Host plant variation in mortality of the leaf-folding sawfly on the arroyo willow

Abstract. 1. We tested the hypothesis that survival and sources of mortality of the leaf-folding sawfly ( Phyllocolpa sp.) varied significantly among host plants of the arroyo willow ( Salix lasiolepis Bent ham).
2. Survival of the leaf folder differed among field and potted willows in a common environment in two of three cases, and sources of mortality differed among plants in four of five cases.
3. Egg mortality differed among field plants but not among the potted willow plants.
4. Larval mortality and parasitism differed among field and potted willows in 2 years, and appeared to be compensatory mortality sources.
5. Leaf folder density among plants was not generally correlated with percentage of leaf folds with no egg (galls formed but no subsequent oviposition), percentage survival, or percentage mortality, indicating a general lack of density dependence.
6. The proportion of folds with no egg oviposited differed significantly among field and potted plants, and was only correlated with survival or sources of mortality in one of three years.
7. The data support the hypothesis that host plant genotype affects the interaction of the leaf folder with its natural enemies, and thus represents a three trophic level interaction.     

Genetic and phenotypic variation in diet breadth in a generalist herbivore

Summary The genetic and plastic components of polyphagy were investigated in a population ofLymantria dispar, the gypsy moth. A simple genetic experiment assessed the expression of (1) genetic variability in life history traits within each of four environments, (2) genetic variability in diet breadth, expressed as a change in the ranks of family performance across hosts, and (3) homeostasis (equivalent performance by a family across hosts) versus phenotypic plasticity (variable performance by a family across hosts). Sibs from each of 14 families, randomly selected from a single population, were reared on four diets: two natural hosts — chestnut and red oak, and two synthetic hosts — a standard laboratory diet and a low-protein version of this diet. Average population performance, measured in terms of development time and pupal weight, was better on standard laboratory diet than on low-protein diet, and was equal on chestnut and red oak for pupal weight, but better on chestnut oak for development time. Average population performance provided no information about the genetic component of host use ability. The gypsy moth expressed genetic variation in development time within each host environment and in pupal weight within natural host environments. Phenotypic plasticity was expressed by a significant number of families in development time and pupal weight across synthetic hosts and, to a lesser extent, across natural hosts. It was only across natural hosts that genetic variation in diet breadth was expressed, and this was confined to females. Genetic variability in diet breadth may be maintained in this species as a consequence of the unpredictability of its food sources.     

Host plant choice in the comma butterfly-larval choosiness may ameliorate effects of indiscriminate oviposition

In most phytophagous insects, the larval diet strongly affects future fitness and in species that do not feed on plant parts as adults, larval diet is the main source of nitrogen. In many of these insect-host plant systems, the immature larvae are considered to be fully dependent on the choice of the mothers, who, in turn, possess a highly developed host recognition system. This circumstance allows for a potential mother-offspring conflict, resulting in the female maximizing her fecundity at the expense of larval performance on suboptimal hosts. In two experiments, we aimed to investigate this relationship in the polyphagous comma butterfly, Polygonia c-album, by comparing the relative acceptance of low- and medium-ranked hosts between females and neonate larvae both within individuals between life stages, and between mothers and their offspring. The study shows a variation between females in oviposition acceptance of low-ranked hosts, and that the degree of acceptance in the mothers correlates with the probability of acceptance of the same host in the larvae. We also found a negative relationship between stages within individuals as there was a higher acceptance of lower ranked hosts in females who had abandoned said host as a larva. Notably, however, neonate larvae of the comma butterfly did not unconditionally accept to feed from the least favorable host species even when it was the only food source. Our results suggest the possibility that the disadvantages associated with a generalist oviposition strategy can be decreased by larval participation in host plant choice.     

The effects of host plant species and larval density on immune function in the polyphagous moth Spodoptera littoralis

Immune functions are costly, and immune investment is usually dependent on the individual''s condition and resource availability. For phytophagous insects, host plant quality has large effects on performance, for example growth and survival, and may also affect their immune function. Polyphagous insects often experience a large variation in quality among different host plant species, and their immune investment may thus vary depending on which host plant species they develop on. Larvae of the polyphagous moth Spodoptera littoralis have previously been found to exhibit density‐dependent prophylaxis as they invest more in certain immune responses in high population densities. In addition, the immune response of S. littoralis has been shown to depend on nutrient quality in experiments with artificial diet. Here, I studied the effects of natural host plant diet and larval density on a number of immune responses to understand how host plant species affects immune investment in generalist insects, and whether the density‐dependent prophylaxis could be mediated by host plant species. While host plant species in general did not mediate the density‐dependent immune expression, particular host plant species was found to increase larval investment in certain functions of the immune system. Interestingly, these results indicate that different host plants may provide a polyphagous species with protection against different kinds of antagonisms. This insight may contribute to our understanding of the relationship between preference and performance in generalists, as well as having applied consequences for sustainable pest management.     

Host growth conditions regulate the plasticity of horizontal and vertical transmission in Holospora undulata,a bacterial parasite of the protozoan Paramecium caudatum

Abstract.— A parasite might be prohibited from investing simultaneously in horizontal (infection of new hosts) and vertical (infection of the current host's offspring) transmission because of developmental, physiological, or evolutionary costs and constraints. Rather, these constraints may select for adaptive phenotypic plasticity, where the parasite uses the transmission pathway that maximizes transmission in the current ecological and epidemiological conditions. By varying environmental conditions for the host's replication, we investigated the plasticity of vertical and horizontal transmission of Holospora undulata , a micronucleus-specific bacterial parasite of the protozoan Paramecium caudatum . We observed a negative correlation between the host's growth rate and the parasite's investment in horizontal transmission. In rapidly dividing hosts, the parasite remained in the reproductive stage and was passed on vertically to the daughter nuclei during mitotic division of the Paramecium . In contrast, at low or negative growth rates of the host, the parasite's reproductive forms differentiated into infectious forms, the agents of horizontal transmission. Furthermore, in treatments that were initiated with a high proportion of individuals harboring horizontally transmitted infectious forms, rapid replication resulted in a switch back from predominantly horizontal to almost exclusively vertical transmission. These results suggest a trade-off between the efficacies of vertical and horizontal transmission, with the parasite switching to horizontal transmission only if conditions for host replication, and thus vertical transmission, deteriorate.     

Ecosystem consequences of selective feeding of an insect herbivore: palatability–decomposability relationship revisited

1. The relationship between leaf palatability and litter decomposability is critical to understanding the effects of selective feeding by herbivores on decomposition processes, and several studies have reported that there is a positive relationship between them. 2. However, palatability is not always positively correlated with decomposability, because of species‐specific feeding adaptation of herbivores to host plants. Moreover, the effects of selective feeding by herbivores on soil decomposition processes should be understood in terms of the inputs of leaf litter and excrement. 3. The present study examined the relationships between leaf palatability and the decomposability of litter and frass, using Lymantria dispar Linnaeus and 15 temperate deciduous tree species. 4. Larvae of L. dispar exhibited a clear feeding preference, and subsequently the excreted frass mass differed among tree species. Litter and frass decomposability also differed among tree species, and frass was more rapidly decomposed than litter. There were no positive or negative correlations between palatability and decomposability of litter and frass. 5. These results indicate that L. dispar larvae may accelerate the decomposition process in temperate deciduous forests through selective feeding on plants with relatively low litter decomposability and the production of frass with higher decomposability than the litter.     

Genetic Allee effects on performance, plasticity and developmental stability in a clonal plant

Negative effects of small population size on fitness, so-called Allee effects, may threaten population persistence even in intact habitat remnants. We studied genotypes of 14 isolated populations of the clonal plant Ranunculus reptans, for which molecular genetic (RAPD-) variability is higher for large than for small populations. In a competition-free greenhouse environment vegetative offspring of genotypes from large populations produced more rosettes and flowers, indicating higher fitness. Within-genotype coefficients of variation in performance traits, indicating developmental instability, were lower for genotypes from populations with higher RAPD-variability. In competition with a taller grass, we found relative reduction in leaf length less pronounced for plants from large populations, suggesting higher adaptive plasticity. Our experimental study of a plant with predominantly vegetative reproduction suggests, that negative genetic effects of recent habitat fragmentation, which so far rather were expected in plants with frequent sexual reproduction, are more severe and more common than previously acknowledged.     

Effect of food plant switching by a herbivore on its parasitoid: Cotesia melanoscela development in Lymantria dispar exposed to reciprocal dietary crosses

1. Host plant switching by dispersing early instar lepidopterans could have implications for parasitoid performance, but this possibility has not been evaluated thoroughly. 2. The relative growth rates of Lymantria dispar parasitized by Cotesia melanoscela, and the weight of larvae at the time of parasitoid emergence, were affected most by the second larval food plant consumed. 3. The relative growth rates, pupal weights, weight of larva at the time of parasitoid emergence, and development times of L. dispar were affected significantly by the second larval food plant consumed. 4. Development time and size of Cotesia melanoscela were affected most by the second larval food plant consumed. 5. Parasitoid performance was affected most by the larval host’s relative growth rate and the final weight of the host larva at the time of parasitoid emergence. 6. Host plant switching affected the weight of L. dispar larvae at the time of parasitoid emergence, but the effect of switching per se was not a significant factor in C. melanoscela size or development. 7. Lymantria dispar larvae that fed on Populus as their second host outperformed larvae that fed ultimately on Acer. 8. Parasitoids yielded from L. dispar larvae that fed ultimately on Populus outperformed parasitoids yielded from larvae that fed ultimately on Acer. 9. Per cent mortality of L. dispar due to parasitism and percentage adult C. melanoscela emergence were highest in parasitized larvae fed Populus, poor in hosts fed Acer, and intermediate in switching larvae.     

Toxicity of acephate to larvae of gypsy moth as a function of host plant and bioassay method

We examined toxicity of acephate to third-instar gypsy moth, Lymantria dispar (L.) (Lepidoptera: Lymantriidae), under different conditions of administration method, availability of food to larvae during bioassay, host plant, and activity of detoxifying enzymes. Larvae that had been fed field-collected foliage of white alder (Alnus rhombifolia Nutt.) were less susceptible 48 h after treatment with topically applied acephate if they were allowed to continue feeding on foliage during the bioassay period (LD₅₀= 60.6 μg/g larva ) than if they were not (LD₅₀= 13.5 μg/g larva ). All surviving larvae were replaced on their original food plant after the 48-h bioassay; of these, 14.4% of the larvae not fed during treatment died before pupation, compared with 1.3% of the larvae fed alder during treatment. The LD₅₀ obtained for topically treated larvae reared and treated on Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, (51.1 μg/g larva) was comparable to that obtained for larvae fed alder (60.0 μg/g larva) throughout treatment. Larvae treated orally with acephate, however, were slightly more susceptible when reared on Douglas-fir (LC₅₀, 20.3 ppm ) than when reared on alder (LC₅₀, 27.0 ppm ). Post-treatment mortality in orally treated larvae was 10.3% in those fed alder and 9.5% in those fed Douglas-fir. Higher cytochrome P-450 activities in larvae reared on Douglas-fir apparently did not enhance tolerance to acephate. Both sexes of orally treated larvae took significantly longer to pupate than did controls on both foliage types, as did topically treated males fed Douglas-fir. Pupal weight generally was slightly, but not always significantly, higher in treated than untreated larvae under all dietary and treatment regimes.     

Functional changes in the anatomy of the pharyngeal jaw apparatus of Astatoreochromis alluaudi (Pisces, Cichlidae), and their effects on adjacent structures

Organisms are tightly packed with structures so architectonic interdependency of structures is an obvious aspect of integration. This aspect of functional morphology, however, has received remarkably little attention. The present paper presents an example of the spatial relations among several apparatuses in the head of the cichlid fish, Astatoreochromis alluaudi. It investigates the transformations of these apparatuses and their functions due to a change in the pharyngeal jaw apparatus resulting from a functional shift (insect eating to snail crushing or vice versa ). The volume of the pharyngeal jaw apparatus differs 55% between the insect eating- and the snail eating morph. The increase in volume of the pharyngeal jaw apparatus has an impressive number of spatial effects, both direct and indirect, on other structures. Reallocation of space within the pharyngeal jaw apparatus occurs. Total head volume increases 31% but a reallocation of space is still necessary as the increase of the opercular compartment where the pharyngeal jaw apparatus is situated compensates for only 59% of the volume increase of that. Not all spatial effects do impose constraints. Spatial constraints are avoided when one of the apparatuses can use a topographically different volume of space. The respiratory apparatus shows internal reallocations of space without loss of total volume. The same solution occurs for elements of the expansion apparatus and the buccal savity. The eyes are not influenced. Finally spatial effects can have positive repercussions. The muscles of the oral jaw apparatus increase in size. This may be an example of an epiphenomenon.     

Genotypic and phenotypic variation in transmission traits of a complex life cycle parasite

Characterizing genetic variation in parasite transmission traits and its contribution to parasite vigor is essential for understanding the evolution of parasite life‐history traits. We measured genetic variation in output, activity, survival, and infection success of clonal transmission stages (cercaria larvae) of a complex life cycle parasite (Diplostomum pseudospathaceum). We further tested if variation in host nutritional stage had an effect on these traits by keeping hosts on limited or ad libitum diet. The traits we measured were highly variable among parasite genotypes indicating significant genetic variation in these life‐history traits. Traits were also phenotypically variable, for example, there was significant variation in the measured traits over time within each genotype. However, host nutritional stage had no effect on the parasite traits suggesting that a short‐term reduction in host resources was not limiting the cercarial output or performance. Overall, these results suggest significant interclonal and phenotypic variation in parasite transmission traits that are not affected by host nutritional status.     

Morphological variation of Aechmea distichantha (Bromeliaceae) in a Chaco forest: habitat and size-related effects

Plants show different morphologies when growing in different habitats, but they also vary in their morphology with plant size. We examined differences in sun- and shade-grown plants of the bromeliad Aechmea distichantha with respect to relationships between plant size and variables related to plant architecture, biomass allocation and tank water dynamics. We selected vegetative plants from the understorey and from forest edges of a Chaco forest, encompassing the whole size range of this bromeliad. Plant biomass was positively correlated with most architectural variables and negatively correlated with most biomass allocation variables. Understorey plants were taller and had larger diameters, whereas sun plants had more leaves, larger sheath area, sheath biomass and sheath mass fraction. All tank water-related variables were positively correlated with plant biomass. Understorey plants had a greater projected leaf area, whereas sun plants had higher water content and evaporative area. Plasticity indices were higher for water-related than for allocation variables. In conclusion, there were architectural and biomass allocation differences between sun- and shade-grown plants along a size gradient, which, in turn, affected tank water-related variables.     

Quantifying individual variation in reaction norms: Mind the residual

Phenotypic plasticity is a central topic in ecology and evolution. Individuals may differ in the degree of plasticity (individual‐by‐environment interaction (I × E)), which has implications for the capacity of populations to respond to selection. Random regression models (RRMs) are a popular tool to study I × E in behavioural or life‐history traits, yet evidence for I × E is mixed, differing between species, populations, and even between studies on the same population. One important source of discrepancies between studies is the treatment of heterogeneity in residual variance (heteroscedasticity). To date, there seems to be no collective awareness among ecologists of its influence on the estimation of I × E or a consensus on how to best model it. We performed RRMs with differing residual variance structures on simulated data with varying degrees of heteroscedasticity and plasticity, sample size and environmental variability to test how RRMs would perform under each scenario. The residual structure in the RRMs affected the precision of estimates of simulated I × E as well as statistical power, with substantial lack of precision and high false‐positive rates when sample size, environmental variability and plasticity were small. We show that model comparison using information criteria can be used to choose among residual structures and reinforce this point by analysis of real data of two study populations of great tits (Parus major). We provide guidelines that can be used by biologists studying I × E that, ultimately, should lead to a reduction in bias in the literature concerning the statistical evidence and the reported magnitude of variation in plasticity.     

Evolutionary causes and consequences of consistent individual variation in cooperative behaviour

Behaviour is typically regarded as among the most flexible of animal phenotypic traits. In particular, expression of cooperative behaviour is often assumed to be conditional upon the behaviours of others. This flexibility is a key component of many hypothesized mechanisms favouring the evolution of cooperative behaviour. However, evidence shows that cooperative behaviours are often less flexible than expected and that, in many species, individuals show consistent differences in the amount and type of cooperative and non-cooperative behaviours displayed. This phenomenon is known as ‘animal personality’ or a ‘behavioural syndrome’. Animal personality is evolutionarily relevant, as it typically shows heritable variation and can entail fitness consequences, and hence, is subject to evolutionary change. Here, we review the empirical evidence for individual variation in cooperative behaviour across taxa, we examine the evolutionary processes that have been invoked to explain the existence of individual variation in cooperative behaviour and we discuss the consequences of consistent individual differences on the evolutionary stability of cooperation. We highlight that consistent individual variation in cooperativeness can both stabilize or disrupt cooperation in populations. We conclude that recognizing the existence of consistent individual differences in cooperativeness is essential for an understanding of the evolution and prevalence of cooperation.     

Environmental and genetic interactions reveal FLOWERING LOCUS C as a modulator of the natural variation for the plasticity of flowering in Arabidopsis

The timing of flowering initiation depends strongly on the environment, a property termed as the plasticity of flowering. Such plasticity determines the adaptive potential of plants because it provides phenotypic buffer against environmental changes, and its natural variation contributes to evolutionary adaptation. We addressed the genetic mechanisms of the natural variation for this plasticity in Arabidopsis thaliana by analysing a population of recombinant inbred lines derived from Don‐0 and Ler accessions collected from distinct climates. Quantitative trait locus (QTL) mapping in four environmental conditions differing in photoperiod, vernalization treatment and ambient temperature detected the folllowing: (i) FLOWERING LOCUS C (FLC) as a large effect QTL affecting flowering time differentially in all environments; (ii) numerous QTL displaying smaller effects specifically in some conditions; and (iii) significant genetic interactions between FLC and other loci. Hence, the variation for the plasticity of flowering is determined by a combination of environmentally sensitive and specific QTL, and epistasis. Analysis of FLC from Don identified a new and more active allele likely caused by a cis‐regulatory deletion covering the non‐coding RNA COLDAIR. Further characterization of four FLC natural alleles showed different environmental and genetic interactions. Thus, FLC appears as a major modulator of the natural variation for the plasticity of flowering to multiple environmental factors.     

Host use of a hemiparasitic plant: no trade-offs in performance on different hosts

To examine putative specialization of a hemiparasitic plant to the most beneficial host species, we studied genetic variation in performance and trade-offs between performance on different host species in the generalist hemiparasite, Rhinanthus serotinus. We grew 25 maternal half-sib families of the parasite on Agrostis capillaris and Trifolium pratense and without a host in a greenhouse. Biomass and number of flowers of the parasite were the highest when grown on T. pratense. There were significant interactions between host species and R. serotinus seed-family indicating that the differences in performance on the two hosts and without a host varied among the families. However, we found no significant negative correlations between performance of R. serotinus on the host species or between performance on the two hosts and autotrophic performance. Thus, the genetic factors studied here are not likely to affect the evolution of specialization of R. serotinus to the most beneficial host.     

Does the age of the host plant modulate migratory activity of Plutella xylostella?

Ephemeral plants and most tropical crops are available to herbivore insects as irregular and unpredictable patches of resources. Insects that exploit patched habitats usually have well‐developed migratory ability, and migration is a fundamental process in the cycle of colonization and extinction of the local populations. We ask here whether Plutella xylostella (Lepidoptera: Yponomeutidae) uses the aging process of the host plant to modulate its migratory activity. The insect was reared in the laboratory on the leaves from the middle estratum (fourth expanded leaves) of cabbage (Brassica oleraceae var. capitata) of various ages, and on leaves from three strata of the mature plant. Females that spent their larval phase in young and tender plants lived for a shorter period of time and laid eggs immediately after metamorphosis. In contrast, mature plants, particularly the new leaves from the superior stratum, favored the development of adult phenotypic traits that occur in migratory forms, such as reduced body size, increased longevity and delayed reproductive activity. We suggest that the lesser nutritional quality and the short temporal persistence of mature plants are selective forces favoring individuals that are better prepared to abandon their habitat soon after emergence. Plutella xylostella may use the predictable ontogenetic changes of the ephemeral host plant to modulate its physiological and behavioral migratory ability.     

Development, fecundity and survival of the herbivore Lymantria dispar and the number of plant species in its diet

ABSTRACT.

• 1 Larval gypsy moth Lymantria dispar (L.) mortality depends upon the number and type of plant species consumed as well as the sequence in which they are consumed.
• 2 When a two-species diet is composed of a favourable and an unfavourable species, larval mortality is higher on the two-species diet than on the diet of the favourable species alone.
• 3 When a two species diet is composed of a favourable and unfavourable coniferous species, larval mortality is higher on the single conifer species diet than on the two-species diet.
• 4 Two-species diets, in which one species is a conifer, produce individuals with a greater fecundity than diets of either species alone.
• 5 The influence of diets composed of two favourable species, on development, size and fecundity, depends both on the relative quality of each of the two plant species and which of the two is consumed by older instars.

     

Host plant relationships and life cycles of the aphid genusCryptomyzus

The host plant relationships of the aphid genusCryptomyzus were studied by field sampling and laboratory experiments. Host plant suitability and host plant preference were assessed in the laboratory using parthenogenetic females of the summer generations (exules). Differences in reproductive performance and host plant preference were found between closely related taxa ofC. galeopsidis. Host plant preference was correlated with reproductive performance in these forms ofC. galeopsidis. This indicated that evolutionary divergence has occurred and this justifies their status as separate species. It did not prove possible to induce an improvement in the performance of theC. galeopsidis fromLamium galeobdolon onGaleopsis tetrahit, the host plant of a closely related taxon. The life cycles and host plant relationships ofCryptomyzus are defined, including the previously unknown life cycles ofC. heinzei, C. ballotae and two forms ofC. galeopsidis. The results are discussed in relation to host use and their significance for differentiation between closely related aphid taxa.

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Résumé Les relations des pucerons du genreCryptomyzus avec leurs plantes hôtes ont été étudiées dans la nature et au laboratoire. L'adéquation de la plante et le choix de l'insecte ont été examinés au laboratoire sur des femelles parthénogétiques des générations estivales (exules). Les différences de performances reproductives et de choix des hôtes étaient liées étroitement à la position taxonomique deC. galeopsidis; chez ces formes deC. galeopsidis les choix entre hôtes étaient liés aux performances reproductives. Ceci indique qu'il y a eu une divergence évolutive et justifie leur statut d'espèces distinctes. Ceci ne prouve pas qu'il soit possible d'induire une amélioration des performances deC. galeopsidis deLamium galeobdolon surGaleopsis tetrahit, plante hôte d'un taxon voisin. Les cycles et les relations avec les plantes hôtes de tous les taxa deCryptomyzus ont été précisés, y compris pour les cycles jusqu'alors inconnus deC. heinzei, C. ballotae et de 2 formes deC. galeopsidis. Ces résultats sont discutés en fonction de l'utilisation de l'hôte et de leur signification pour la différenciation de taxa très voisins de pucerons.