die more frequently than upper site larvae during the cold storm. Upper site larvae grew approximately 10% faster than lower site larvae at the lower site and under controlled conditions in the laboratory. These findings indicate that upper and lower site populations were adapted to the local selective regime, which suggest how populations of montane phytopagous insects may adapt to changing elevations.     

Elevation has contrasting effects on avian and mammalian nest traits in the Andean temperate mountains

Nest building is a widespread breeding strategy across taxa. Nest composition and structure can play a critical role in the breeding success and/or adult survival of nest‐building vertebrates. Although nest traits are expected to vary adaptively across elevational gradients, few studies address this relationship. We studied the variation in nest traits (composition and structure) across elevation for two taxa with two different functions in the Andean temperate forests of southern Chile: a bird (Aphrastura spinicauda, Furnariidae, 170 breeding nests) and a marsupial mammal (Dromiciops gliroides, Microbiotheriidae, 91 winter torpor nests). For A. spinicauda, we further assessed how nest traits influenced clutch size and hatching success. Both species used fewer types of nest materials (items) with increasing elevation, and a greater proportion of leaves were used in highland compared to lowland forests. Aphrastura spinicauda used feathers and hair, and D. gliroides used bryophytes more frequently in lowland forests. The mass and volume of nests decreased with increasing elevation for A. spinicauda and increased for D. gliroides. Nest traits had subsequent fitness consequences for A. spinicauda, such that: (i) greater cup volume and depth were associated with larger clutch sizes, (ii) more items used during nest building were linked to improved clutch size at high elevation only, and (iii) nest wall thickness was negatively associated with hatching success. Thus, in temperate mountain ecosystems, elevation may be an important factor influencing nest‐building behaviour for cavity‐using vertebrates. However, the direction of the elevational effects may vary among taxa and nest functions in these ecosystems.     

Relationship between Survival Rate of Avian Artificial Nests and Forest Vegetation Structure along a Tropical Altitudinal Gradient on Mount Cameroon

Nest predation is assumed to be an important factor driving avian life histories. Altitudinal gradients offer valuable study systems to investigate how avian nest predation risk varies between bird populations. In this study, a hypothesis postulating an increase in avian nest survival rate with elevation as a result of decreasing predation pressure was experimentally tested along an altitudinal gradient (300‐2250 m) in West‐Central Africa. Three types of artificial nests (cup‐shrub, cup‐ground and bare‐ground) were used along the altitudinal gradient. Overall, elevation had no effect on the daily survival rate (DSR) of the artificial nests. However, there was a significant elevation‐nest type interaction. Daily survival rate for cup‐shrub nests decreased significantly with elevation, but for cup‐ground and bare‐ground nests, elevation had no significant effect. We tested the effects of the same vegetation parameters (tree density, herb and shrub layer coverage, and canopy openness) on the DSR of different nest types to understand how different vegetation layers or combinations of them affect DSR. Daily survival rate for bare‐ground nests significantly decreased with increasing canopy openness, and was positively influenced by coverage of herb layer and tree density. For cup‐shrub nests, DSR increased significantly with increasing shrub layer coverage. Finally, for cup‐ground nests, we found a positive effect of shrub coverage and canopy openness on DSR. In summary, we found that different forest vegetation layers affect predation risk of different nest types along elevations on Mt. Cameroon.     

Testing the enemies hypothesis in forest stands: the important role of tree species composition

Numerous studies conducted in agro-ecosystems support the enemies hypothesis, which states that predators and parasites are more efficient in controlling pest densities in polycultures than in monocultures. Few similar studies, however, have been conducted in forest ecosystems, and we do not yet have evidence as to whether the enemies hypothesis holds true in forests. In a 2-year study, we investigated whether the survival of autumnal moth (Epirrita autumnata) larvae and pupae differs between silver birch monocultures and two-species mixtures of birch with black alder, Norway spruce and Scots pine. We placed young larvae on birch saplings and monitored their survival until the end of the larval period, when we checked whether they had been parasitized. After the larvae had pupated, pupal survival was tested in a field trial. In 2002, the larvae disappeared earlier and their overall survival was lower in birch–pine mixtures than in other stand types. In 2003, survival probability was lowest in birch–pine stands only during the first week and there were no differences between stands in overall survival. Larval parasitism was not affected by tree species composition. Pupal weight and pupal survival were likewise not affected by stand type. Among the predators, wood ants were more abundant on birches growing in birch–pine mixtures than in other stand types probably because colonies of myrmecophilic aphids were common on pines. In contrast, spider numbers did not differ between stand types. Ant exclusion by means of a glue ring around the birch trunk increased larval survival, indicating that ants are important predators of the autumnal moth larvae; differences in larval survival between stands are probably due to differential ant predation. Our results provide only partial support for the enemies hypothesis, and suggest that it is both tree species composition and species diversity which affect herbivore survival and predation.     

Salt marsh elevation is a strong determinant of nest‐site selection by Clapper Rails in Georgia,USA

As sea levels rise, birds nesting in coastal marshes will be particularly vulnerable to increased tidal inundation. Understanding how marsh birds select their nesting habitat along the elevational gradient of these marshes will provide insight into how these species might be affected by rising sea levels. Clapper Rails (Rallus crepitans) are coastal marsh‐nesting birds whose nests are vulnerable to flooding, but it is not clear if they select for habitat along the elevational gradient or only use other habitat cues. Our objective was to determine if Clapper Rails select higher‐elevation nest sites, while also controlling for selection of other habitat variables at both landscape and territory levels, by comparing nest habitat to habitat in other areas of territories and at random points in the marsh landscape. At the landscape level, Clapper Rails did not exhibit selection for the elevational gradient, with nests and random points at similar elevations. At the territory level, however, nest‐site selection was most influenced by elevation and plant height, with Clapper Rails selecting nest sites with higher elevations and in areas with taller plants. However, the strength of the elevation effect was uncertain, suggesting the importance of precise elevation measurements in the field. Given this selection for higher‐elevation nest sites, Clapper Rails may be somewhat resilient to increased tidal inundation. However, the potential for increased intra‐ and interspecific competition for high‐elevation marshes should make conservation of these habitats a priority.     

Evidence that predator satiation may restrict the spatial spread of a tussock moth (Orgyia vetusta) outbreak

Western tussock moths (Orgyia vetusta Bdv., Lymantriidae) infest one stand of bush lupine (Lupinus arboreus Sims, Fabaceae) heavily and several other stands very lightly at the Bodega Marine Reserve (Sonoma Co., Calif., USA). We found that the disappearance rates of experimentally placed larvae and pupae were consistently lower in the outbreak area than in non-outbreak areas. For pupae but not larvae, this difference was removed by using tanglefoot to repel nonflying predators. However, the major nonflying predator of pupae, the ant Formica lasioides, was no more abundant in non-outbreak areas than in the outbreak area. We found inverse density-dependence in the rate of attack by F. lasioides on experimental pupae, suggesting this generalist predator is satiated within the outbreak area, but preys more heavily on the moth where the moth is sparse.     

Nest microhabitats and tree size mediate shifts in ant community structure across elevation in tropical rainforest canopies

Declines or mid-elevation peaks in invertebrate diversity with elevation are often attributed to climate and geometric constraints. However, vegetation structure may also drive diversity patterns, especially for tree-dwelling species, via its effects on microhabitat use and competitive interactions. Here we investigate these effects on the diversity and community structure of tree-nesting ants over elevation. We exhaustively sampled ant nests in 1254 trees within continuous plots of primary rainforest at low (200 m a.s.l.), mid (900 m a.s.l.) and high (1800 m a.s.l.) elevation in Papua New Guinea. Ant diversity, nest abundance and tree occupancy peaked at mid-elevation. Although host tree diversity also peaked at mid-elevation, there was low specialisation of ant species to tree species at all elevations. Mid-elevation trees hosted more species, more nests and a greater diversity of nest types than trees of a similar size at low or high elevation. Tree size and nest microhabitat use were the strongest predictors of species composition, explaining twice as much of the variability in the communities than elevation. At mid to high elevation there were proportionally fewer large nests than in the lowlands, with an increase in smaller nests in live hollow twigs and epiphytes. There was high species turnover between elevations, and between trees within elevations. Species co-occurrence patterns within trees differed with tree size, and with elevation. In large trees species tended to co-occur at random at low and high elevation, but co-occurred more often than expected by chance at mid elevation, indicating an elevational shift in competitive interactions. We conclude that the more extreme diurnal temperatures at higher elevations, combined with increased epiphyte availability, drive ants to nest in more insulated microhabitats. This results in smaller colony sizes and a decrease in interspecific competition, thereby boosting species co-existence at mid elevation.     

Susceptibility of potato tuber moth (Lepidoptera: Gelechiidae) to postharvest gamma irradiation

Gamma irradiation doses applied to inhibit potato sprouting were tested against potato tuber moth larval and pupal stages. Young larvae and pupae were more susceptible to irradiation injuries than older ones. When larvae of different ages were exposed to doses > or = 100 Gy, only 13-35% pupated, but no adult emergence was obtained. The exposure of 1-1.5- or 3-3.5-d-old pupae to 150 Gy induced high level of sterility and remarkable reduction in female mating ability and fecundity, whereas the reduction was less noticeable for 5-5.5-d-old pupae. Postharvest gamma irradiation of potatoes could be considered as an efficient control approach against potato tuber moth larval and pupal infestations.     

Avian life history variation along altitudinal gradients: an example with cardueline finches

Elevation has long been considered a major influence on the evolution of life-history traits. Most elevation-induced variation in life history traits has been attributed to changes in climate, duration of breeding season, predation, and food limitation. I use a phylogenetic approach to show that life histories are closely associated with breeding elevation in extant cardueline finches. Finches at high elevations had smaller clutches, fewer broods, and longer incubation periods. Neither food limitation nor nest predation appear to readily account for this strong elevational variation in cardueline life histories. However, juvenile survival may be greater at higher elevations as a result of prolonged parental care and shorter natal dispersal and can potentially compensate for reduced fecundity in high-elevation finches. Received: 28 September 1996 / Accepted: 24 March 1997     

Bird predation as a selective pressure on the immature stages of the cabbage butterflies, Pieris rapae and P. brassicae

The importance of bird predation as a selective pressure on the eggs, larvae and pupae of the cabbage butterflies, Pieris rapae L. and P. brassicae L. has been investigated in a rural garden and an allotment. Birds are the main predators of all stages in a well-tended garden whereas in a field the eggs and young larvae are mainly preyed upon by arthropods. The species of avian predators vary according to the stage of development and species of prey. Consequently the relative survival rate of P. rupae and P. brussicue varies with stage of development. This changing pattern of survival has in turn acted as a selective pressure on their two main braconid parasites. Many aspects of the size, shape, colour and behaviour of the larvae and pupae of both species of Pieris appear to be adaptations to the selective pressures exerted by bird predation.