Polyphagy and adult body size in geometrid moths

A comparative study of body size evolution in moths: evidence of correlated evolution with feeding and phenology-related traits

Interspecific variation in body size is one of the most popular topics in comparative studies. Despite recent advances, little is known about the patterns and processes behind the evolution of body size in insects. Here, we used a robust data set comprising all geometrid moth species occurring in Northern Europe to examine the evolutionary associations involving body size and several life-history traits under an explicitly phylogenetic framework. We provided new insights into the interactive effects of life-history traits on body size and evidence of correlated evolution. We further established the sequence of trait evolution linking body size with the life-history traits correlated with it. We found that most (but not all) of the studied life-history traits, to some extent, influenced interspecific variation in body size, but interactive effects were uncommon. Both bi- and multivariate phylogenetic analyses indicated that larger species tend to be nocturnal flyers, overwinter in the larval stage, feed on the foliage of trees rather than herbs, and have a generalist feeding behaviour. We found evidence of correlated evolution involving body size with overwintering stage, host-plant growth form, and dietary specialization. The examination of evolutionary transitions within the correlated evolution models signalled that overwintering as larvae commonly preceded the evolution of large sizes, as did feeding on tree foliage and the generalist feeding behaviour. By showing that both body size and all life-history traits correlated with it evolve at very slow rates, we caution against uncritical attempts to propose causal explanations for respective associations based on contemporary ecological settings.

Understanding the joint evolution of dispersal and host specialisation using phytophagous arthropods as a model group

Theory generally predicts that host specialisation and dispersal should evolve jointly. Indeed, many models predict that specialists should be poor dispersers to avoid landing on unsuitable hosts while generalists will have high dispersal abilities. Phytophagous arthropods are an excellent group to test this prediction, given extensive variation in their host range and dispersal abilities. Here, we explore the degree to which the empirical literature on this group is in accordance with theoretical predictions. We first briefly outline the theoretical reasons to expect such a correlation. We then report empirical studies that measured both dispersal and the degree of specialisation in phytophagous arthropods. We find a correlation between dispersal and levels of specialisation in some studies, but with wide variation in this result. We then review theoretical attributes of species and environment that may blur this correlation, namely environmental grain, temporal heterogeneity, habitat selection, genetic architecture, and coevolution between plants and herbivores. We argue that theoretical models fail to account for important aspects, such as phenotypic plasticity and the impact of selective forces stemming from other biotic interactions, on both dispersal and specialisation. Next, we review empirical caveats in the study of this interplay. We find that studies use different measures of both dispersal and specialisation, hampering comparisons. Moreover, several studies do not provide independent measures of these two traits. Finally, variation in these traits may occur at scales that are not being considered. We conclude that this correlation is likely not to be expected from large-scale comparative analyses as it is highly context dependent and should not be considered in isolation from the factors that modulate it, such as environmental scale and heterogeneity, intrinsic traits or biotic interactions. A stronger crosstalk between theoretical and empirical studies is needed to understand better the prevalence and basis of the correlation between dispersal and specialisation.

Gut microbiota metabolic potential correlates with body size between mulberry-feeding lepidopteran pest species

BACKGROUND

Many insect pests rely on microbial symbionts to obtain nutrients or for defence, thereby allowing them to exploit novel food sources and degrade environmental xenobiotics, including pesticides. Although Lepidoptera is one of the most diverse insect taxa and includes important agricultural pests, lepidopteran microbiotas, particularly functional traits, have not been studied widely. Here, we provide a comprehensive characterization of the gut microbiota across multiple mulberry-feeding lepidopteran species, resolving both community structure and metabolic potential.

RESULTS

Our results indicate abundant bacteria inside the gut of larval Lepidoptera. However, even though they were fed the same diet, the structures of the bacterial communities differed in four major mulberry pest species, suggesting host-specific effects on microbial associations. Community-level metabolic reconstructions further showed that although taxonomic composition varied greatly, carbohydrate and amino acid metabolism and membrane transporter were key functional capabilities of the gut bacteria in all samples, which may play basic roles in the larval gut. In addition, principal coordinate analysis (PCoA) of gut bacterial-predicted gene ontologies revealed specialized features of the microbiota associated with these mulberry pests, which were divided into two distinct clusters (macrolepidopterans and microlepidopterans). This pattern became even more prominent when further Lepidoptera species were involved.

CONCLUSIONS

A suite of gut microbiota metabolic functions significantly correlated with larval size; the metabolism of terpenoids and polyketides, xenobiotics biodegradation and metabolism were specifically enriched in large species, while small larvae had enhanced nucleotide metabolism. Our report paves the way for uncovering the correlation between host phenotype and microbial symbiosis in this notorious insect pest group. © 2019 Society of Chemical Industry.

Effects of Temperature and Photoperiod on the Immature Development in Cassida rubiginosa Müll. and C. stigmatica Sffr. (Coleoptera: Chrysomelidae)

Tortoise beetles (Cassida and related genera) are a large cosmopolitan group that includes several pests of agricultural crops and natural enemies of weeds but their biology and ecology remain poorly known. Using a set of environmental chambers, we address simultaneous effects of temperature and photoperiod on immature development and adult body mass in two European species, C. rubiginosa and C. stigmatica. Consistent with its broader distribution range, the former species is less susceptible to low rearing temperatures, develops faster and has a larger body mass than the latter. However, C. rubiginosa seems to be less adapted to late-season conditions as a short-day photoperiod accelerates its immature development to a lesser extent than it does in C. stigmatica, which nevertheless results in greater larval mortality and slightly but significantly smaller adults. By contrast, in C. stigmatica, which is more likely to encounter late-season conditions due to its slower life cycle, short-day acceleration of development is achieved at no cost to survivorship and final body mass. The experiment with C. stigmatica was repeated during two consecutive years with different methods and the main results proved to be well reproducible. In addition, laboratory results for C. rubiginosa agree with field data from literature.

Pheromone production, male abundance, body size, and the evolution of elaborate antennae in moths

The males of some species of moths possess elaborate feathery antennae. It is widely assumed that these striking morphological features have evolved through selection for males with greater sensitivity to the female sex pheromone, which is typically released in minute quantities. Accordingly, females of species in which males have elaborate (i.e., pectinate, bipectinate, or quadripectinate) antennae should produce the smallest quantities of pheromone. Alternatively, antennal morphology may be associated with the chemical properties of the pheromone components, with elaborate antennae being associated with pheromones that diffuse more quickly (i.e., have lower molecular weights). Finally, antennal morphology may reflect population structure, with low population abundance selecting for higher sensitivity and hence more elaborate antennae. We conducted a phylogenetic comparative analysis to test these explanations using pheromone chemical data and trapping data for 152 moth species. Elaborate antennae are associated with larger body size (longer forewing length), which suggests a biological cost that smaller moth species cannot bear. Body size is also positively correlated with pheromone titre and negatively correlated with population abundance (estimated by male abundance). Removing the effects of body size revealed no association between the shape of antennae and either pheromone titre, male abundance, or mean molecular weight of the pheromone components. However, among species with elaborate antennae, longer antennae were typically associated with lower male abundances and pheromone compounds with lower molecular weight, suggesting that male distribution and a more rapidly diffusing female sex pheromone may influence the size but not the general shape of male antennae.

To speciate, or not to speciate? Resource heterogeneity, the subjectivity of similarity, and the macroevolutionary consequences of niche-width shifts in plant-feeding insects

Coevolutionary studies on plants and plant-feeding insects have significantly improved our understanding of the role of niche shifts in the generation of new species. Evolving plant lineages essentially constitute moving islands and archipelagoes in resource space, and host shifts by insects are usually preceded by colonizations of novel resources. Critical to hypotheses concerning ecological speciation is what happens immediately before and after colonization attempts: if an available plant is too similar to the current host(s), it simply will be incorporated into the existing diet, but if it is too different, it will not be colonized in the first place. It thus seems that the probability of speciation is maximized when alternative hosts are at an 'intermediate' distance in resource space. In this review, I wish to highlight the possibility that resource similarity and, thus, the definition of 'intermediate', are subjective concepts that depend on the herbivore lineage's tolerance to dietary variation. This subjectivity of similarity means that changes in tolerance can either decrease or increase speciation probabilities depending on the distribution of plants in resource space: insect lineages with narrow tolerances are likely to speciate by 'island-hopping' on young, species-rich plant groups, whereas more generalized lineages could speciate by shifting among resource archipelagoes formed by higher plant taxa. Repeated and convergent origins of traits known to broaden or to restrict host-plant use in multiple different insect groups provide opportunities for studying how tolerance and resource heterogeneity may interact to determine speciation rates.

Interactions between ecological traits and host plant type explain distribution change in noctuid moths

The ecological traits of species determine how well a species can withstand threats to which it is exposed. If these predisposing traits can be identified, species that are most at risk of decline can be identified and an understanding of the processes behind the declines can be gained. We sought to determine how body size, specificity of larval host plant, overwintering stage, type of host plant, and the interactions of these traits are related to the distribution change in noctuid moths. We used data derived from the literature and analyzed the effects of traits both separately and simultaneously in the same model. When we analyzed the traits separately, it seemed the most important determinants of distribution change were overwintering stage and type of host plant. Nevertheless, ecological traits are often correlated and the independent effect of each trait may not be seen in analyses in which traits are analyzed separately. When we accounted for other correlated traits, the results were substantially different. Only one trait (body size), but 3 interactions, explained distribution change. This finding suggests that distribution change is not determined by 1 or 2 traits; rather, the effect of the traits depends on other interacting traits. Such complexity makes it difficult to understand the processes behind distribution changes and emphasizes the need for basic ecological knowledge of species. With such basic knowledge, a more accurate picture of the factors causing distribution changes and increasing risk of extinction might be attainable.

Body size and feeding specificity: macrolepidoptera in Britain

Within a geographic assemblage, large-bodied species of macrolepidopteran moths tend, on average, to be less host-specific than small-bodied. Five possible explanations for this pattern are identified, based respectively on (i) phylogenetic relationships between species, (ii) latitudinal gradients in body size and feeding specificity, (iii) the relationship between range size and body size, (iv) larger body size as a buffer from environmental variation, and (v) the relationship between endophagous host associations and small body size. These mechanisms are tested using data for British macrolepidoptera and also evaluated using evidence from the literature at large. Although some of their assumptions are found to be justified, there is no significant support for any single mechanism. This lack of evidence for previously proposed mechanisms is discussed in the light of a recently proposed alternative explanation which combines theories of host quality and host defence mechanisms.Copyright 1998 The Linnean Society of London

Forager size and ecology of Acromyrmex coronatus and other leaf-cutting ants in Costa Rica

I compare forager size and foraging ecology of the leaf-cutting ant Acromyrmex coronatus (Fabricius) with published data on three other leaf-cutter species in Costa Rica, Atta cephalotes (L.), Acromyrmex octospinosus (Reich), and Acromyrmex volcanus Wheeler. Intra-and interspecific differences in forager size in these leaf-cutting ants appear to reflect the economics of harvesting different preferred resources. Ac. coronatus colonies have relatively small foragers (mean mass=3.4±1.4 mg) that cut almost exclusively the thin, soft leaves and other parts of small herbaceous plants. Similarly, small A. cephalotes colonies have small foragers (3.3±1.0 mg) that attack the leaves of small herbaceous plants. In contrast, mature A. cephalotes colonies have a wider sizerange of foragers (7.3±4.1 mg) that primarily attack the leaves of trees, with larger foragers cutting thicker, tougher leaves. In A. cephalotes, the match of forager size to leaf type (both ontogenetically and behaviorally) increases foraging efficiency. Extreme forager polymorphism in mature A. cephalotes colonies appears to broaden the diversity of tree species that they can exploit efficiently. Ac. octospinosus and Ac. volcanus both have large, relatively monomorphic foragers (13.3±4.2 mg and 30.6±4.3 mg, respectively) that typically scavenge for pieces of fallen vegetation, such as dead leaves, fruit, and flowers, in addition to cutting herbs. The large foragers of Ac. octospinosus and Ac. volcanus appear to be well suited as generalist foragers, able to cut or collect any desirable vegetation encountered. Ac. coronatus is similar to A. cephalotes in other ways. Both Ac. coronatus and A. cephalotes establish and maintain cleared trunk trails for foraging, and both have minima workers that "hitchhike" on the loads carried by foragers, apparently serving to protect the larger foragers from attack by phorid flies. Trunk trails and hitchhikers are not known for Ac. octospinosus and Ac. volcanus. That A. coronatus and A. cephalotes show little overlap in geographic distribution within Costa Rica may relate both to differences in habitat requirements and to interspecific competition.