Acceptance and suitability of novel trees for <Emphasis Type="Italic">Orthotomicus erosus</Emphasis>, an exotic bark beetle in North America

To predict whether an herbivorous pest insect will establish in a new area, the potential host plants must be known. For invading bark beetles, adults must recognize and accept trees suitable for larval development. The preference-performance hypothesis predicts that adults will select host species that maximize the fitness of their offspring. We tested five species of North American conifers and one angiosperm for adult acceptance and suitability for reproduction of the Mediterranean pine engraver, Orthotomicus erosus (Wollaston). Red pine, Pinus resinosa Aiton, and white spruce, Picea glauca (Moench) Voss, were accepted by adult beetles and suitable for reproduction to the extent of adult replacement. Others, such as balsam fir, Abies balsamea (L.) Mill., eastern hemlock, Tsuga canagensis (L.) Carrière, and tamarack, Larix laricina (Du Roi) Koch, were acceptable but unsuitable. The presence of tree species that are acceptable to adults but unsuitable for reproduction may affect the ability of O. erosus to establish across North America.     

Amphibian lipid levels at metamorphosis correlate to post-metamorphic terrestrial survival

In organisms that have complex life cycles, factors in the larval environment may affect both larval and adult traits. For amphibians, the postmetamorphic transition from the aquatic environment to terrestrial habitat may be a period of high juvenile mortality. We hypothesized that lipid stores at metamorphosis may affect an animal’s success during this critical transition period. We examined variation in total lipid levels among years and sites in recently metamorphosed individuals of two pond-breeding salamander species, the marbled salamander (Ambystoma opacum) and the mole salamander (A. talpoideum), with limited data for one anuran species (southern leopard frog, Rana sphenocephala). Lipid levels were allometrically related to body size and ranged from 1.9 to 23.8% of body dry mass. The two salamander species differed in lipid allocation patterns, with A. opacum apportioning a higher percentage of total lipid reserves into fat bodies than A. talpoideum. Species differences in lipid allocation patterns may primarily reflect that large metamorphs will mature as one-year olds, and, regardless of species, will alter lipid compartmentalization accordingly. We used mark–recapture data obtained at drift fences encircling breeding ponds for 13 A. opacum cohorts to estimate the proportion of postmetamorphic individuals that survived to breed (age 1–4) and the mean age at first reproduction. Regression models indicated that size-corrected lipid level at metamorphosis (i.e., lipid residuals), and to a lesser extent rainfall following metamorphosis, was positively related to adult survival. Snout-vent length at metamorphosis was negatively related to age at first reproduction. We suggest that lipid stores at metamorphosis are vital to juvenile survival in the months following the transition from aquatic to terrestrial habitat, and that a trade-off shaped by postmetamorphic selection in the terrestrial habitat exists between allocation to energy stores versus structural growth in the larval environment.     

Linking aquatic and terrestrial food webs – Odonata in boreal systems

1. It is increasingly realised that aquatic and terrestrial systems are closely linked. We investigated stable isotope variations in Odonata species, putative prey and basal resources of aquatic and terrestrial systems of northern Mongolia during summer. 2. In permanent ponds, δ¹³C values of Odonata larvae were distinctly lower than those of putative prey, suggesting that body tissue comprised largely of carbon originating from isotopically light carbon sources. Presumably, prey consumed during autumn and winter when carbon is internally recycled and/or methanotrophic bacteria form an important basal resource of the food web. In contrast, in a temporary pond, δ¹³C values of Odonata larvae were similar to those of putative prey, indicating that their body carbon originated mainly from prey species present. 3. Changes in δ¹⁵N and δ¹³C values between larvae and adults were species specific and reflected differential replacement of the larval isotopic signature by the terrestrial diet of adult Odonata. The replacement was more pronounced in Odonata species of permanent ponds than in those of the temporary pond, where larvae hatched later in the year. Replacement of larval carbon varied between tissues, with wings representing the larval isotopic signature whereas thoracic muscles and eggs reflected the δ¹⁵N and δ¹³C values of the terrestrial diet of adults. 4. The results suggest that because of their long larval development, Odonata species of permanent ponds carry the larval signature, which is partly replaced during their terrestrial life. Terrestrial prey forms the basis for egg production and thus the next generation of aquatic larvae. In temporary ponds, in contrast, Odonata species rely on prey from a single season, engage in a prolonged aquatic phase and hatch later, leaving less time to acquire terrestrial prey resources for offspring production. Stable isotope analysis provided important insights into the food webs of the waterbodies and their relationship to the terrestrial system.     

SEASONAL DECLINES IN OFFSPRING FITNESS AND SELECTION FOR EARLY REPRODUCTION IN NYMPH-OVERWINTERING GRASSHOPPERS

In this study, I examine the effects of natural and experimentally induced variation in life cycle timing on offspring fitness in Arphia sulphurea and Chortophaga viridifasciata, to understand the selective pressures shaping phenology in these two species of nymph-overwintering grasshoppers. Because these species lack embryonic diapause, hatching varies over a two month range under natural conditions. I used a cold treatment to delay hatching of some egg pods and extend the natural range of hatching dates. Due to the shorter time for growth and poorer growing conditions late in the fall, late-hatching nymphs of both species grew to a smaller size before winter and suffered higher overwinter mortality, compared to early nymphs. In addition, late nymphs that did survive the winter became reproductive later in the following year's breeding season. Size- dependent mortality of offspring during the winter is a strong selective pressure favoring early reproduction in these species. Female adult life history traits appear responsive to the seasonal declines in offspring fitness, in that late-maturing females began reproducing sooner after adult maturation and reproduced at a more rapid rate, even at the expense of having shorter adult longevity and producing fewer total egg pods. Experimental manipulations were crucial in understanding the fitness consequences of intrapopulation variation in the timing of specific life-cycle events for these species.     

Paedomorphosis in <Emphasis Type="Italic">Ambystoma talpoideum</Emphasis>: effects of initial body size variation and density

Facultative paedomorphosis is the ability of a salamander to either metamorphose into a terrestrial, metamorphic adult or retain a larval morphology to become a sexually mature paedomorphic adult. It has been hypothesized that density and initial body size variation within populations are instrumental in cueing metamorphosis or paedomorphosis in salamanders, yet few studies have adequately tested these hypotheses in long-term experiments. Beginning in the spring of 2004, 36 experimental ponds were used to manipulate three body size variation levels (low, medium, high) and two density levels (low, high) of Ambystoma talpoideum larvae. Larvae were individually marked using visible implant elastomers and collected every 2 weeks in order to measure snout–vent length and mass. Bi-nightly sampling was used to collect new metamorphs as they appeared. Analysis revealed significant effects of density, size variation and morph on body size of individuals during the summer. Individuals that metamorphosed during the fall and following spring were significantly larger as larvae than those becoming paedomorphic across all treatments. These results support the Best-of-a-Bad-Lot hypothesis, which proposes that the largest larvae metamorphose in order to escape unfavorable aquatic habitats. Large larvae may metamorphose to leave aquatic habitats, regardless of treatment, due to the colder climate and lower productivity found in Kentucky, which is in the northern-most part of A. talpoideum’s range. By maintaining a long-term experiment, we have provided evidence for the transition of both larvae and paedomorphs into metamorphs during fall and spring metamorphosis events. Furthermore, the production of similar morphs under different environmental conditions observed in this research suggests that the ecological mechanisms maintaining polyphenisms may be more diverse that first suspected. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Morphology and biology of the flower‐visiting water scavenger beetle genus Rygmodus (Coleoptera: Hydrophilidae)

Hydrophilidae (water scavenger beetles) is well known as an aquatic beetle family; however, it contains ca. 1,000 secondarily terrestrial species derived from aquatic ancestors. The New Zealand endemic genus Rygmodus White is a member of the hydrophilid subfamily Cylominae, which is the early‐diverging taxon of the largest terrestrial lineage (Cylominae + Sphaeridiinae) within the Hydrophilidae. In this paper we report that Rygmodus beetles are pollen‐feeding flower visitors as adults, but aquatic predators as larvae. Based on analyses of gut contents and a summary of collecting records reported on museum specimen labels, adult Rygmodus beetles are generalists feeding on pollen of at least 13 plant families. Rygmodus adult mouthparts differ from those of other (saprophagous) hydrophilid beetles in having the simple scoop‐like apex and mola with roughly denticulate surface, resembling the morphology found in pollen‐feeding staphylinid beetles. Larvae were found along the sides of streams, under stones and in algal mats and water‐soaked moss; one collected larval specimen was identified using DNA barcoding of two molecular markers, mitochondrial cytochrome oxidase 1 (cox1) and nuclear histone 3 (H3). Larvae of two species, Rygmodus modestus and Rygmodus sp., are described in detail and illustrated; they closely resemble ambush‐type predatory larvae of the hydrophilid tribe Hydrophilini in the head morphology. Rygmodus is the only known hydrophilid beetle with adults and larvae inhabiting different environments.     

Seasonal variation in parental care, offspring development, and reproductive success in the burying beetle, Nicrophorus vespillo

1. Beetles of the genus Nicrophorus reproduce on small vertebrate carcasses that they bury in the soil to provide the larvae with food. Usually, both parents cooperate in brood care by feeding and guarding their progeny. 2. In pairs of the common European species N. vespillo, the duration of care depended on the time of year when the beetles reproduced. Both in 1990 and in 1991, male and female parents stayed longer with their broods when reproduction started in spring than when reproduction started in early or late summer. This was probably due to the longer development time of the larvae caused by lower temperatures in spring, because laboratory experiments suggested a strong influence of temperature on both the duration of brood care and offspring development. 3. The number of adult offspring produced by a beetle pair did not vary among different times of the year. 4. The median time required for offspring development, measured as time from burial of the carcass to emergence of young adults, was between 62 and 84 days. When the beetles reproduced in late summer, only about three-quarters of the offspring left the soil and hibernated as adults. The remaining offspring stayed underground and adults appeared on the soil surface the following spring. They still showed the flexible cuticle typical of newly-hatched beetles, suggesting that they may have overwintered in a pre-adult stage.     

The Effects of Temperature and Diet during Development,Adulthood, and Mating on Reproduction in the Red Flour Beetle

The effects of different temperatures and diets experienced during distinct life stages are not necessarily similar. The silver-spoon hypothesis predicts that developing under favorable conditions will always lead to better performing adults under all adult conditions. The environment-matching hypothesis suggests that a match between developmental and adult conditions will lead to the best performing adults. Similar to the latter hypothesis, the beneficial-acclimation hypothesis suggests that either developing or acclimating as adults to the test temperature will improve later performance under such temperature. We disentangled here between the effect of growth, adult, and mating conditions (temperature and diet) on reproduction in the red flour beetle (Tribolium castaneum), in reference to the reproduction success rate, the number of viable offspring produced, and the mean offspring mass 13 days after mating. The most influential stage affecting reproduction differed between the diet and temperature experiments: adult temperature vs. parental growth diet. Generally, a yeast-rich diet or warmer temperature improved reproduction, supporting the silver-spoon hypothesis. However, interactions between life stages made the results more complex, also fitting the environment-matching hypothesis. Warm growth temperature positively affected reproduction success, but only when adults were kept under the same warm temperature. When the parental growth and adult diets matched, the mean offspring mass was greater than in a mismatch between the two. Additionally, a match between warm adult temperature and warm offspring growth temperature led to the largest offspring mass. These findings support the environment-matching hypothesis. Our results provide evidence for all these hypotheses and demonstrate that parental effects and plasticity may be induced by temperature and diet.     

The ontogeny of the olfactory system in ceratophryid frogs (Anura,Ceratophryidae)

The aquatic‐to‐terrestrial shift in the life cycle of most anurans suggests that the differences between the larval and adult morphology of the nose are required for sensory function in two media with different physical characteristics. However, a better controlled test of specialization to medium is to compare adult stages of terrestrial frogs with those that remain fully aquatic as adults. The Ceratophryidae is a monophyletic group of neotropical frogs whose diversification from a common terrestrial ancestor gave rise to both terrestrial (Ceratophrys, Chacophrys) and aquatic (Lepidobatrachus) adults. So, ceratophryids represent an excellent model to analyze the morphology and possible changes related to a secondary aquatic life. We describe the histomorphology of the nose during the ontogeny of the Ceratophryidae, paying particular attention to the condition in adult stages of the recessus olfactorius (a small area of olfactory epithelium that appears to be used for aquatic olfaction) and the eminentia olfactoria (a raised ridge on the floor of the principal cavity correlated with terrestrial olfaction). The species examined (Ceratophrys cranwelli, Chacophrys pierottii, Lepidobatrachus laevis, and L. llanensis) share a common larval olfactory organ composed by the principal cavity, the vomeronasal organ and the lateral appendix. At postmetamorphic stages, ceratophryids present a common morphology of the nose with the principal, middle, and inferior cavities with characteristics similar to other neobatrachians at the end of metamorphosis. However, in advanced adult stages, Lepidobatrachus laevis presents a recessus olfactorius with a heightened (peramorphic) development and a rudimentary (paedomorphic) eminentia olfactoria. Thus, the adult nose in Lepidobatrachus laevis arises from a common developmental ‘terrestrial’ pathway up to postmetamorphic stages, when its ontogeny leads to a distinctive morphology related to the evolutionarily derived, secondarily aquatic life of adults of this lineage.     

A simple,heat‐sterilizable artificial diet excluding animal‐derived ingredients for adult blowfly,Lucilia sericata

Artificial diets have been developed for Lucilia sericata (Meigen) blowfly larvae; however, diets for adults have not yet been developed. An adult diet that excludes animal tissues and animal‐derived ingredients and promotes not only ovarian development, but also oviposition, would aid in basic research and maggot debridement therapy. We have successfully developed artificial diets that exclude animal tissues and animal‐derived ingredients for L. sericata adults. The outcomes of the diets were comparable with those of a beef liver diet in terms of oviposition, adult survival and number of offspring.     

Poor Nutritional Conditions During the Early Larval Stage Reduce Risk‐Taking Activities of Fire Salamander Larvae (Salamandra salamandra)

Environmental conditions experienced early in life have been shown to significantly affect growth trajectories at later stages in many vertebrate species. Amphibians typically have a biphasic life history, with an aquatic larval phase during early development and a subsequent terrestrial adult phase after completed metamorphosis. Thus, the early conditions have an especially strong impact on the future survival and fitness of amphibians. We studied whether early nutritional conditions affect the behavioural reaction of fire salamander larvae (Salamandra salamandra) before completion of metamorphosis. Fire salamander larvae reared under rich nutritional conditions were heavier and larger, displayed better body condition overall throughout the first three month of life and metamorphosed earlier compared with larvae raised under poor nutritional conditions. Specifically, we tested whether larvae reared under these different conditions differed with respect to their risk‐taking behaviour and activity. We found no differences in the activity of larvae with respect to their experienced early food conditions. However, larvae reared under poor nutritional conditions hid significantly more often in a risk‐taking test than larvae reared under rich food conditions. This increase in shelter‐seeking behaviour might be an adaptation to reduce the risk of larval drift or an adaptation to compensate for physiological deficits in part by appropriate behavioural reactions. Our results indicate that environmental conditions, such as food availability, may lead to different behavioural strategies.     

Female‐biased offspring sex ratios in birds at a mercury‐contaminated river

Mercury is a ubiquitous environmental pollutant that can negatively impact physiology and behavior of vertebrates, causing sub‐lethal changes in condition and reducing fitness. Here we examine its effect on offspring sex ratio. Previous studies demonstrate the ability of environmental contaminants to skew sex ratios in wild populations toward the production of females, and research in humans has demonstrated a decrease in male births following mercury exposure. We therefore hypothesized that female birds inhabiting the floodplain of a mercury‐contaminated river would produce broods more biased towards the production of females relative to birds from uncontaminated areas. We examined complete broods of three species: the aquatic‐feeding belted kingfisher Megaceryle alcyon, the terrestrial‐feeding eastern bluebird Sialia sialis, and the tree swallow Tachycineta bicolor, which feeds from both aquatic and terrestrial sources. Nestling sex ratios were shifted toward the production of females in all three species on mercury‐contaminated sites when compared to uncontaminated reference sites. These results may be explained by endocrine disruption or the Trivers–Willard theory of sex allocation. Our study is the first to examine the impact of mercury on offspring sex ratios in birds, and therefore contributes to our understanding of the potential for this persistent biomagnifying contaminant to alter fitness and effective population size in wildlife.     

On the virtue of being the first born: the influence of date of birth on fitness in the mosquitofish, Gambusia affinis

We found in an earlier study that mosquitofish (Gambusia affinis and G. holbrooki) ceased reproduction in the late summer, long before the end of warm weather, stored fat, then utilized reserves to survive the winter and initiate reproduction the following spring. We hypothesized that this pattern of fat utilization was a life history adaptation that enabled the fish to acquire food resources in the autumn then allocate them to reproduction the following spring when the fitness of the young would be greater. Here we evaluate one aspect of this hypothesis by evaluating the probability of survival to maturity and fecundity of young as a function of date of birth. We placed cohorts comprising eight to ten litters of young born early‐, mid‐ or late in the reproductive season in replicate field enclosures. The entire experiment was repeated in two different years. Early‐born young had a significantly higher probability of survival to maturity but did not differ in fecundity relative to the last cohort of the season. Early‐born young also attained maturity early enough to reproduce in their year of birth while late‐born young had to overwinter before reproduction. The fitness consequences to the mother of either producing one more litter of young at the end of the season, versus instead storing fat and reproducing the following spring are not as determinate as are the effects of date of birth on offspring fitness. Females most often gain fitness by not producing one last litter and instead over‐wintering. If, however, the overwinter survival of offspring is not influenced by their size at the end of the season, then a female's fitness could be enhanced by producing one more litter late in the season. If instead the probability of overwinter survival is strongly influenced by the size of offspring at the end of the season, then our results suggest that a female gains more by deferring reproduction and storing for overwinter survival and reproduction the following spring.     

Landscape context influences the abundance of amphibians and the strength of their food web interactions in small ponds

Many species, including most amphibians, undergo an ontogenetic niche shift (ONS) from an aquatic larval stage to a terrestrial adult stage. We use the ratio of aquatic to terrestrial habitat in a landscape as a tool to understand the influence of landscape context on the population growth of ONS species. The aquatic to terrestrial ratio (ATR) of habitats can be viewed as an analog to the influence of resource ratios on the population growth of consumers and depends on the degree to which each habitat type limits the growth of a given population. Population growth rates of shorter‐lived species tend to be more limited by demographic rates in early (aquatic) life stages. As a result, increasing the ATR should lead to a higher total population size in the landscape (and higher densities in the terrestrial habitat), but have little influence on the density of individuals in any given aquatic habitat. Alternatively, population growth rates of longer‐lived species tend to be more limited by demographic rates in later (terrestrial) life stages and increasing the ATR should have little influence on the total population size in the landscape, but decrease the density of individuals in any given aquatic habitat. We show that among‐landscape variation in the breeding‐pond densities of three widespread amphibians with contrasting life histories is consistent with this framework. Within‐pond densities of Pseudacris crucifer, a species with short‐lived adults, were not influenced by ATR, whereas within‐pond densities of Hyla versicolor, a longer‐lived member of the same family (Hylidae), declined as ATR increased. Ambystoma maculatum, a long‐lived salamander, also had lower densities in ponds with higher ATR. Because A. maculatum larvae are important predators in ponds, we use structural equation modeling to show that landscape context (ATR) can moderate community structure via direct (amphibian abundances) and indirect (prey species richness) effects.     

Host‐Plant Choice Behavior at Multiple Life‐Cycle Stages: The Roles of Mobility and Early Growth in Decision‐Making

The choice of food plants often assumes critical importance for a herbivore. Although many studies have investigated host‐plant choice behavior, few have examined preferences (vs. growth and survival) at multiple stages of the life cycle, notwithstanding the importance of identifying the critical stage(s) in an animal’s life history. Fern moths Herpetogramma theseusalis (Lepidoptera: Crambidae) provide an excellent opportunity to test host‐plant choice at several stages. Fern moth larvae feed on distantly related ferns, sensitive Onoclea sensibilis and marsh fern Thelypteris palustris, and adults oviposit on both species. We examined newly hatched larvae, overwintered larvae and ovipositing females to test hypotheses predicting when host‐plant choice takes place (overwintering and mobility hypotheses: overwintering stage determines choice of substrate vs. most mobile stage chooses) and the basis for choice (optimal oviposition and enemy‐free space hypotheses: resource producing highest fecundity vs. lowest losses to enemies). We also evaluated the hypothesis that host‐associated fitness trade‐offs explain host specialization. Only ovipositing females, the most mobile stage, exhibited a clear preference (for marsh fern), consistent with the mobility hypothesis. However, their preference for marsh fern fits neither the optimal oviposition hypothesis nor the enemy‐free space hypothesis; although some larvae initially grew faster on marsh fern, adults reared from the two ferns did not differ significantly in mass and experienced marginally lower parasitism on sensitive fern. Thus, we found no host‐associated fitness trade‐offs. Overwintering losses in marsh fern plots exceeded those in sensitive fern, and mixed plots supported the most overwintered larvae. Preference for marsh fern suggests that early success drives host‐plant choice, an advantage that later disappears. Temporal variability may prevent closer fits to the hypotheses, because both ferns provide the moths with acceptable resources throughout their life cycles.     

Life cycle polyphenism as a factor affecting ecological divergence within <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis>

Polyphenism, which allows a single genotype to express multiple discrete phenotypes in response to environmental cues, is an adaptive trait in heterogeneous environments. Pond hydroperiod is an important ecological parameter affecting amphibian life history, and variation in local pond hydrology has been hypothesized to play a role in species divergence via changes in polyphenism. The eastern newt (Notophthalmus viridescens) expresses life cycle polyphenism. Larvae develop along three possible pathways: metamorphosis to aquatic lunged adult via a terrestrial juvenile (eft) stage, metamorphosis directly to an aquatic lunged adult, or maturation directly to an aquatic gilled adult without metamorphosis (i.e., paedomorphosis). Subspecies of N. viridescens vary in their polyphenic patterns, suggesting possible adaptation to different environments. However, no studies have experimentally tested how genetic and environmental components contribute to the observed differences among subspecies and whether such differences may facilitate divergence. We tested whether adaptation to local pond hydrology via polyphenic changes existed among subspecies by rearing larvae of three subspecies (N. v. dorsalis, N. v. louisianensis, and N. v. viridescens) along three hydroperiod regimes (short, long, and constant) in outdoor artificial ponds. We found that larval N. v. viridescens obligately metamorphosed to efts under all hydroperiods, whereas N. v. dorsalis and N. v. louisianensis exhibited plasticity: larvae metamorphosed to efts under drying conditions but metamorphosed directly to aquatic adults or became paedomorphic in constant water. Also, N. v. viridescens metamorphosed to efts faster and at a smaller body size than the other two subspecies. These data suggest that subspecies of N. viridescens are adapted to different pond hydroperiods, supporting the potential for polyphenism to facilitate divergence. Canalizing selection for certain alternative phenotypes within a single species in which other populations remain plastic may play an important role in the initiation of ecological divergence.     

Differential vulnerability to predation and refuge use in competing larval salamanders

The aquatic larvae of two species of salamanders coexist as a result of differences in their competitive abilities: Ambystoma talpoideum is a superior aggressor, whereas A. maculatum is a superior forager. I examined the behavioral mechanisms that permit these species to coexist with their predatory congener, A. opacum. I asked whether the two prey species differ in their vulnerability to predation and in their use of structural and spatial refugia when under the risk of predation; such inter-specific variation may allow predation to contribute indirectly to prey coexistence. Larval A. maculatum (the superior forager) was more vulnerable to predation by A. opacum than was A. talpoideum, and only the latter species significantly increased its use of structural refugia (leaf litter) in the presence of the predator. In pond enclosures, both species of prey exhibited diel patterns of microhabitat use; significantly more larvae occupied shallow regions of enclosures during the day and migrated to deeper water (a spatial refugium) at night. However, when considered separately, neither (1) the presence of a predatory larval A. opacum nor (2) an increased density of intra- and interspecific competitors significantly altered this habitat shift for either prey species. Rather, diel microhabitat usage in A. talpoideum was significantly affected by an interaction between predator presence and competitor density. My results demonstrate the importance of refugia to coexistence in this predator-prey assemblage. Furthermore, predation by A. opacum may mediate prey competition; that is, preferential consumption of A. maculatum may reduce the competitive impact of this superior forager on A. talpoideum, thus enhancing their coexistence.     

Filling ephemeral ponds affects development and phenotypic expression in Ambystoma talpoideum

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Allee effect in larval resource exploitation in Drosophila: an interaction among density of adults, larvae, and micro-organisms

Abstract 1. Aggregation pheromones can evolve when individuals benefit from clustering. Such a situation can arise with an Allee effect, i.e. a positive relationship between individual fitness and density of conspecifics. Aggregation pheromone in Drosophila induces aggregated oviposition. The aim of the work reported here was to identify an Allee effect in the larval resource exploitation by Drosophila melanogaster, which could explain the evolution of aggregation pheromone in this species. 2. It is hypothesised that an Allee effect in D. melanogaster larvae arises from an increased efficiency of a group of larvae to temper fungal growth on their feeding substrate. To test this hypothesis, standard apple substrates were infested with specified numbers of larvae, and their survival and development were monitored. A potential beneficial effect of the presence of adult flies was also investigated by incubating a varying number of adults on the substrate before introducing the larvae. Adults inoculate substrates with yeast, on which the larvae feed. 3. Fungal growth was related negatively to larval survival and the size of the emerging flies. Although the fungal growth on the substrate was largely reduced at increased larval densities, the measurements of fitness components indicated no Allee effect between larval densities and larval fitness, but rather indicated larval competition. 4. In contrast, increased adult densities on the substrates prior to larval development yielded higher survival of the larvae, larger emerging flies, and also reduced fungal growth on the substrates. Hence, adults enhanced the quality of the larval substrate and significant benefits of aggregated oviposition in fruit flies were shown. Experiments with synthetic pheromone indicated that the aggregation pheromone itself did not contribute directly to the quality of the larval resource. 5. The interaction among adults, micro‐organisms, and larval growth is discussed in relation to the consequences for total fitness.