John T. Lill

Host specificity of Lepidoptera in tropical and temperate forests

For numerous taxa, species richness is much higher in tropical than in temperate zone habitats. A major challenge in community ecology and evolutionary biogeography is to reveal the mechanisms underlying these differences. For herbivorous insects, one such mechanism leading to an increased number of species in a given locale could be increased ecological specialization, resulting in a greater proportion of insect species occupying narrow niches within a community. We tested this hypothesis by comparing host specialization in larval Lepidoptera (moths and butterflies) at eight different New World forest sites ranging in latitude from 15° S to 55° N. Here we show that larval diets of tropical Lepidoptera are more specialized than those of their temperate forest counterparts: tropical species on average feed on fewer plant species, genera and families than do temperate caterpillars. This result holds true whether calculated per lepidopteran family or for a caterpillar assemblage as a whole. As a result, there is greater turnover in caterpillar species composition (greater β diversity) between tree species in tropical faunas than in temperate faunas. We suggest that greater specialization in tropical faunas is the result of differences in trophic interactions; for example, there are more distinct plant secondary chemical profiles from one tree species to the next in tropical forests than in temperate forests as well as more diverse and chronic pressures from natural enemy communities.

Feeny revisited: condensed tannins as anti-herbivore defences in leaf-chewing herbivore communities of Quercus

Abstract.  1. Community level oak–tannin–insect patterns have been largely unexplored since Paul Feenys ground‐breaking research. Two hypotheses were tested for Quercus velutina and Q. alba in the Missouri Ozarks: abundance and richness of leaf‐chewing herbivores are negatively correlated with foliar condensed tannin concentrations and variation in condensed tannin concentrations explains variation in herbivore community structure.

The global distribution of diet breadth in insect herbivores

Significance Dietary specialization determines an organism’s resource base as well as impacts on host or prey species. There are important basic and applied reasons to ask why some animals have narrow diets and others are more generalized, and if different regions of the Earth support more specialized interactions. We investigated site-specific host records for more than 7,500 species of insect herbivores. Although host specialists predominate, the proportion of specialists is affected by the diversity of hosts and shifts globally, supporting predictions of more exclusive tropical interactions. These results not only affect our understanding of the ecology of food webs, but also have implications for how they respond to environmental change, as well as for ecosystem management and restoration. Understanding variation in resource specialization is important for progress on issues that include coevolution, community assembly, ecosystem processes, and the latitudinal gradient of species richness. Herbivorous insects are useful models for studying resource specialization, and the interaction between plants and herbivorous insects is one of the most common and consequential ecological associations on the planet. However, uncertainty persists regarding fundamental features of herbivore diet breadth, including its relationship to latitude and plant species richness. Here, we use a global dataset to investigate host range for over 7,500 insect herbivore species covering a wide taxonomic breadth and interacting with more than 2,000 species of plants in 165 families. We ask whether relatively specialized and generalized herbivores represent a dichotomy rather than a continuum from few to many host families and species attacked and whether diet breadth changes with increasing plant species richness toward the tropics. Across geographic regions and taxonomic subsets of the data, we find that the distribution of diet breadth is fit well by a discrete, truncated Pareto power law characterized by the predominance of specialized herbivores and a long, thin tail of more generalized species. Both the taxonomic and phylogenetic distributions of diet breadth shift globally with latitude, consistent with a higher frequency of specialized insects in tropical regions. We also find that more diverse lineages of plants support assemblages of relatively more specialized herbivores and that the global distribution of plant diversity contributes to but does not fully explain the latitudinal gradient in insect herbivore specialization.

Revisiting the evolution of ecological specialization, with emphasis on insect–plant interactions

Ecological specialization is a fundamental and well-studied concept, yet its great reach and complexity limit current understanding in important ways. More than 20 years after the publication of D. J. Futuyma and G. Morenos oft-cited, major review of the topic, we synthesize new developments in the evolution of ecological specialization. Using insect-plant interactions as a model, we focus on important developments in four critical areas: genetic architecture, behavior, interaction complexity, and macroevolution. We find that theory based on simple genetic trade-offs in host use is being replaced by more subtle and complex pictures of genetic architecture, and multitrophic interactions have risen as a necessary framework for understanding specialization. A wealth of phylogenetic data has made possible a more detailed consideration of the macroevolutionary dimension of specialization, revealing (among other things) bidirectionality in transitions between generalist and specialist lineages. Technological advances, including genomic sequencing and analytical techniques at the community level, raise the possibility that the next decade will see research on specialization spanning multiple levels of biological organization in non-model organisms, from genes to populations to networks of interactions in natural communities. Finally, we offer a set of research questions that we find to be particularly pressing and fruitful for future research on ecological specialization.

Impact of plant architecture versus leaf quality on attack by leaf-tying caterpillars on five oak species

Because shelter-building herbivorous insect species often consider structural features of their host plants in selecting construction sites, their probability of attack is likely to be a function of some combination of plant architectural traits and leaf quality factors. We tested the hypothesis that plant architecture, in the form of the number of touching leaves, influences interspecific variation in attack by leaf-tying caterpillars in five species of sympatric Missouri oaks (Quercus). We compared colonization on control branches, in which both architecture and leaf quality were potentially important, with colonization on experimental branches for which we controlled for the effects of architecture by creating equal numbers of artificial ties. Colonization of artificial ties was highly correlated with natural colonization on neighboring control branches, suggesting that leaf quality factors and not architecture influenced interspecific variation in attack by leaf-tying caterpillars. Of the leaf quality factors measured (water, protein-binding capacity, nitrogen, specific leaf area, pubescence, and toughness), nitrogen was the most explanatory. With the exception of white oak, natural leaf tie colonization was positively correlated with nitrogen availability (ratio of nitrogen to protein-binding capacity), and negatively correlated with protein-binding capacity of leaf extracts. Both host plant species and subgenus oak influenced the community composition of leaf-tying caterpillars and the non-tying symbionts colonizing the ties. Host plant differences in leaf nitrogen content were positively correlated with pupal weight of one of two caterpillar species reared on all five host plant species. Thus, interspecific differences in nitrogen, nitrogen availability, and protein-binding capacity of leaf extracts are the best predictors at this time of interspecific differences in attack by leaf-tying caterpillars, in turn affecting their success on individual host plants in the laboratory.

Cascading effects of host size and host plant species on parasitoid resource allocation

1. The bottom‐up factors that determine parasitoid host use are an important area of research in insect ecology. Host size is likely to be a primary cue for foraging parasitoids due to its potential influence on offspring development time, the risk of multiparasitism, and host immunocompetence. Host size is mediated in part by host‐plant traits that influence herbivore growth and potentially affect a herbivores quality as a host for parasitoids.

6 - Microhabitat Manipulation: Ecosystem Engineering by Shelter-Building Insects

Plant-feeding insects that construct shelters on their food plants provide ample opportunities for examining the impacts of allogenic ecosystem engineering on natures most diverse group of organisms, the arthropods. Shelters serve as habitats for a variety of plant-dwelling arthropods that exploit a range of available resources within these constructs. The engineering impact of multiple shelter types constructed on the same plant is another area that warrants investigation. Many plants host a variety of different shelter types as well as other types of plant modifiers. All of these structures have the potential to increase habitat heterogeneity, but it is not known whether their engineering effects are additive or nonadditive. This chapter concludes that the ease of manipulation of these constructs relative to other types of ecosystem engineers holds great promise for addressing both system-specific questions and more general questions posed by theoreticians and those seeking to integrate ecosystem engineering more fully into ecological and evolutionary studies.

Hemocyte Density Increases with Developmental Stage in an Immune-Challenged Forest Caterpillar

The cellular arm of the insect immune response is mediated by the activity of hemocytes. While hemocytes have been well-characterized morphologically and functionally in model insects, few studies have characterized the hemocytes of non-model insects. Further, the role of ontogeny in mediating immune response is not well understood in non-model invertebrate systems. The goals of the current study were to (1) determine the effects of caterpillar size (and age) on hemocyte density in naïve caterpillars and caterpillars challenged with non-pathogenic bacteria, and (2) characterize the hemocyte activity and diversity of cell types present in two forest caterpillars: Euclea delphinii and Lithacodes fasciola (Limacodidae). We found that although early and late instar (small and large size, respectively) naïve caterpillars had similar constitutive hemocyte densities in both species, late instar Lithacodes caterpillars injected with non-pathogenic E. coli produced more than a twofold greater density of hemocytes than those in early instars. We also found that both caterpillar species contained plasmatocytes, granulocytes and oenocytoids, all of which are found in other lepidopteran species, but lacked spherulocytes. Granulocytes and plasmatocytes were found to be strongly phagocytic in both species, but granulocytes exhibited a higher phagocytic activity than plasmatocytes. Our results strongly suggest that for at least one measure of immunological response, the production of hemocytes in response to infection, response magnitudes can increase over ontogeny. While the underlying raison d’ être for this improvement remains unclear, these findings may be useful in explaining natural patterns of stage-dependent parasitism and pathogen infection.

Enemy-Free Space for Parasitoids

ABSTRACT Natural enemies often cause significant levels of mortality for their prey and thus can be important agents of natural selection. It follows, then, that selection should favor traits that enable organisms to escape from their natural enemies into “enemy-free space” (EFS). Natural selection for EFS was originally proposed as a general force in structuring ecological communities, but more recently has become conceptually narrow and is typically only invoked when studying the evolutionary ecology of host plant use by specialized insect herbivores. By confining the application of EFS to specialist herbivores, its potential value to community and evolutionary ecology has been marginalized. As a first step toward exploring the potential explanatory power of EFS in structuring ecological niches of higher trophic-level organisms, we consider host use by parasitoids. Here, we present three distinct mechanisms from our studies of caterpillar host—parasitoid interactions suggesting that parasitoids may be under selection to exploit traits of their hosts and the plants on which those hosts feed to garner EFS for their developing offspring. The neglect of EFS as a top—down selective force on host use by parasitoids may be a serious limitation to basic and applied ecology, given the great diversity of parasitoids and their significance in controlling herbivore populations in both natural and managed ecosystems. Parasitoids and other mesopredators represent excellent candidates for further developments of EFS theory and testing of its broader importance.

Review of Parasitoid Wasps and Flies (Hymenoptera, Diptera) Associated with Limacodidae (Lepidoptera) in North America, with a Key to Genera

Abstract. Hymenopteran and dipteran parasitoids of slug moth caterpillars (Lepidoptera: Limacodidae) from North America are reviewed, and an illustrated key to 23 genera is presented. Limacodid surveys and rearing were conducted during the summer months of 2004–2009 as part of research on the ecology and natural history of Limacodidae in the mid-Atlantic region of the U.S.A. Parasitoid rearing involved a combination of collecting naturally occurring larvae in the field (at least 14 host species) and placing out large numbers of “sentinel” larvae derived from laboratory colonies of three host species. Species in the following families are documented from limacodids in North America as primary or secondary parasitoids (number of genera for each family in parentheses; number of genera included in key but not reared through this research in brackets): Chalcididae ([1]; Hymenoptera: Chalcidoidea), Eulophidae (3; Chalcidoidea), Pteromalidae ([1]; Chalcidoidea), Trichogrammatidae (1; Chalcidoidea), Braconidae (3 [1]; Hymenoptera: Ichneumonoidea), Ichneumonidae (7 [3]; Ichneumonoidea), Ceraphronidae (1; Hymenoptera: Ceraphronoidea), Trigonalidae (2; Hymenoptera: Trigonaloidea), Bombyliidae ([1]; Diptera: Asilioidea), and Tachinidae (3; Oestroidea). We recovered 20 of 28 genera known to attack limacodids in North America. Records discerned through rearing in the mid-Atlantic region are augmented with previously published host-parasitoid relationships for Limacodidae in North America north of Mexico. New records are reported for the following parasitoids (total new records in parentheses): Uramya limacodis (Walker) (1), U. pristis (Townsend) (5), Austrophorocera spp. (6), Ceraphron sp. (1), Alveoplectrus lilli Gates (1), Playplectrus americana (Girault) (10), Pediobius crassicornis (Thomson) (1), Trichogramma (1), Mesochorus discitergus (Say) (1), Hyposoter fugitivus (Say) (1), and Isdromas lycaenae (Howard) (5). The male of Platyplectrus americana (Hymenoptera: Eulophidae) is redescribed, and the female is described for the first time. Incidental and miscellaneous host-parasitoid associations are discussed, and it is concluded that most of these records are likely parasitoids of contaminants accidentally introduced during the limacodid rearing process. Triraphis eupoeyiae (Ashmead), new combination, is transferred from Rogas (Hymenoptera: Braconidae).