Foliar damage, parasitoids and indirect competition: a test using herbivores of birch

ABSTRACT.

• 1 The hypothesis that leaf-chewing caterpillars and leaf miners indirectly compete via the influence of foliar damage on parasitoids was experimentally tested using the leaf miner Coleophora serratella (L.) and a complex of folivorous caterpillars on birch.
• 2 Separate experiments tested the effects of manipulating artificial or caterpillar-induced leaf damage on parasitism of leaf miners at several spatial scales.
• 3 Parasitism of C. serratella by specialist and generalist parasitoids was independent of the extent of either collateral or leaf-miner damage, whether experimentally or naturally induced.
• 4 Support in the literature for the hypothesis is restricted to a single case, and only on a limited spatial scale; thus, the idea that collateral damage directs parasitoid-mediated competition among herbivores remains highly speculative.

     

Foraging strategies in orb-spinning spiders: Ambient light and silk decorations in Argiope aetherea Walckenaer (Araneae: Araneoidea)

Abstract Many species of orb-spinning spiders construct silk decorations within the structure of the orb-web. The evolutionary significance of these decorations is poorly understood, but the silk decorations of many species reflect UV light, suggesting that they may function to attract insects. In these species, relatively more silk decoration may be required under dimmer light conditions in order to maintain a constant UV-reflecting signal, and hence level of insect attraction. We investigated experimentally whether the orb-spinning spider Argiope aetherea adjusts the amount of silk decoration added to the web according to light conditions. Consistent with the prey-attracting function, we found that spiders adjusted the quantity of silk decoration to their webs, adding more silk decoration when the web was located in dim light rather than bright light.     

Can floral field margins improve pollination and seed production in coriander Coriandrum sativum L. (Apiaceae)?

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Carcass Decoration Changes Web Structure and Prey Capture Rate in an Orb-Web Spider, Cyclosa mulmeinensis (Araneae, Araneidae)

Cyclosa spiders attach prey carcasses as decorations to their webs, but the functions of the carcasses are unclear and controversial. We investigated distinctive features of these webs in the field and conducted prey-capture experiments in the lab using the orb-web spider Cyclosa mulmeinensis. Webs with attached decoration had a significantly narrower mesh width than those without decoration and a higher degree of vertical asymmetry in the web’s shape. In the laboratory, webs without decorations trapped significantly more prey, even though other features of the webs were nearly identical. These results suggest that web decorations do not attract prey in this species, but might play other roles such as blinding predators to the spider’s presence.     

Adaptive trade-offs of leaf folding in Dichomeris caterpillars on goldenrods

Abstract.

• 1 Leaf folding is a common behaviour among caterpillars that has many potential benefits such as creation of a favourable microclimate and protection from predation or dislodgment.
• 2 One cost of leaf folding which has not previously been quantified is the reduction in growth rate that might result from the energetic expenditures of producing silk and applying it to fold leaves.
• 3 On caged goldenrods in field and forest habitats, early-instar Dichomeris leuconotella (Busck) caterpillars that were repeatedly forced to spin new leaf refuges (either folds or webs of silk) actually had higher growth rates than caterpillars left undisturbed.
• 4 This surprising result apparently reflected the fact that disturbed caterpillars chose relatively young green leaves for new refuges, whereas undisturbed caterpillars tended to remain in their webs or folds on ageing leaves.
• 5 In a glasshouse experiment using plants that underwent little senescence, growth rates were unaffected by the amout of refuge-making effort, indicating that the costs of such effort were either negligible or easily compensated for by these caterpillars.
• 6 On glasshouse plants, caterpillars periodically forced to change refuges had higher disappearance rates than those left undisturbed, despite the absence of predators. Wandering or falling from plants is evidently a major hazard of frequent refuge changes.
• 7 Thus, early-instar Dichomeris caterpillars face a trade-off in which risks of predation or loss of contact with the host favour the observed low rate of refuge changes, but in which growth rates could actually be improved with more refuge changes.

     

Host‐size decisions of female parasitoid wasps seeking hidden hosts

1. Choice of host size may play a critical role in parasitoid success, a task that takes on added complications when dealing with concealed hosts, but most such studies of insect behaviour have only taken place in the laboratory. 2. This study investigates the success of a wasp (Alabagrus texanus: Braconidae) in finding host caterpillars Herpetogramma theseusalis (Crambidae) of the most effectively handled size hidden in shelters, in both the field and the laboratory. 3. First, the study tested wasp preference and success in parasitizing large, middle‐sized and small caterpillars (> 5, 3–5, < 3 mm) presented in the open, one at a time, in the laboratory. The wasps attacked (inserted or attempted to insert their ovipositor) a higher proportion of middle‐sized (3–5 mm) caterpillars compared with either small (< 3 mm) or large (> 5 mm) caterpillars. Naïve wasps attacked large caterpillars more often than did experienced wasps. Wasps responded to increasing caterpillar size by increasing the number of legs used to pin their prey rather than by increasing handling time. 4. The frequencies of visits to shelters in the field containing a majority of either large or middle‐sized caterpillars were then compared, followed by a test providing the wasps with similar choices under controlled laboratory conditions. Wasps most frequently visited shelters containing a majority of middle‐sized caterpillars both in the field and under controlled laboratory conditions. 5. The combined results confirmed that the wasps can size‐select their hosts both in the field and in laboratory tests.     

Green‐brown polymorphism in alpine grasshoppers affects body temperature

1. Ectothermic animals depend on external heat sources for pursuing their daily activities. However, reaching sufficiently high temperature can be limiting at high altitudes, where nights are cold and seasons short. We focus on the role of a green‐brown color polymorphism in grasshoppers from alpine habitats. The green‐brown polymorphism is phylogenetically and spatially widespread among Orthopterans and the eco‐evolutionary processes that contribute to its maintenance have not yet been identified.
2. We here test whether green and brown individuals heat up to different temperatures under field conditions. If they do, this would suggest that thermoregulatory capacity might contribute to the maintenance of the green‐brown polymorphism.
3. We recorded thorax temperatures of individuals sampled and measured under field conditions. Overall, thorax temperatures ranged 1.7–42.1°C. Heat up during morning hours was particularly rapid, and temperatures stabilized between 31 and 36°C during the warm parts of the day. Female body temperatures were significantly higher than body temperatures of males by an average of 2.4°C. We also found that brown morphs were warmer by 1.5°C on average, a pattern that was particularly supported in the polymorphic club‐legged grasshopper Gomphocerus sibiricus and the meadow grasshopper Pseudochorthippus parallelus.
4. The difference in body temperature between morphs might lead to fitness differences that can contribute to the maintenance of the color polymorphism in combination with other components, such as crypsis, that functionally trade‐off with the ability to heat up. The data may be of more general relevance to the maintenance of a high prevalence polymorphism in Orthopteran insects.

     

Chemical convergence between a guild of facultative myrmecophilous caterpillars and host plants

1. Ants exert strong selective pressure on herbivorous insects, although some caterpillars can live in symbiosis with them using chemical defensive strategies.
2. We investigated the adaptive resemblance of cuticular hydrocarbons (CHCs) in multitrophic systems involving a guild of facultative myrmecophilous caterpillar species (Lepidoptera: Lycaenidae), tending ants (Hymenoptera: Formicidae), and host plants from three families. We hypothesised that the CHCs of the caterpillars would resemble those of their host plants (chemical camouflage).
3. We analysed CHCs using gas chromatography/mass spectrometry. Morisita's similarity index (SI) was used to compare CHC profiles of caterpillar species with different types of ant associations (commensal or mutualistic), ants, and host plants.
4. We found strong convergence between caterpillars' CHCs and plants, especially for commensal species that do not provide secretion rewards for ants. Moreover, we found unexpected chemical convergence among mutualistic (trophobiotic) caterpillar species that offer caloric reward secretions to ants.
5. These results show that the studied caterpillars acquire CHCs through their diet and that they vary according to host plant species and type of ant association (commensalism or mutualism). This ‘chemical camouflage’ of myrmecophilous caterpillars may have arisen as a defensive strategy allowing coexistence with ants on plants, whereas ‘chemical conspicuousness’ may have evolved in the context of honest signalling between mutualistic partners.
6. We suggest the existence of chemical mimicry among myrmecophilous species, especially between mutualistic caterpillars. Cuticular chemical mixtures can play a key adaptive role in decreasing ant attacks and increasing caterpillar survival in multimodal sensory systems.

     

Invasive plant population and herbivore identity affect latex induction

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Community structure and antifungal activity of actinobacteria in a fungus-growing termite

1. Fungus-growing termites cultivate the fungal mutualist Termitomyces as their main food source; however, how fungus-growing termites protect Termitomyces from threats is still unclear. In this study, we investigated the actinobacterial communities in Odontotermes formosanus individuals and their fungal combs.
2. Moreover, the antifungal activities of the isolated actinobacteria were tested. 16 S rRNA gene sequencing results indicated that the actinobacteria in O. formosanus and its fungal combs belong to 5 classes, 17 orders, 40 families, and 84 genera.
3. The relative abundance of Coriobacteriia in the nymphs, young workers, old workers, and soldiers was higher than that in the queens and fungal combs, and the relative abundance of class Actinobacteria in the queens and the fungal combs was higher than that in the nymphs, young workers, old workers, and soldiers.
4. Based on antifungal bioassays, 3 strains of Amycolatopsis and 2 strains of Streptomyces isolated from old workers had strong inhibitory activity against Xylaria angulosa but weak inhibitory activity against Termitomyces sp.
5. These results indicated that the actinobacteria of O. formosanus may contribute to protecting termite fungal food via their asymmetric antifungal activities.

     

Behaviour of specialist and generalist caterpillars on plantain (Plantago lanceolata)

Abstract.

• 1 Foraging patterns of specialist (Junonia coenia Hubner: Nymphalidae) and generalist (Spilosoma congrua Wlk.: Arctiidae) caterpillars on five genotypes of plantain (Plantago lanceolata L.: Plantaginaceae) were examined in an experimental garden.
• 2 Movement by the specialists reflected declining availability of new leaves. When direct sunlight was available, these caterpillars were usually exposed to it.
• 3 Although the generalists also preferred new leaves, they spent less than 50% of their time on the plantain and changed location more frequently than the specialists. They often hid at the base of plants or under leaves.
• 4 Plant genotype influenced the apparency of the specialists and damage by the herbivores.

     

Effects of predation risk and plant resistance on Manduca sexta caterpillar feeding behaviour and physiology

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Temporal and geographic variation in parasitoid attack with no evidence for ant protection of the Melissa blue butterfly,Lycaeides melissa

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Lack of susceptibility of soil‐inhabiting Platyprepia virginalis caterpillars,a native arctiid,to entomopathogenic nematodes in nature

Entomopathogenic nematodes (EPNs) can kill and regulate populations of soil‐inhabiting insects, but studies evaluating these interactions in native ecosystems are rare. The objective of this study was to examine the effects of EPNs on a non‐agricultural caterpillar, Platyprepia virginalis (Boisduval) (Lepidoptera: Arctiidae), under natural conditions. Platyprepia virginalis caterpillars live in litter on the soil surface feeding beneath bush lupine during summer, autumn, and winter. Initial laboratory assays revealed that the caterpillars were vulnerable to at least two species of EPNs with which they co‐occur in the coastal prairie in northern California (USA). In contrast to laboratory assays, caterpillars survived exposure to prairie soil containing EPNs under natural conditions in field assays. To better understand the divergence between laboratory and field results for this native caterpillar, we used sentinel insects [Galleria mellonella L. (Lepidoptera: Pyralidae)] to identify particular locations where EPNs were present in the field. Platyprepia virginalis caterpillars were caged at these sites but again showed no evidence of susceptibility to EPNs. Platyprepia virginalis caterpillars reduce their exposure to EPNs by spending their time in and above the litter rather than contacting the soil when given the choice in nature. We conclude that P. virginalis is unlikely to serve as a reservoir for EPNs and that nematodes are unlikely to be important mortality factors for P. virginalis in this natural system.     

The effect of host developmental stage at parasitism on sex‐related size differentiation in a larval endoparasitoid

• 1 For their larval development, parasitoids depend on the quality and quantity of resources provided by a single host. Therefore, a close relationship is predicted between the size of the host at parasitism and the size of the emerging adult wasp. This relationship is less clear for koinobiont than for idiobiont parasitoids.
• 2 As size differentiation in host species exhibiting sexual size dimorphism (SSD) is likely to occur already during larval development, in koinobiont larval endoparasitoids the size of the emerging adult may also be constrained based on the sex of the host caterpillar.
• 3 Sex‐specific growth trajectories were compared in unparasitised Plutella xylostella caterpillars and in second and fourth instar hosts that were parasitised by the solitary larval koinobiont endoparasitoid Diadegma semiclausum. Both species exhibit SSD, where females are significantly larger than males.
• 4 Healthy female P. xylostella caterpillars developed significantly faster than their male conspecifics. Host regulation induced by D. semiclausum parasitism depended on the instar attacked. Parasitism in second‐instar caterpillars reduced growth compared to healthy unparasitised caterpillars, whereas parasitism in fourth‐instar caterpillars arrested development. The reduction in growth was most pronounced in hosts producing male D. semiclausum.
• 5 Parasitism itself had the largest impact on host growth. SSD in the parasitoid is mainly the result of differences in growth rate of the parasitoid–host complex producing male and female wasps and differences in exploitation of the host resources. Female wasps converted host biomass more efficiently into adult biomass than males.

     

Drought and stand susceptibility to attacks by the European spruce bark beetle: A remote sensing approach

1. Several time-series analyses have demonstrated that after extreme summer drought bark beetle damage increased. However, studies predicting stand susceptibility over large spatial extents are limited by technical constraints in obtaining detailed, spatially-explicit data on infestation spot occurrence.
2. Using a unique dataset of georeferenced bark beetle infestation data, we tested whether the spatial variation of local growing conditions of forest stands, topography, and landscape variables modified the local occurrence of Ips typographus infestations after a severe hot drought in Central Europe.
3. Bark beetle infestation occurrence depended on soil-related aridity intensity, elevation, slope, and soil conditions. We showed that elevation interacted with growing conditions and topography. At low elevations, spruce forests growing on flat areas and wetter soils were more sensitive to the infestations. On the contrary, forests on steep slopes and soils with low water availability were rarely attacked. At the landscape scale, bark beetle damage increased with host tree cover but decreased with compositional diversity.
4. Our findings are generally consistent with the growth-differentiation balance hypothesis that predicts that trees growing under chronic dry conditions tend to be more resistant against biotic disturbances.
5. Spruce stands at low elevations located in homogeneous landscapes dominated by spruce were those more exposed to bark beetles in the initial phase of a drought-induced outbreak.

     

Signaling by decorating webs: luring prey or deterring predators?

Many organisms convey false signals to mislead their prey orpredators. Some orb-weaving spiders build conspicuous structureson webs called decorations. Web decorations and spider colorationsare both suggested to be important signals involved in interactionsbetween spiders and other organisms. There are several hypothesesabout the functions of signaling by decorations, among whichprey attraction had received much support, but empirical evidenceregarding predator defense is controversial. In this study,we conducted field experiments to investigate the effects ofspider decoration and coloration on insect interception ratesof webs built by Argiope aemula and to evaluate whether presenceof decorations may decrease predation risk of spiders. Decoratedwebs with spiders present had the highest prey interceptionrate, followed by undecorated webs with spiders, and then undecoratedwebs without spiders. Such results indicated that decorationsof Argiope spiders functioned as visual lures, and so did spiders'bright body colorations. In the field, almost all wasp attackevents occurred on medium-sized spiders rather than on largeones. Moreover, medium-sized Arg. aemula on decorated webs receivedfar more attacks than those on undecorated webs. Results ofthis study thus show that the signals conveyed by decorationscan visually lure prey but at the cost of an increased predationrisk. Received 20 March 2007; revised 3 August 2007; accepted 5 August 2007.     

<Emphasis Type="Italic">Argiope bruennichi</Emphasis> shows a drinking-like behaviour in web hub decorations (Araneae,Araneidae)

As stationary predators, araneid spiders that lack protective retreats are especially vulnerable to abiotic influences. Species of the genus Argiope permanently remain at the hub of their orb webs and are thereby exposed to desiccating circumstances. Like other land arthropods, spiders must balance their hygric status. Beside desiccation avoidance behaviours, they can manage this balance by water gain through either liquefied prey items or direct ingestions of free water. Drinking-like behaviours are sparely documented for Araneids. We observed Argiope bruennichi ingesting accumulated water droplets from the silk-overstitched web hub, a part of the web decoration, and subsequently tested whether this behaviour is a regular feature of this species. In 50% of our observations, spiders that had been sprayed with water actively searched the hub decoration for water droplets and ingested them. The behavioural elements were very stereotypic among the tested individuals. Significantly, A. bruennichi females only searched the covered web hubs for water, even though the entire web was moistened. These data suggest that hub decorations of A. bruennichi might have an adaptive significance by helping to maintain a balance of water metabolism, adding yet another element to the spirited debate about the functional significance of web decorations.     

Sex-dependent body mass and respiratory responses on Euschistus heros individuals resistant to imidacloprid

1. In insect-resistant populations, adaptive fitness is intimately related to the physiological responses of males and females to different stressors.
2. By using laboratory-selected imidacloprid-resistant individuals of the Neotropical brown stink bug Euschistus heros, one of the most prevalent pests in soybean fields in the Neotropical region, we evaluated whether resistance to imidacloprid (synergized by piperonyl butoxide—PBO) would differentially affect the longevity, body mass and respiratory responses of E. heros males and females.
3. We used an imidacloprid-susceptible E. heros strain (ImiSusc), whose individuals were reared under controlled and imidacloprid-free conditions for over a decade, to select the PBO-synergized imidacloprid-resistant strain (ImiPBOSel).
4. After eight generations using PBO-synergized imidacloprid, our results revealed that resistant individuals exposed only to dry residues of imidacloprid (tarsal contact) could survive at concentrations as high as 168 μg a.i./cm², which is equivalent to 40-fold the label recommendation for field applications. Furthermore, ImiPBOSel males and females exhibited higher longevity, higher body mass and elevated respiratory activity than ImiSusc individuals.
5. Altogether, the findings described here showed that imidacloprid resistance can select E. heros individuals with better metabolic capacities, which can complicate the management of this pest under field conditions.

     

Preference,performance, and impact of the water-lily aphid on multiple species of duckweed

1. The role of herbivores in driving the structure of freshwater macrophyte communities remains poorly understood in comparison with terrestrial ecosystems. For instance, although duckweed (subfamily Lemnoideae) are globally distributed, can be locally highly abundant and ecologically dominant, and are of growing economic importance, their interactions with herbivores remain understudied.
2. To address how herbivores may impact duckweed species composition, we here experimentally quantify the preference and performance of a common duckweed herbivore, the water-lily aphid (Rhopalosiphum nymphaeae) on four widespread duckweed species.
3. Our two-way choice experiments reveal that aphids display a preference for Spirodela polyrhiza > Landoltia punctata = Lemna minor > > Wolffia brasiliensis. These results are rarely influenced by natal host species.
4. By evaluating the growth of aphid populations on each duckweed species, we find that preference may be adaptive in certain ecological conditions.
5. Quantifying the population growth rate of duckweed growing in the presence and absence of aphids revealed differential tolerance of herbivory across duckweed species.
6. This study shows that aphids, through preferential feeding and significant differential effects on duckweed growth, can have a significant impact on duckweed population dynamics and potentially community composition.