Foraging activity of leaf‐cutting ants changes light availability and plant assemblage in Atlantic forest

1. Leaf‐cutting ants (LCAs) have often been denoted as ecosystem engineers because of their multifarious effects on the vegetation, particularly via nest‐driven environmental changes. However, the non‐trophic impacts of LCAs on forest dynamics via foliage harvesting across sizeable foraging zones (so‐called associated ecosystem engineering) are still poorly investigated. 2. Here, light availability and sapling assemblages were assessed within foraging areas and ant‐free control zones of 16 Atta cephalotes colonies located in a large remnant of Atlantic forest in northeastern Brazil. 3. Canopy openness and total light transmission were 1.4 and 1.6 times higher in foraging zones than in control areas. In parallel, sapling density and species richness decreased constantly from control to foraging zone plots. Additionally, shade‐tolerant species exhibited reduced abundance across foraging zones. A non‐metric multidimensional scaling ordination based on taxonomic similarity primarily segregated foraging zone and control plots; foraging zone plots converged to be more similar to each other as well. Finally, some plant species emerged as indicators of LCA‐free zones. 4. These results suggest that LCA foraging activity in the forest canopy directly increases the light availability and indirectly affects the recruitment and the structure of local plant assemblages. 5. Such a biologically significant effect on the light environment and its cascades confirms LCAs as potent ecosystem engineers, particularly as a plant assembly force, which operates beyond the spatial reach of their well‐described nest effects.     

Parallel foraging cycles for different resources in leaf‐cutting ants: a clue to the mechanisms of rhythmic activity

1. Leaf‐cutting ants display regular diel cycles of foraging, but the regulatory mechanisms underlying these cycles are not well known. There are, however, some indications in the literature that accumulation of leaf tissue inside a nest dampens recruitment of foragers, thereby providing a negative feedback that can lead to periodic foraging. We investigated two foraging cycles occurring simultaneously in an Atta colombica colony, one involving leaf harvesting and the other exploiting an ephemeral crop of ripe fruit. 2. Leaf harvesting followed a typical diel pattern of a 10–12 h foraging bout followed by a period of inactivity, while fruit harvesting occurred continuously, but with a regular pre‐dawn dip in activity that marked a 24 h cycle. 3. Although the results of the present study are drawn from a single field colony, the difference found is consistent with a mechanism of negative feedback regulation acting in parallel on two resources that differ in their rates of distribution and processing, creating cycles of formation and depletion of material caches. 4. This hypothesis should provoke further interest from students of ant behaviour and some simple manipulative experiments that would begin to test it are outlined. Any role of resource caches in regulating foraging by Atta colonies may have similarities to the logistics of warehouse inventories in human economic activity.     

Foraging in highly dynamic environments: leaf‐cutting ants adjust foraging trail networks to pioneer plant availability

Major shifts in the availability of palatable plant resources are of key relevance to the ecology of leaf‐cutting ants in human‐modified landscapes. However, our knowledge is still limited regarding the ability of these ants to adjust their foraging strategy to dynamic environments. Here, we examine a set of forest stand attributes acting as modulating forces for the spatiotemporal architecture of foraging trail networks developed by Atta cephalotes L. (Hymenoptera: Formicidae: Attini). During a 12‐month period, we mapped the foraging systems of 12 colonies located in Atlantic forest patches with differing size, regeneration age, and abundance of pioneer plants, and examined the variation in five trail system attributes (number of trails, branching points, leaf sources, linear foraging distance, and trail complexity) in response to these patch‐related variables. Both the month‐to‐month differences (depicted in annual trail maps) and the steadily accumulating number of trails, trail‐branching points, leaf sources, and linear foraging distance illustrated the dynamic nature of spatial foraging and trail complexity. Most measures of trail architecture correlated positively with the number of pioneer trees across the secondary forest patches, but no effects from patch age and size were observed (except for number of leaf sources). Trail system complexity (measured as fractal dimension; Df index) varied from 1.114 to 1.277 along the 12 months through which ant foraging was monitored, with a marginal trend to increase with the abundance of pioneer stems. Our results suggest that some leaf‐cutting ant species are able to generate highly flexible trail networks (via fine‐tuned adjustment of foraging patterns), allowing them to profit from the continuous emergence/recruitment of palatable resources.     

Plant palatability to leaf‐cutter ants (Atta laevigata) and litter decomposability in a Neotropical woodland savanna

Although leaf‐cutter ants have been recognized as the dominant herbivore in many Neotropical ecosystems, their role in nutrient cycling remains poorly understood. Here we evaluated the relationship between plant palatability to leaf‐cutter ants and litter decomposability. Our rationale was that if preference and decomposability are related, and if ant consumption changes the abundance of litter with different quality, then ant herbivory could affect litter decomposition by affecting the quality of litter entering the soil. The study was conducted in a woodland savanna (cerrado denso) area in Minas Gerais, Brazil. We compared the decomposition rate of litter produced by trees whose fresh leaves have different degrees of palatability to the leaf‐cutter ant Atta laevigata. Our experiments did not indicate the existence of a significant relationship between leaf palatability to A. laevigata and leaf‐litter decomposability. Although the litter mixture composed of highly palatable plant species showed, initially, a faster decay rate than the mixture of poorly palatable species, this difference was no longer visible after about 6 months. Results were consistent regardless of whether litter invertebrates were excluded or not from litter bags. Similarly, experiments comparing the decomposition rate of litter from pairs of related plant species also showed no association between plant palatability and decomposition. Decomposition rate of the more palatable species was faster, slower or similar to that of the less palatable species depending upon the particular pair of species being compared. We suggest that the traits that mostly influence the decomposition rate of litter produced by cerrado trees may not be the same as those that influence plant palatability to leaf‐cutter ants. Atta laevigata select leaves of different species based – at least in part – on their nitrogen content, but N content was a poor predictor of the decomposition rates of the species we studied.     

Extended phenotypes and foraging restrictions: ant nest entrances and resource ingress in leaf‐cutting ants

Several factors may restrict the acquisition of food to below the levels predicted by the optimization theory. However, how the design of structures that animals build for foraging restricts the entry of food is less known. Using scaling relationships, we determined whether the design of the entrances of leaf‐cutting ant nests restricts resource input into the colony. We measured nests and foraging parameters in 25 nests of Atta cephalotes in a tropical rain forest. Ant flux was reduced to up to 60% at nest entrances. The width of all entrances per nest increased at similar rates as nest size, but the width of nest entrances increased with the width of its associated trail at rates below those expected by isometry. The fact that entrance widths grow slower than trail widths suggests that the enlargement of entrance holes does not reach the dimensions needed to avoid delays when foraging rates are high and loads are big. The enlargement of nest entrances appears to be restricted by the digging effort required to enlarge nest tunnels and by increments in the risk of inundation, predator/parasitoid attacks and microclimate imbalances inside the nest. The design of the extended phenotypes can also restrict the ingress of food into the organisms, offering additional evidence to better understand eventual controversies between empirical data and the foraging theory. Abstract in Spanish is available with online material.     

Drought stress drives intraspecific choice of food plants by Atta leaf‐cutting ants

Leaf‐cutting ants (LCA) are polyphagous and dominant herbivores throughout the Neotropics that carefully select plant individuals or plant parts to feed their symbiotic fungus. Although many species‐specific leaf traits have been identified as criteria for the choice of food plants, the factors driving intraspecific herbivory patterns in LCA are less well studied. Herein, we evaluate whether or not drought‐stressed native plants are a preferred food source using free‐living colonies of two leaf‐cutting ants, Atta sexdens L. (Hymenoptera: Formicidae: Attini), in combination with five plant species, Ocotea glomerata Nees (Lauraceae), Lecythis lurida S. A. Mori (Lecythidaceae), Miconia prasina DC (Melastomataceae), Tovomita brevistaminea Engl. (Clusiaceae), and Tapirira guianensis Aubl. (Anacardiaceae), and Atta cephalotes L., in combination with two plant species, O. glomerata and Licania tomentosa Benth. (Chrysobalanaceae). In dual‐choice bioassays, ants removed about three times more leaf area from drought‐stressed plants compared to control plants. Both leaf‐cutting ant species consistently preferred drought‐stressed plants for all species tested, except T. guianensis. The mean acceptability index – expressing the preference for one of two options on a scale of 0 to 1 – of drought‐stressed plants ranged from 0.65 to 0.86 across plant species, and the preference did not differ significantly among the tested plant species. Our results suggest that selection of drought‐stressed individuals is a general feature of food plant choice by leaf‐cutting ants irrespective of ant or plant species. As human‐modified forest assemblages across the Neotropics are increasingly prone to drought stress, the documented preference of Atta for drought‐stressed plants may have tangible ecological implications.     

Human disturbance promotes herbivory by leaf‐cutting ants in the Caatinga dry forest

Anthropogenic disturbances are known to modify plant–animal interactions such as those involving the leaf‐cutting ants, the most voracious and proliferating herbivore across human‐modified landscapes in the Neotropics. Here, we evaluate the effect of chronic anthropogenic disturbance (e.g., firewood collection, livestock grazing) and vegetation seasonality on foraging area, foliage availability in the foraging area, leaf consumption and herbivory rate of the leaf‐cutting ant Atta opaciceps in the semiarid Caatinga, a mosaic of dry forest and scrub vegetation in northeast Brazil. Contrary to our initial expectation, the foraging area was not affected by either disturbance intensity or the interaction between season and disturbance intensity. However, leaf consumption and herbivory rate were higher in more disturbed areas. We also found a strong effect of seasonality, with higher leaf consumption and herbivory rate in the dry season. Our results suggest that the foraging ecology of leaf‐cutting ants is modulated by human disturbance and seasonality as these two drivers affect the spectrum and the amount of resources available for these ants in the Caatinga. Despite the low productivity of Caatinga vegetation, the annual rates of biomass consumption by A. opaciceps are similar to those reported from other leaf‐cutting ants in rain forests and savannas. This is made possible by maintaining high foraging activity even in the peak of the dry season and taking benefit from any resource available, including low‐quality items. Such compensation highlights the adaptive capacity of LCA to persist or even proliferate in human‐modified landscapes from dry to rain forests.     

The foraging decisions of a central place foraging seabird in response to fluctuations in local prey conditions

During reproduction, seabirds need to balance the demands of self- and offspring-provisioning within the constraints imposed by central place foraging. To assess behavioral adjustments and tolerances to these constraints, we studied the feeding tactics and reproductive success of common murres (also known as common guillemots) Uria aalge , at their largest and most offshore colony (Funk Island) where parents travel long distances to deliver a single capelin Mallotus villosus to their chicks. We assessed changes in the distance murres traveled from the colony, their proximate foraging locations and prey size choice during two successive years in which capelin exhibited an order of magnitude decrease in density and a shift from aggregated (2004) to dispersed (2005) distributions. When capelin availability was low (2005), parental murres increased their maximum foraging distances by 35% (60 to 81 km) and delivered significantly larger capelin to chicks, as predicted by central place foraging theory. Murres preferred large (>140 mm) relative to small capelin (100–140 mm) in both years, but unexpectedly this preference increased as the relative density of large capelin decreased. We conclude that single prey-loading murres target larger capelin during long foraging trips as parents are 'forced' to select the best prey for their offspring. Low fledgling masses suggest also that increased foraging time when capelin is scarce may come at a cost to the chicks (i.e. fewer meals per day). Murres at this colony may be functioning near physiological limits above which further or sustained adjustments in foraging effort could compromise the life-time reproductive success of this long-lived seabird.     

Density‐mediated indirect interactions alter host foraging behaviour of parasitoids without altering foraging efficiency

1. Foraging decisions of parasitoids are influenced by host density via density‐mediated indirect interactions. However, in the parasitoid's environment, non‐suitable herbivores are also present. These non‐hosts also occur in different densities, which can affect a parasitoid's foraging behaviour. 2. The influence of non‐host densities can be expressed during the first phase of the foraging process, when parasitoids use plant volatiles to locate plants infested by their host. They may also play a role during the second phase, when parasitoids use infochemicals from the host and plant to locate, recognise and accept the host. 3. By using laboratory and field setups, it was studied whether the density of non‐host herbivores influences these two phases of the foraging behaviour of the parasitoid Cotesia glomerata as well as the parasitoid's efficiency to find its host, Pieris brassicae caterpillars. 4. The findings show that a high non‐host density, regardless of the species used, negatively affected parasitoid preference for host‐infested plants, but that the behaviour on the plant and the total host‐finding efficiency of the parasitoids were not influenced by non‐host density. 5. These results are discussed in the context of density‐mediated indirect interactions.     

The ‘truck‐driver’ effect in leaf‐cutting ants: how individual load influences the walking speed of nest‐mates

The foraging behaviour of social insects is highly flexible because it depends on the interplay between individual and collective decisions. In ants that use foraging trails, high ant flow may entail traffic problems if different workers vary widely in their walking speed. Slow ants carrying extra‐large loads in the leaf‐cutting ant Atta cephalotes L. (Hymenoptera: Formicidae) are characterized as ‘highly‐laden’ ants, and their effect on delaying other laden ants is analyzed. Highly‐laden ants carry loads that are 100% larger and show a 50% greater load‐carrying capacity (i.e. load size/body size) than ‘ordinary‐laden’ ants. Field manipulations reveal that these slow ants carrying extra‐large loads can reduce the walking speed of the laden ants behind them by up to 50%. Moreover, the percentage of highly‐laden ants decreases at high ant flow. Because the delaying effect of highly‐laden ants on nest‐mates is enhanced at high traffic levels, these results suggest that load size might be adjusted to reduce the negative effect on the rate of foraging input to the colony. Several causes have been proposed to explain why leaf‐cutting ants cut and carry leaf fragments of sizes below their individual capacities. The avoidance of delay in laden nest‐mates is suggested as another novel factor related to traffic flow that also might affect load size selection The results of the presennt study illustrate how leaf‐cutting ants are able to reduce their individual carrying performance to maximize the overall colony performance.     

Taking a trip to the shelf: Behavioral decisions are mediated by the proximity to foraging habitats in the black‐legged kittiwake

For marine top predators like seabirds, the oceans represent a multitude of habitats regarding oceanographic conditions and food availability. Worldwide, these marine habitats are being altered by changes in climate and increased anthropogenic impact. This is causing a growing concern on how seabird populations might adapt to these changes. Understanding how seabird populations respond to fluctuating environmental conditions and to what extent behavioral flexibility can buffer variations in food availability can help predict how seabirds may cope with changes in the marine environment. Such knowledge is important to implement proper long‐term conservation measures intended to protect marine predators. We explored behavioral flexibility in choice of foraging habitat of chick‐rearing black‐legged kittiwakes Rissa tridactyla during multiple years. By comparing foraging behavior of individuals from two colonies with large differences in oceanographic conditions and distances to predictable feeding areas at the Norwegian shelf break, we investigated how foraging decisions are related to intrinsic and extrinsic factors. We found that proximity to the shelf break determined which factors drove the decision to forage there. At the colony near the shelf break, time of departure from the colony and wind speed were most important in driving the choice of habitat. At the colony farther from the shelf break, the decision to forage there was driven by adult body condition. Birds furthermore adjusted foraging behavior metrics according to time of the day, weather conditions, body condition, and the age of the chicks. The study shows that kittiwakes have high degree of flexibility in their behavioral response to a variable marine environment, which might help them buffer changes in prey distribution around the colonies. The flexibility is, however, dependent on the availability of foraging habitats near the colony.     

The fungal cultivar of leaf‐cutter ants produces specific enzymes in response to different plant substrates

Herbivores use symbiotic microbes to help derive energy and nutrients from plant material. Leaf‐cutter ants are a paradigmatic example, cultivating their mutualistic fungus Leucoagaricus gongylophorus on plant biomass that workers forage from a diverse collection of plant species. Here, we investigate the metabolic flexibility of the ants’ fungal cultivar for utilizing different plant biomass. Using feeding experiments and a novel approach in metaproteomics, we examine the enzymatic response of L. gongylophorus to leaves, flowers, oats or a mixture of all three. Across all treatments, our analysis identified and quantified 1766 different fungal proteins, including 161 putative biomass‐degrading enzymes. We found significant differences in the protein profiles in the fungus gardens of subcolonies fed different plant substrates. When provided with leaves or flowers, which contain the majority of their energy as recalcitrant plant polymers, the fungus gardens produced more proteins predicted to break down cellulose: endoglucanase, exoglucanase and β‐glucosidase. Further, the complete metaproteomes for the leaves and flowers treatments were very similar, while the mixed substrate treatment closely resembled the treatment with oats alone. This indicates that when provided a mixture of plant substrates, fungus gardens preferentially break down the simpler, more digestible substrates. This flexible, substrate‐specific enzymatic response of the fungal cultivar allows leaf‐cutter ants to derive energy from a wide range of substrates, which likely contributes to their ability to be dominant generalist herbivores.     

A review of the occurrence of inter‐colony segregation of seabird foraging areas and the implications for marine environmental impact assessment

Understanding the determinants of species’ distributions is a fundamental aim in ecology and a prerequisite for conservation but is particularly challenging in the marine environment. Advances in bio‐logging technology have resulted in a rapid increase in studies of seabird movement and distribution in recent years. Multi‐colony studies examining the effects of intra‐ and inter‐colony competition on distribution have found that several species exhibit inter‐colony segregation of foraging areas, rather than overlapping distributions. These findings are timely given the increasing rate of human exploitation of marine resources and the need to make robust assessments of likely impacts of proposed marine developments on biodiversity. Here we review the occurrence of foraging area segregation reported by published tracking studies in relation to the density‐dependent hinterland (DDH) model, which predicts that segregation occurs in response to inter‐colony competition, itself a function of colony size, distance from the colony and prey distribution. We found that inter‐colony foraging area segregation occurred in 79% of 39 studies. The frequency of occurrence was similar across the four seabird orders for which data were available, and included species with both smaller (10–100 km) and larger (100–1000 km) foraging ranges. Many predictions of the DDH model were confirmed, with examples of segregation in response to high levels of inter‐colony competition related to colony size and proximity, and enclosed landform restricting the extent of available habitat. Moreover, as predicted by the DDH model, inter‐colony overlap tended to occur where birds aggregated in highly productive areas, often remote from all colonies. The apparent prevalence of inter‐colony foraging segregation has important implications for assessment of impacts of marine development on protected seabird colonies. If a development area is accessible from multiple colonies, it may impact those colonies much more asymmetrically than previously supposed. Current impact assessment approaches that do not consider spatial inter‐colony segregation will therefore be subject to error. We recommend the collection of tracking data from multiple colonies and modelling of inter‐colony interactions to predict colony‐specific distributions.     

Latitudinal trends in foliar oils of eucalypts: Environmental correlates and diversity of chrysomelid leaf‐beetles

Eucalypts are characterized by their oleaginous foliage, yet no one has considered the universality of oil expression or its ecological associations and implications for biodiversity. Published literature on the oils of 66 eucalypts was combined with geographic distribution information contained in the Australian National Herbarium (ANHSIR) database to investigate continent‐scale changes in oil yield and composition. The exposure to fire and rainfall of each eucalypt was considered in reference to Walker's data on fire frequency and Australian Bureau of Meteorology 97‐year records of rainfall variability. Host collection records for 69 species of chrysomelid leaf‐beetle were collated from entomologists to consider patterns of association with a subset of 16 eucalypts. Eucalypts endemic to the seasonally arid, sub‐tropical to tropical climates of northern Australia have less oleaginous and aromatic leaves than species endemic to the mesic, temperate climates of the southern parts of the continent. Maximum oil yield and the concentrations of cineole and pinene were positively correlated with minimum fire interval but not with rainfall variability. Low oil contents in more northerly distributed species may facilitate persistence in highly fire‐prone habitats. There were no patterns in the diversity of chrysomelid leaf‐beetles with either the oil yield or the concentrations of 1,8‐cineole or α‐pinene in their hosts. When taken in consideration with the apparent strategy of eucalypts to tolerate insect herbivory, current evidence augurs against high concentrations of cineole or pinene acting alone as antibiotic plant secondary metabolites.     

To be or not to be faithful: flexible fidelity to foraging trails in the leaf‐cutting ant Acromyrmex lobicornis

1. Ants using trails to forage have to select between two alternative routes at bifurcations, using two, potentially conflicting, sources of information to make their decision: individual experience to return to a previous successful foraging site (i.e. fidelity) and ant traffic. In the field, we investigated which of these two types of information individuals of the leaf‐cutting ant Acromyrmex lobicornis Emery use to decide which foraging route to take. 2. We measured the proportion of foraging ants returning to each trail of bifurcations the following day, and for 4–7 consecutive days. We then experimentally increased ant traffic on one trail of the bifurcation by adding additional food sources to examine the effect of increased ant traffic on the decision that ants make. 3. Binomial tests showed that for 62% of the trails, ant fidelity was relatively more important than ant traffic in deciding which bifurcation to follow, suggesting the importance of previous experience. 4. When information conflict was generated by experimentally increasing ant traffic along the trail with less foraging activity, most ants relied on ant traffic to decide. However, in 33% of these bifurcations, ants were still faithful to their trail. Thus, there is some degree of flexibility in the decisions that A. lobicornis make to access food resources. 5. This flexible fidelity results in individual variation in the response of workers to different levels of ant traffic, and allows the colony to simultaneously exploit both established and recently discovered food patches, aiding efficient food gathering.     

The Multiple Impacts of Leaf‐Cutting Ants and Their Novel Ecological Role in Human‐Modified Neotropical Forests

Herbivory has been identified as a potent evolutionary force, but its ecological impacts have been frequently underestimated. Leaf‐cutting ants represent one of the most important herbivores of the Neotropics and offer an interesting opportunity to address the role played by herbivorous insects through a perspective that embraces population‐ to ecosystem‐level effects. Here we: (1) qualitatively summarize the multiple ways leaf‐cutting ants interact with food plants and their habitats and elucidate the ultimate outcome of such interactions at multiple organization levels; (2) update our understanding of leaf‐cutting ant‐promoted disturbance regimes; and (3) examine potential ecological roles by leaf‐cutting ants within the context of human‐modified landscapes to guide future research agendas. First, we find that leaf‐cutting ants show that some herbivorous insects are able to generate ecologically important disturbance regimes via non‐trophic activities. Second, impacts of leaf‐cutting ants can be observed at multiple spatio‐temporal scales and levels of biological organization. Third, ecosystem‐level effects from leaf‐cutting ants are ecosystem engineering capable not only of altering the abundance of other organisms, but also the successional trajectory of vegetation. Finally, effects of leaf‐cutting ants are context‐dependent, species‐specific, and synergistically modulated by anthropogenic interferences. Future research should examine how leaf‐cutting ants respond to deforestation and influence remaining vegetation in human‐modified landscapes. By promoting either heterogeneity or homogeneity, leaf‐cutting ants operate not only as agricultural pests but also as ecological key players.     

Between‐individual variation drives the seasonal dynamics in the trophic niche of a Neotropical marsupial

The dynamics of population niches result from the variation in resource use within individuals and also from the variation between individuals. The prevalence of one mechanism or the other leads to competing hypotheses about the major mechanisms underlying the empirical observations of the contraction/expansion dynamics of the trophic niche in natural populations. In this study, we investigated how within‐ and between‐individual variation in resource use shapes the food niche dynamics of the woolly mouse opossum, Marmosa paraguayana (Didelphimorphia: Didelphidae), in a remnant of the highly seasonal Cerrado in south‐eastern Brazil. To do so, we analysed the faecal samples of live‐trapped individuals to determine their diets within the wet and dry seasons. In addition to a seasonal shift in the composition of the diet, the population trophic niche was significantly wider during the dry season than the wet season. This expansion resulted from larger between‐individual variation in the dry season that was not related to sex preferences, whereas the individual niche widths did not significantly increase from the wet to the dry seasons. Our findings add to the growing list of animal populations that show individual‐level variation in resource use. Furthermore, these results represent a pattern of individual‐level response to seasonal changes that is different from patterns reported for other organisms. We suggest that a pathway to build more realistic foraging models and produce more accurate predictions on population and community dynamics is to consider between‐individual variation and short‐term niche dynamics.     

Associative learning of host presence in non‐host environments influences parasitoid foraging

1. Parasitoids are known to utilise learning of herbivore‐induced plant volatiles (HIPVs) when foraging for their herbivorous host. In natural situations these hosts share food plants with other, non‐suitable herbivores (non‐hosts). Simultaneous infestation of plants by hosts and non‐hosts has been found to result in induction of HIPVs that differ from host‐infested plants. Each non‐host herbivore may have different effects on HIPVs when sharing the food plant with hosts, and thus parasitoids may learn that plants with a specific non‐host herbivore also contain the host. 2. This study investigated the adaptive nature of learning by a foraging parasitoid that had acquired oviposition experience on a plant infested with both hosts and different non‐hosts in the laboratory and in semi‐field experiments. 3. In two‐choice preference tests, the parasitoid Cotesia glomerata shifted its preference towards HIPVs of a plant–host–non‐host complex previously associated with an oviposition experience. It could, indeed, learn that the presence of its host is associated with HIPVs induced by simultaneous feeding of its host Pieris brassicae and either the non‐host caterpillar Mamestra brassicae or the non‐host aphid Myzus persicae. However, the learned preference found in the laboratory did not translate into parasitisation preferences for hosts accompanying non‐host caterpillars or aphids in a semi‐field situation. 4. This paper discusses the importance of learning in parasitoid foraging, and debates why observed learned preferences for HIPVs in the laboratory may cancel out under some field experimental conditions.     

High densities of leaf‐tiers in open habitats are explained by host plant architecture

1. Crown architecture remains one of the least studied plant traits that influence plant–herbivore interactions. The hypotheses that dense crown architecture of mountain birches from open habitats favours leaf‐tying caterpillars through bottom‐up and/or top‐down effects associated with high leaf connectivity were tested. 2. Population densities of leaf‐tying herbivores in open (industrial barren and seashore) habitats were three times as high as in the shaded (forest) habitats. An experimental increase in leaf density by branch binding did not affect foliar consumption by free‐living herbivores but increased consumption by leaf‐tiers. 3. The specific leaf weight was lower in shaded habitats and in bound branches, but branch binding did not influence either the foliar concentrations of carbon and nitrogen or the pupal weight of the most abundant leaf‐tier, Carpatolechia proximella Hbn. (Lepidoptera: Gelechiidae). 4. Caterpillars of C. proximella build several shelters during their lifetime and spend a considerable amount of time outside the shelter, where they excrete most of their faeces. In bound branches, caterpillars built new shelters more frequently than in control branches, and consumed less foliar biomass per shelter. 5. Mortality from parasitoids in bound branches was half that in the control, presumably because the complex environment disrupted parasitoid searching behaviour and/or because of lower damage to leaves from which the shelters were built. 6. It is concluded that the crown architecture associated with high leaf connectivity decreases mortality risks from natural enemies both outside and inside the shelter. Compact and dense crowns of host plants may at least partly explain high population densities of leaf‐tiers in open habitats.     

Edge‐mediated reduction of phorid parasitism on leaf‐cutting ants in a Brazilian Atlantic forest

Previous studies have shown that leaf‐cutting ant populations benefit greatly from living in or near the edges of the Brazilian Atlantic forest. One of the mechanisms responsible for this rise in population density is an edge‐mediated increase of pioneer plants, resulting in increased food availability for the ants (i.e., less bottom‐up control). Here, we hypothesized that the release from natural enemies (i.e., less top‐down control) may also contribute to the phenomenon. We investigated whether parasitism of phorid flies on leaf‐cutting ants decreases in colonies located along the forest edge vs. the interior of a large tract of Atlantic forest in northeastern Brazil. For this, we assessed abundance and rates of oviposition attack by phorids in bimonthly intervals over a period of 1 year in 10 adult colonies of Atta cephalotes (L.) (Hymenoptera: Formicidae: Myrmicinae), five at the forest edge and five in the forest interior. The number of phorids attracted by ants at edge colonies was 40% lower than that at interior colonies. The temporal variation in phorid attraction was also significant, with approximately 35% fewer flies in the dry months as compared to the rainy months. As a result of lower phorid abundance, ant workers of edge colonies suffered three times fewer oviposition attacks than those of interior colonies. There was a tendency for fewer attacks during dry months, but the difference in the temporal variation was not significant. Our findings suggest that edge creation contributes to increased leaf‐cutting ant abundance, not only via the attenuation of bottom‐up forces, but also through an environmentally triggered depression of parasitoid abundance/efficiency, possibly because of adverse environmental conditions in edge habitats.