Effects of group size and pine defence chemicals on Diprionid sawfly survival against ant predation

The defence chemicals and behavioural adaptations (gregariousness and active defensive behaviour) of pine sawfly larvae may be effective against ant predation. However, previous studies have tested their defences against very few species of ants, and few experiments have explored ant predation in nature. We studied how larval group size (groups of 5 and 20 in Neodiprion sertifer and 10, 20 and 40 in Diprion pini) and variation in levels of defence chemicals in the host tree (Scots pine, Pinus sylvestris) affect the survival of sawfly larvae. Food preference experiments showed that ants do eat sawfly larvae, although they are not their most preferred food item. According to our results, ant predation significantly increases the mortality rate of sawfly larvae. Larval mortality was minor on pine tree branches where ant traffic was excluded. We also found that a high resin acid concentration in the host tree significantly decreased the mortality of D. pini larvae when ants were present. However, there was no such relationship between the chemical concentrations of the host tree and larval mortality for N. sertifer. Surprisingly, grouping did not help sawfly larvae against ant predation. Mortality risk was the same for all group sizes. The results of the study seemingly contradict previous understanding of the effectiveness of defence mechanisms of pine sawfly against ant predation, and suggest that ants (Formica exsecta in particular) are effective predators of sawfly larvae.An erratum to this article can be found at     

History,geography and host use shape genomewide patterns of genetic variation in the redheaded pine sawfly (Neodiprion lecontei)

Divergent host use has long been suspected to drive population differentiation and speciation in plant‐feeding insects. Evaluating the contribution of divergent host use to genetic differentiation can be difficult, however, as dispersal limitation and population structure may also influence patterns of genetic variation. In this study, we use double‐digest restriction‐associated DNA (ddRAD) sequencing to test the hypothesis that divergent host use contributes to genetic differentiation among populations of the redheaded pine sawfly (Neodiprion lecontei), a widespread pest that uses multiple Pinus hosts throughout its range in eastern North America. Because this species has a broad range and specializes on host plants known to have migrated extensively during the Pleistocene, we first assess overall genetic structure using model‐based and model‐free clustering methods and identify three geographically distinct genetic clusters. Next, using a composite‐likelihood approach based on the site frequency spectrum and a novel strategy for maximizing the utility of linked RAD markers, we infer the population topology and date divergence to the Pleistocene. Based on existing knowledge of Pinus refugia, estimated demographic parameters and patterns of diversity among sawfly populations, we propose a Pleistocene divergence scenario for N. lecontei. Finally, using Mantel and partial Mantel tests, we identify a significant relationship between genetic distance and geography in all clusters, and between genetic distance and host use in two of three clusters. Overall, our results indicate that Pleistocene isolation, dispersal limitation and ecological divergence all contribute to genomewide differentiation in this species and support the hypothesis that host use is a common driver of population divergence in host‐specialized insects.     

Developmental responses to variable diet composition in a butterfly: the role of nitrogen,carbohydrates and genotype

Understanding the evolution of herbivore–plant interactions requires detailed information on proximate responses to relevant dietary variability and genetic variance, if any, associated with these responses. We measured the behavioral and developmental responses of Pieris rapae larvae to variation in the nitrogen (N) and carbohydrate (CARB) content of chemically‐defined diets, using differences in the average responses of sibling groups to estimate underlying genetic variance. Larval P. rapae responded to dietary reductions in both N and CARB with increased feeding, but these responses were inadequate to compensate for dietary N deficiencies within the range of N found in host plants. As a result, larvae on reduced N diets exhibited lower relative growth rates and longer development times, whereas larvae on reduced CARB diets maintained normal developmental trajectories. We also report evidence for genetic variation underlying (a) compensatory feeding responses to CARB and N availability, (b) N‐driven variation in growth rate and (c) CARB‐driven variation in larval duration. Our results highlight N availability as a key factor in the growth and development of this herbivorous butterfly, with CARB availability being less constraining and sufficiently addressed by changes to larval consumption rate. Furthermore, our study reveals standing genetic variance associated with larval responses to macronutrient availability, suggesting continued potential for herbivore–plant co‐evolution in this system despite a putative history of strong directional selection.     

Host plant influence on chemical defense in conifer sawflies (Hymenoptera: Diprionidae)

Host diet affects the defensive efficacy of Neodiprion sawflies. In laboratory assays with wood ants (Formica obscuripes), secretions from larvae reared on Pinus banksiana were the most repellent, while those from P. resinosa feeders were the least so. This was explained diterpene resin acid content, but not total monoterpene content. The terpene content of regurgitant generally reflected dietary concentrations. Compounds were sequestered nonselectively by larvae. Host-based differences in defense persisted at the behavioral level. P. banksiana feeders regurgitated greater volumes of fluid and were less likely to be disabled or killed by ants in one-on-one interactions than were larvae fed on P. resinosa. The defensive advantages of host diet conflicted with developmental requirements. N. sertifer reared on P. banksiana (the best diet for defense in all cases) had lower cocoon weights (a correlate of fecundity) than those reared on other diets, and had prolonged larval development compared to insects fed P. sylvestris. No such tradeoff was detected in N. lecontei. Larvae of both species strongly preferred P. banksiana over P. resinosa in feeding choice assays.     

Landscape‐scale life‐history gradients in New Zealand freshwater fish

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Induced tree response to herbivory influences the foraging preference and performance of a conifer‐defoliating sawfly

Herbivory often alters the growth and development of woody plants and can thereby render hosts less susceptible to subsequent herbivores. We carried out field surveys and experiments to investigate how previous herbivory influences adult egg lay, larval feeding preference, and associated survival of the yellowheaded spruce sawfly, Pikonema alaskensis (Rohwer) (Hymenoptera: Tenthredinidae), within crowns of black spruce, Picea mariana (Mill.) Britton et al. (Pinaceae). Pikonema alaskensis females laid nearly twice as many eggs, and late‐instar larvae consumed nearly twice as much foliar biomass, on undefoliated vs. defoliated branches. The major factor driving the lower incidence of egg lay on defoliated branches was a significant reduction in the availability of preferred size classes of shoots. In general, adult females preferred medium‐sized shoots, whereas late instars preferred large shoots; previous defoliation generally decreased the availability of medium and large shoots. In a field bioassay, late‐instar survival generally declined with increasing defoliation, which again corresponded to significant reductions in shoot length on defoliated branches. The tendency of P. alaskensis to avoid shoots on severely defoliated branches is likely to increase the time lag between herbivory and the feedback effects of induction and could thus contribute to fluctuations in sawfly population density during outbreak. Our results differ from numerous past studies of closely related diprionid sawflies in conifers, most of which have reported weak or positive effects of previous herbivory on host susceptibility; we attribute the observed responses of spruce to the unusual sawfly feeding preference of P. alaskensis for developing conifer foliage.     

Competitive interactions between parasitoid larvae and the evolution of gregarious development

We report experiments using two closely related species of alysiine braconids directed at understanding how gregarious development evolved in one subfamily of parasitoid wasps. Theoretical models predict that once siblicide between parasitoid wasps has evolved, it can only be lost under stringent conditions, making the transition from solitary to gregarious development exiguous. Phylogenetic studies indicate, however, that gregariousness has independently arisen on numerous occasions. New theoretical models have demonstrated that if gregarious development involves reductions in larval mobility, rather than a lack of fighting ability (as in the older models), the evolution of gregariousness is much more likely. We tested the predictions of the older tolerance models (gregariousness based on non-fighting larval phenotypes) and the reduced mobility models (gregariousness based on non-searching larval phenotypes) by observing larval movement and the outcome of interspecific competition between Aphaereta genevensis (solitary) and A. pallipes (gregarious) under multiparasitism. Differences in larval mobility matched the prediction of the reduced mobility model of gregarious development, with the solitary A. genevensis having larvae that are much more mobile. The proportion of hosts producing the solitary species significantly declined after subsequent exposure to females of the gregarious species. This contradicts the prediction of the older models (fighting vs non-fighting phenotypes), under which any competitive interactions between solitary and gregarious larvae will result in a highly asymmetrical outcome, as the solitary species should be competitively superior. The observed outcome of interspecific competition offers evidence, with respect to this subfamily, in favour of the new models (searching vs non-searching phenotypes).     

Wheat stem sawfly‐infested plants benefit from parasitism of the herbivorous larvae

1 Parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck and their herbivorous host the wheat stem sawfly Cephus cinctus Norton, a pest of wheat Triticum aestivum, were investigated for yield in T. aestivum grown in the field. 2 Wheat stem sawfly‐infested stems had a higher yield potential than uninfested stems. However, final reproductive output was not significantly different between ears on infested stems that supported complete larval development compared with ears on uninfested stems. 3 Stems containing parasitized larvae and stems containing larvae that died before completing their development had a higher mean number of seeds and seed weight, when accounting for number of fertile spikelets of each ear, than either infested with live larvae and uninfested stems. 4 The results obtained suggest that larval feeding prevented infested stems from attaining their yield potential, and that the negative impact of the pest on wheat yield was reduced when late instar sawfly larvae were parasitized. Even though some feeding occurs before parasitism, this early damage has a comparatively low impact on yield. 5 This is the first study to show a yield benefit and enhanced plant fitness due to the wheat stem sawfly parasitoids B. cephi and B. lissogaster. This results from the maintenance of increased seed number and seed weight in the higher yielding stems that are preferentially infested by this pest.     

Effects of neem oil on feeding behaviour and development of the pear sawfly, Caliroa cerasi

Neem oil deterred feeding by pear sawfly, Caliroa cerasi L., larvae (Hymenoptera: Tenthredinidae), both in choice and in no-choice cherry leaf disk bioassays. Deterrence was greater in the choice tests, with 50% inhibition occurring at 0.49% aqueous neem oil compared with 1.11% in the no-choice tests. Antifeedant activity towards pear sawfly larvae is slightly less than has been observed for Lepidoptera, but is higher than deterrence to other insects such as aphids. Topical application of neem oil to sawfly larvae resulted in reduced feeding, increased mortality, and a trend towards slower development. Most larval mortality after neem treatment resulted from incomplete subsequent moults. The potential utilization of neem insecticides for control of pear sawfly in tree fruit pest management is discussed.     

Phylogeography and population genetics of pine butterflies: Sky islands increase genetic divergence

The sky islands of southeastern Arizona (AZ) mark a major transition zone between tropical and temperate biota and are considered a neglected biodiversity hotspot. Dispersal ability and host plant specificity are thought to impact the history and diversity of insect populations across the sky islands. We aimed to investigate the population structure and phylogeography of two pine‐feeding pierid butterflies, the pine white (Neophasia menapia) and the Mexican pine white (Neophasia terlooii), restricted to these “islands” at this transition zone. Given their dependence on pines as the larval hosts, we hypothesized that habitat connectivity affects population structure and is at least in part responsible for their allopatry. We sampled DNA from freshly collected butterflies from 17 sites in the sky islands and adjacent high‐elevation habitats and sequenced these samples using ddRADSeq. Up to 15,399 SNPs were discovered and analyzed in population genetic and phylogenetic contexts with Stacks and pyRAD pipelines. Low genetic differentiation in N. menapia suggests that it is panmictic. Conversely, there is strong evidence for population structure within N. terlooii. Each sky island likely contains a population of N. terlooii, and clustering is hierarchical, with populations on proximal mountains being more related to each other. The N. menapia habitat, which is largely contiguous, facilitates panmixia, while the N. terlooii habitat, restricted to the higher elevations on each sky island, creates distinct population structure. Phylogenetic results corroborate those from population genetic analyses. The historical climate‐driven fluxes in forest habitat connectivity have implications for understanding the biodiversity of fragmented habitats.     

Spatiotemporal occurrence and feeding habits of tonguefish,Cynoglossus lighti Norman, 1925, larvae in Ariake Bay,Japan

Spatiotemporal occurrence and feeding habits of tonguefish (Cynoglossus lighti Norman, 1925) larvae were investigated in an offshore area (>5 m in depth) of the inner part of Ariake Bay, Japan. All specimens were symmetric, free‐swimming larvae. Although their seasonal abundance and distribution in the study site varied from year to year, spawning started in June and the larval abundance was high in August and September with a wide distribution in the inner part of the bay. Both present and previous study results strongly suggest that larvae may settle primarily in the estuary and near‐shore areas of Ariake Bay after their wide distribution in the offshore area of the bay during the free‐swimming stage. Larvae showed a clear feeding rhythm in which they fed on prey mainly during the daytime. Larvae fed exclusively on copepods, and identified prey were mostly Paracalanidae (mainly Parvocalanus crassirostris), Microsetella norvegica, and Oithona davisae. Pre‐metamorphosis larvae fed primarily on Paracalanidae and O. davisae, whereas O. davisae formed a smaller proportion of the early‐metamorphosis diet. In early metamorphoses, larvae fed preferentially on Paracalanidae and M. norvegica.     

Ontogenetic and individual diet variation in amphibian larvae across an environmental gradient

1. Variation among individuals within size or age classes can have profound effects on community dynamics and food‐web structure. We investigated the potential influence of habitat disturbance on intrapopulation niche variation. 2. Amphibians occupy a range of lentic habitats from short‐hydroperiod intermittent ponds to long‐hydroperiod permanent ponds. We quantified ontogenetic diet variation and individual specialisation in wood frog tadpoles (Lithobates sylvaticus) and blue‐spotted salamander larvae (Ambystoma laterale) to investigate the influence of hydroperiod on population niche width across a natural hydroperiod gradient using stable isotope and gut content analyses. In one of the few tests using larval forms, we tested the niche variation hypothesis, which predicts that populations with larger niche widths also have increased individual variation. 3. Our results support the niche variation hypothesis, indicating that more generalised populations exhibit higher within‐individual diet variation. We report gradual changes in the relative importance of diet items, decreased dietary overlap and increased trophic position in L. sylvaticus throughout development. A. laterale became more enriched in δ¹³C and increased in δ¹⁵N throughout its larval period. We did not find a relationship between hydroperiod and niche parameters, indicating that niches are conserved across heterogeneous habitats. In contrast to most documented cases, we estimated low levels of individual specialisation in amphibian larvae. 4. Amphibians are an important link between aquatic and terrestrial ecosystems, whereby diet shifts can influence food‐web structure by altering energy flow pathways and the trophic position of higher consumers, ultimately changing food‐chain length.     

Effects of different food on growth and survival of first‐feeding taimen Hucho taimen (Pallas, 1773) larvae

A study was conducted to compare growth and survival of Hucho taimen larvae from 21 to 76 days after hatch (DAH) fed one of three diets: formulated feed alone (group F); a co‐feeding diet of water fleas, tubifex and formulated feed (group C); or live food of water fleas and tubifex (group L), and to investigate the potential use of dietary L‐alanyl‐L‐glutamine (L‐AG) in larval taimen for a more nutritious starter diet. Triplicate groups of 5000 fish were randomly assigned to each aquarium provided with water from a flow‐through system, and fed to apparent satiation. The results show that larvae can feed efficiently on floating crumbled particles of formulated feed. Weight gain of larvae fed only formulated feed was significantly lower than other groups at 34 DAH (P < 0.05). At the end of the experiment, weight gain reached the highest value in group F and was lowest in group L (P < 0.05). Condition factor reached the highest values in group F and lowest in group C (P < 0.05). Specific growth rate was in accordance with weight gain at 76 DAH. Survival showed no differences among the groups (P > 0.05). In conclusion, H. taimen larvae can be fed formulated feed alone and L‐AG may be used as a feeding attractant during the weaning process, which should lead to a better understanding in the rearing improvement in the feeding of larvae.     

Fitness costs of phoretic nematodes in the burying beetle,Nicrophorus vespilloides

Nicrophorusvespilloides is a social beetle that rears its offspring on decomposing carrion. Wild beetles are frequently associated with two types of macrobial symbionts, mites, and nematodes. Although these organisms are believed to be phoretic commensals that harmlessly use beetles as a means of transfer between carcasses, the role of these symbionts on N. vespilloides fitness is poorly understood. Here, we show that nematodes have significant negative effects on beetle fitness across a range of worm densities and also quantify the density‐dependent transmission of worms between mating individuals and from parents to offspring. Using field‐caught beetles, we provide the first report of a new nematode symbiont in N. vespilloides, most closely related to Rhabditoides regina, and show that worm densities are highly variable across individuals isolated from nature but do not differ between males and females. Next, by inoculating mating females with increasing densities of nematodes, we show that worm infections significantly reduce brood size, larval survival, and larval mass, and also eliminate the trade‐off between brood size and larval mass. Finally, we show that nematodes are efficiently transmitted between mating individuals and from mothers to larvae, directly and indirectly via the carcass, and that worms persist through pupation. These results show that the phoretic nematode R. regina can be highly parasitic to burying beetles but can nevertheless persist because of efficient mechanisms of intersexual and intergenerational transmission. Phoretic species are exceptionally common and may cause significant harm to their hosts, even though they rely on these larger species for transmission to new resources. However, this harm may be inevitable and unavoidable if transmission of phoretic symbionts requires nematode proliferation. It will be important to determine the generality of our results for other phoretic associates of animals. It will equally be important to assess the fitness effects of phoretic species under changing resource conditions and in the field where diverse interspecific interactions may exacerbate or reduce the negative effects of phoresy.     

Influence of induced water stress in ponderosa pine on pine sawflies

Summary Three levels of water stress were induced on pole-size ponderosa pine (Pinus ponderosa) to determine the influence of plant moisture stress on oviposition, survival, and growth of two species of pine sawfly (Neodiprion fulviceps and N. autumnalis). It was found that water stress affected oviposition and subsequent egg survival but not larval development or survival. Stress had a negative effect on early season oviposition (N. fulviceps) and a positive effect on late season oviposition (N. autumnalis). Egg hatch was different between species and years and among treatment levels. Larval development, feeding, and survival were not affected by water stress. Overall, the effect of stress was not sufficient to explain population outbreaks of sawflies. Several hypotheses are presented as possible explanations for the observed experimental results.     

The spatial structure of variation in salamander survival,body condition and morphology in a headwater stream network

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Oviposition choice and larval performance of Neomusotima conspurcatalis on leaflet types of the invasive fern,Lygodium microphyllum

Lygodium microphyllum (Cavanilles) R Brown (Lygodiaceae), Old World climbing fern, invades wildlands in central and southern Florida, USA, and causes considerable habitat disturbance. Efforts to develop an effective biological control strategy have focused on several folivorous insects and a leaf‐galling mite from southeast Asia and Australia. Neomusotima conspurcatalis Warren (Lepidoptera: Crambidae) is a foliage‐feeding moth that specializes on Lygodium species. First released in 2008, N. conspurcatalis successfully established in Florida and is dispersing throughout the L. microphyllum‐invaded range. We investigated whether N. conspurcatalis display any preference (oviposition or larval feeding) between two leaflet morphologies, i.e., fertile spore‐producing vs. sterile leaflets. Furthermore, we examined whether oviposition and no‐choice diets on either fern leaflet type conferred any possible selective advantages (e.g., weight, emergence success) to the offspring. Finally, we analyzed leaf nitrogen and carbon content and leaf volatiles to gauge differences between the leaflet types. In choice and no‐choice scenarios, larvae consume approximately the same amount of leaf area and display no preference between fertile and sterile leaflets. However, females deposited significantly more eggs on fertile than on sterile leaflets. Oviposition choice had no effect on larval development times, pupal weights, or successful eclosure. Nitrogen and carbon content did not differ between fertile and sterile samples, but sterile leaflets contain more 1‐octen‐3‐ol, which may act as an herbivory deterrent. The propagule attack imposed by N. conspurcatalis feeding on fertile leaflets is unlikely to provide adequate control, but could be beneficial as part of a suite of biological control agents and other control methods.     

Does group feeding by toxic prey confer a defensive benefit? Aristolochic acid content,group size and survival of first‐instar pipevine swallowtail (Battus philenor L.) larvae

1. Aggregative feeding of larvae is widespread in the Lepidoptera, and many hypotheses have been proposed to explain the adaptive significance of this trait. Group feeding occurs disproportionately more in species with aposematic, chemically defended larvae compared with species with cryptic, non‐chemically defended larvae, consistent with the hypothesis that group feeding provides an enhanced aposematic signal to natural enemies. Most species characterised as having chemically defended larvae are cryptic during the first instar, when they are most highly aggregated and most vulnerable to predation. 2. The benefits of group feeding in terms of decreased predation were explored for first‐instar larvae of the pipevine swallowtail, Battus philenor, a species that sequesters aristolochic acids from its Aristolochia host plant and exhibits aposematism in later instars and as adults. We found that groups of larvae with experimentally enhanced aristolochic acid content had significantly lower survivorship due to predation both in the field and in laboratory experiments compared with groups of larvae without enhanced chemical defence. 3. A laboratory experiment found that aristolochic acid does not deter the generalist predator Hippodamia convergens. 4. No evidence was found that was consistent with the hypothesis that group feeding and increased sequestered chemical defence interact to decrease rates of larval mortality in non‐aposematic, first‐instar larvae. Future research on chemical defence, aposematism, and aggregative feeding should continue to appreciate that particular chemical defences and feeding behaviours are not universally effective against all natural enemies.     

Quantifying Neodiprion sertifer nucleopolyhedrovirus DNA from insects,foliage and forest litter using the quantitative real‐time polymerase chain reaction

• 1 Neodiprion sertifer nucleopolyhedrovirus (NeseNPV) is widely used as a viral bio‐insecticide against larvae of the European pine sawfly N. sertifer (Geoff.) (Hymenoptera: Diprionidae), which is one of the most harmful defoliators of pines in Northern Europe. A major obstacle to studying this pathogenic virus in nature is the difficulty of confirming and quantifying the presence of NeseNPV.
• 2 In the present study, we developed real‐time polymerase chain reaction (PCR) primers, based on the caspid gene 39 sequence, for the specific and quantitative detection of NeseNPV. The quantitative real‐time PCR (qPCR) assay can detect virus from any substrate tested, including different insect life stages (egg, larval, adult), pine foliage, and litter or ground vegetation. The reproducible detection limit for the real‐time assay is 0.013 pg of viral DNA (0.013×10^?12 g), corresponding to 136 viral genomes or approximately one to seven virus occlusion bodies per sample.
• 3 qPCR is a specific, quantitative, sensitive, reliable and flexible procedure, and is a good supplement to conventional microscopy‐ or bioassay‐based methods for detection of the virus. We have used qPCR to quantify the level of NeseNPV in samples collected in the field after aerial application of the virus, and demonstrated significantly higher virus levels in sawfly larvae from sprayed areas compared with unsprayed control areas 4 weeks after spraying.
• 4 This qPCR assay can be used to determine important aspects of the biology of NeseNPV (e.g. virus levels in different insect life stages and in their microhabitats on pine foliage and in forest litter).