On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.     

Increased moth herbivory associated with environmental stress of pinyon pine at local and regional levels

 Using 6 years of observational and experimental data, we examined the hypothesis that water and nutrient stress increase the susceptibility of pinyon pine (Pinus edulis) to the stem- and cone-boring moth (Dioryctria albovittella). At two geographic levels, a local scale of 550 km² and a regional scale of 10,000 km², moth herbivory was strongly correlated with an edaphic stress gradient. At a local scale, from the cinder soils of Sunset Crater to nearby sandy-loam soils, nine of ten soil macro- and micronutrients, and soil water content were lowest in cinder-dominated soils. Herbivore damage was six times greater on trees growing in the most water and nutrient deficient site at Sunset Crater compared to sites with well-developed soils. Percentage silt-clay content of soil, which was highly positively correlated with soil nutrient and soil moisture at a local scale, accounted for 56% of the variation in herbivory at a regional scale among 22 sites. Within and across sites, increased stem resin flow was positively associated with reduced moth attack. On the basis of moth distribution across a stress gradient, we predicted that pinyons growing in highly stressful environments would show increased resistance to herbivores if supplemented with water and/or nutrients. We conducted a 6-year experiment at a high-stress site where individual trees received water only, fertilizer only, and water + fertilizer. Relative to control trees, stem growth and resin flow increased in all three treatments, but only significantly in the water + fertilizer treatment. Although there was no significant difference in herbivore damage among these three treatments, there was an overall reduction in herbivore damage on all treatment trees combined, compared to control trees. This experiment suggests that release from stress leads to increased resistance to insect attack and is consistent with our observational data. While other studies have predicted that short-term stress will result in herbivore outbreaks, our studies extend this prediction to chronically stressed host populations. Finally, while flush-feeders are not predicted to respond positively to stressed host plants, we found a positive association between herbivore attack and stressed pinyon populations. Received: 25 December 1995 / Accepted: 15 August 1996     

The influence of extrafloral nectaries on parasitism of an insect herbivore

The extrafloral nectaries of many plants promote ant defense against insect herbivores. We examined the influence of extrafloral nectaries on the levels of parasitism of a generalist insect herbivore, the gypsy moth (Lymantria dispar L.). Larvae and pupae of the moth were collected from trees with and without extrafloral nectaries growing in the same forests in South Korea and reared to evaluate parasitism. More parasitism occurred on plants with extrafloral nectaries in seven of the nine season-long collections at the six sites and in four out of five collecting periods. Parasitism was higher on the four main genera of plants with extrafloral nectaries than on any of five main genera of plants without extrafloral nectaries. There was no difference in parasitoid richness; nine species occurred in each group, eight of which were the same. There was a positive and almost significant correlation between the abundance of plants with extrafloral nectaries and the parasitism of gypsy moth at the sites. Extrafloral nectaries may reduce herbivory by inducing more parasitism of the insect herbivores that attack plants bearing the glands.     

Predicting the outcomes of a tri‐trophic interaction between an indigenous parasitoid and an exotic herbivorous pest and its host plants

Understanding the novel ecological interactions that result from biological invasions is a critical issue in modern ecology and evolution as well as pest management. Introduced herbivorous insects may interact with native plants and indigenous natural enemies, creating novel tri‐trophic interactions. To help predict the potential outcomes of novel interactions, we investigated the behavioural and physiological responses of an indigenous generalist parasitoid (Habrobracon gelechiae) to an introduced generalist herbivore (the light brown apple moth, Epiphyas postvittana) and its new host plants in California. We first examined the parasitoid's host location and acceptance on a range of nine common host plants of the moth representing distinctly different geographic origins and morphologies (to examine the effect of a known toxic plant on the parasitoid's performance, an additional toxic plant species was also tested that the moth consumes in the laboratory but does not naturally attack). The parasitoid was able to locate the host larvae on all plants equally well, although clutch size was affected by host plant. We then determined fitness of the moth and the parasitoid on four representative plants. The moth larvae suffered higher mortality and a slower developmental rate on the known toxic plant than on the other three plants, but the parasitoid's fitness correlates did not differ between the host food plants. These results show a high level of plasticity in the indigenous generalist parasitoid in its ability to exploit the exotic host on a wide range of host plants, generating an invasion‐driven novel tri‐trophic interaction.     

Evolutionary indirect effects of biological invasions

Just as ecological indirect effects can have a wide range of consequences for community structure and ecosystem function, theory suggests that evolutionary indirect effects can also influence community dynamics and the outcome of species interactions. There is little empirical evidence documenting such effects, however. Here, I use a multi-generation selection experiment in the field to investigate: (1) how the exotic plant Medicago polymorpha and the exotic insect herbivore Hypera brunneipennis affect the evolution of anti-herbivore resistance traits in the native plant Lotus wrangelianus and (2) how observed Lotus evolutionary responses to Hypera alter interactions between Lotus and other members of the herbivore community. In one of two study populations, I document rapid evolutionary changes in Lotus resistance to Hypera in response to insecticide treatments that experimentally reduced Hypera abundance, and in response to Medicago-removal treatments that also reduced Hypera abundance. These evolutionary changes in response to Hypera result in reduced attack by aphids. Thus, an evolutionary change caused by one herbivore species alters interactions with other herbivore taxa, an example of an eco-evolutionary feedback. Given that many traits mediate interactions with multiple species, the effects of evolutionary changes in response to one key biotic selective agent may often cascade through interaction webs to influence additional community members.     

Comparative analysis of quantitative trait loci controlling glucosinolates,myrosinase and insect resistance in Arabidopsis thaliana

Evolutionary interactions among insect herbivores and plant chemical defenses have generated systems where plant compounds have opposing fitness consequences for host plants, depending on attack by various insect herbivores. This interplay complicates understanding of fitness costs and benefits of plant chemical defenses. We are studying the role of the glucosinolate-myrosinase chemical defense system in protecting Arabidopsis thaliana from specialist and generalist insect herbivory. We used two Arabidopsis recombinant inbred populations in which we had previously mapped QTL controlling variation in the glucosinolate-myrosinase system. In this study we mapped QTL controlling resistance to specialist (Plutella xylostella) and generalist (Trichoplusia ni) herbivores. We identified a number of QTL that are specific to one herbivore or the other, as well as a single QTL that controls resistance to both insects. Comparison of QTL for herbivory, glucosinolates, and myrosinase showed that T. ni herbivory is strongly deterred by higher glucosinolate levels, faster breakdown rates, and specific chemical structures. In contrast, P. xylostella herbivory is uncorrelated with variation in the glucosinolate-myrosinase system. This agrees with evolutionary theory stating that specialist insects may overcome host plant chemical defenses, whereas generalists will be sensitive to these same defenses.     

Effect of generalist insect herbivores on introduced Lepidium draba (Brassicaceae): implications for the enemy release hypothesis

The enemy release hypothesis (ERH) states that decreased regulation by natural enemies allows plants to increase in distribution, abundance and vigour following their introduction into an exotic range. Invasive plants rarely escape herbivory entirely, and for hoary cress [ Lepidium draba L. (Brassicaceae)] it has been demonstrated that generalist insect abundance is greater in its introduced North American range than in the native European range. We assessed the role of increased generalist herbivory on hoary cress using representatives of four important herbivore niches commonly found in the introduced range. We experimentally examined the density dependent impact of these herbivores individually and in combination on hoary cress in a series of greenhouse experiments. We found that defoliation of the oligophagous diamondback moth Plutella xylostella (L.) (Lep., Plutellidae) had the strongest and most consistent impact, while damage by the stem-mining weevil Ceutorhynchus americanus Buchanan (Col., Curculionidae) tended to have the highest per capita effect. Plant response to feeding by the oligophagous crucifer flea beetle Phyllotreta cruciferae (Goeze) (Col., Chrysomelidae) was minor despite obvious feeding damage, and the impact of the polyphagous tarnished plant bug Lygus hesperus Knight (Het., Miridae) was negligible. In multiple-species experiments, herbivore impacts were usually additive. In general, we found that hoary cress can tolerate high densities of oligophagous insect herbivory and effectively resisted attack by the polyphagous L. hesperus, but also the oligophagous C. americanus . Our results indicate that a combination of plant resistance and tolerance allows hoary cress to withstand increased generalist herbivore load in its introduced range, consistent with the predictions of the ERH.     

Direct and indirect chemical defence of pine against folivorous insects

The chemical defence of pine against herbivorous insects has been intensively studied with respect to its effects on the performance and behaviour of the herbivores as well as on the natural enemies of pine herbivores. The huge variety of terpenoid pine components play a major role in mediating numerous specific food web interactions. The constitutive terpenoid pattern can be adjusted to herbivore attack by changes induced by insect feeding or oviposition activity. Recent studies on folivorous pine sawflies have highlighted the role of induced pine responses in herbivore attack and have demonstrated the importance of analysing the variability of pine defence and its finely tuned specificity with respect to the herbivore attacker in a multitrophic context.     

Travel costs, oviposition behaviour and the dynamics of insect–plant systems

Insect attack can have major consequences for plant population dynamics. We used individually based simulation models to ask how insect oviposition behaviour influences persistence and potential stability of an herbivore–plant system. We emphasised effects on system dynamics of herbivore travel costs and of two kinds of behaviour that might evolve to mitigate travel costs: insect clutch size behaviour (whether eggs are laid singly or in groups) and female aggregation behaviour (whether females prefer or avoid plants already bearing eggs). Travel costs that increase as plant populations drop lead to inverse density dependence of plant reproduction under herbivore attack. Female clutch size and aggregation behaviours also strongly affect system dynamics. When females lay eggs in large clutches or aggregate their clutches, herbivore damage varies strongly among plants, providing probabilistic refuges that permit plant reproduction and persistence. However, the population dynamics depend strongly on whether insect behaviour is fixed or responds adaptively to plant population size: when (and only when) females increase clutch size or aggregation as plants become rare, refuges from herbivory weaken at high plant density, creating inverse density dependence in plant reproduction. Both herbivore travel costs themselves, and also insect behaviour that might evolve in response to travel costs, can thus create plant density dependence—a basic requirement for regulation of plant populations by their insect herbivores.     

Herbivore‐induced poplar cytochrome P450 enzymes of the CYP71 family convert aldoximes to nitriles which repel a generalist caterpillar

Numerous plant species emit volatile nitriles upon herbivory, but the biosynthesis as well as the relevance of these nitrogenous compounds in plant–insect interactions remains unknown. Populus trichocarpa has been shown to produce a complex blend of nitrogenous volatiles, including aldoximes and nitriles, after herbivore attack. The aldoximes were previously reported to be derived from amino acids by the action of cytochrome P450 enzymes of the CYP79 family. Here we show that nitriles are derived from aldoximes by another type of P450 enzyme in P. trichocarpa. First, feeding of deuterium‐labeled phenylacetaldoxime to poplar leaves resulted in incorporation of the label into benzyl cyanide, demonstrating that poplar volatile nitriles are derived from aldoximes. Then two P450 enzymes, CYP71B40v3 and CYP71B41v2, were characterized that produce aliphatic and aromatic nitriles from their respective aldoxime precursors. Both possess typical P450 sequence motifs but do not require added NADPH or cytochrome P450 reductase for catalysis. Since both enzymes are expressed after feeding by gypsy moth caterpillars, they are likely to be involved in herbivore‐induced volatile nitrile emission in P. trichocarpa. Olfactometer experiments showed that these volatile nitriles have a strong repellent activity against gypsy moth caterpillars, suggesting they play a role in induced direct defense against poplar herbivores.     

Molecular phylogeny and population structure of the codling moth (Cydia pomonella) in Central Europe: I. Ancient clade splitting revealed by mitochondrial haplotype markers

The codling moth (Cydia pomonella L., Tortricidae, Lepidoptera) is an important pest of pome fruit with global distribution. It has adapted successfully to different habitats by forming various ecotypes and populations, often termed strains, which differ among each other in several morphological, developmental, and physiological features. Many strains of Cydia pomonella have developed resistance against a broad range of chemically different pesticides. Obviously, pesticide-resistant strains must have a genetic basis inherent to the gene pool of codling moth populations, and this deserves our particular attention. The primary intention of the present study was to contribute novel information regarding the evolutionary phylogeny and phylogeography of codling moth populations in Central Europe. In addition, we aimed at testing the hypothesis that differential biological traits and response patterns towards pesticides in codling moth populations may be reflected at a mitochondrial DNA level. In particular, we wanted to test if pesticide resistance in codling moths is associated repeatedly and independently with more than one mitochondrial haplotype. To this end, we analyzed mitochondrial DNA and constructed phylogenetic trees based on three mitochondrial genes: cytochrome oxidase I (COI), the A+T-rich region of the control region (CR), and the nicotinamide adenine dinucleotide dehydrogenase subunit 5 (ND5). The results indicate that Central European populations of Cydia pomonella are clearly divided in two ancient clades. As shown by means of a molecular clock approach, the splitting of the two clades can be dated to a time period between the lower and middle Pleistocene, about 1.29-0.20 million years ago. It is assumed that the cyclic changes of warm and cold periods during Pleistocene may have lead to the geographic separation of codling moth populations due to glaciation, giving rise to the formation of the two separate refugial clades, as already shown for many other European animal species. Due to their inclination towards developing novel detoxification gene variants, codling moth individuals from both clades independently and multifariously may have developed pesticide resistance, and this process may be ongoing. During their more recent evolutionary history, natural events such as the gradual disappearance of climate-specific geographic barriers, as well as human-aided dispersal in recent historic times, may have allowed codling moth haplotypes from the original clades to interbreed and completely merge again, creating a globally successful insect species with a gene pool capable of responding to novel selective challenges by rapid adaptation.     

Colonization of a host tree by herbivorous insects under a changing climate

Climate warming has been predicted to increase the abundance of herbivorous insects. Together with concurrent poleward shifts in many insect species this may increase herbivore pressure on plants. However, the manner in which plants at higher latitudes become colonized by herbivorous insects in the future is unknown. We established a translocation experiment using 26 micropropagated silver birch Betula pendula genotypes from six populations originating from 60°N to 67°N, to study the susceptibility of the translocated birches to local herbivores. The birches were planted at three different latitudes in Finland (60°N, 62°N and 67°N). We studied the effect of source population and latitudinal translocation on herbivore density, species richness, and community composition among the genotypes growing in the same environmental conditions in two years; 2011 and 2012. The source population explained the variation in the herbivore density only in 2012, whereas latitudinal translocation did not affect herbivore density. Variation in species richness was not explained by the source population or by the latitudinal translocation. At two of the study sites, the similarity of the herbivore communities among the populations decreased with increasing latitudinal distance of the source populations, possibly because birch populations that grow geographically closer to each other are genetically more similar, and therefore support a more similar composition of the arthropod community. All birch genotypes were colonized by local herbivores, suggesting that as herbivores shift their ranges polewards, they are able to colonize novel host‐plant genotypes. This enables compositional changes in insect communities on their host plants in the future, which in turn, might affect total herbivory and eventually, plant growth.     

Community heterogeneity and the evolution of interactions between plants and insect herbivores

Plant communities vary tremendously in terms of productivity, species diversity, and genetic diversity within species. This vegetation heterogeneity can impact both the likelihood and strength of interactions between plants and insect herbivores. Because altering plant-herbivore interactions will likely impact the fitness of both partners, these ecological effects also have evolutionary consequences. We review several hypothesized and well-documented mechanisms whereby variation in the plant community alters the plant-herbivore interaction, discuss potential evolutionary outcomes of each of these ecological effects, and conclude by highlighting several avenues for future research. The underlying theme of this review is that the neighborhood of plants is an important determinant of insect attack, and this results in feedback effects on the plant community. Because plants exert selection on herbivore traits and, reciprocally, herbivores exert selection on plant-defense traits, variation in the plant community likely contributes to spatial and temporal variation in both plant and insect traits, which could influence macroevolutionary patterns.     

The population ecology of novel plant–herbivore interactions

Many native herbivores are known to attack exotic plants, and we can expect these interactions to occur with increasing frequency in coming years as invasive plants become naturalized and new invaders arrive in native communities. In some cases, evolutionary biologists and ecologists have learned a great deal from insects adapting to novel hosts. However, there is more to be learned and we suggest that the ecological study of exotic host colonization by native insects has been impeded by a lack of focus in the questions being asked, and also from overlap with other areas of plant–insect ecology, including the study of specialization. In the present paper, a conceptual model is described for the colonization of a novel host‐plant, which focuses on the relationship between occupancy and availability. Occupancy is the fraction of patches of novel hosts that are utilized by an herbivore, and availability is the abundance or presence of a novel host on the landscape. Considering the slope of that relationship (between occupancy and availability), hypotheses are suggested that involve dispersal and, most important, population growth rate of an insect herbivore associated with an exotic host. A focus on the occupancy–availability relationship highlights the strengths and weaknesses of common experimental approaches, such as preference–performance experiments. Suggestions for future work are offered that include integration with evolutionary theory and exploration of more complex demographic and ecological scenarios.     

Specific herbivore-induced volatiles defend plants and determine insect community composition in the field

In response to insect attack, plants release complex blends of volatile compounds. These volatiles serve as foraging cues for herbivores, predators and parasitoids, leading to plant-mediated interactions within and between trophic levels. Hence, plant volatiles may be important determinants of insect community composition. To test this, we created rice lines that are impaired in the emission of two major signals, S-linalool and (E)-β-caryophyllene. We found that inducible S-linalool attracted predators and parasitoids as well as chewing herbivores, but repelled the rice brown planthopper Nilaparvata lugens, a major pest. The constitutively produced (E)-β-caryophyllene on the other hand attracted both parasitoids and planthoppers, resulting in an increased herbivore load. Thus, silencing either signal resulted in specific insect assemblages in the field, highlighting the importance of plant volatiles in determining insect community structures. Moreover, the results imply that the manipulation of volatile emissions in crops has great potential for the control of pest populations.     

Effects of insect attack to stems on plant survival,growth, reproduction and photosynthesis

Studies of insect herbivory have mostly focused on leaf‐feeding even though most woody plant biomass is stem tissue. Attack to stems has the potential to be more detrimental to plant performance than attack to leaves. Here we asked how severe is the impact of insect stem herbivory on plant performance. We quantify the effect of insect stem herbivory via a meta‐analysis of 119 papers in 100 studies (papers by the same authors were treated as the same study). These studies involved 92 plant species and 70 species of insect herbivore (including simulated herbivory). Attack to plant stems reduced plant performance by an average of approximately 22%. Stem herbivory had greatest impacts on plant and branch survival, which was reduced by 63%. Measures of plant reproduction and vegetative biomass were reduced by 33% and 16% respectively, while measurements of photosynthetic rate were not significantly different between plants with and without stem herbivore attack. Stem herbivory led to a decline in leader performance but an increase in performance of laterals, highlighting the importance of plant compensation. Juvenile plants were more severely affected by stem herbivory than adult plants, and studies conducted in greenhouses found more severe effects than studies conducted in the field. Stem herbivory did not have a significant effects on any of the non‐performance responses measured (defence compounds, SLA, root:shoot, phenology and plant carbon and nitrogen). We compare our results with results from various meta‐analyses considering herbivory on other plant parts. The impact of insect herbivory to stems on plant performance appears at least as severe as insect herbivory to roots and leaves, if not more.     

Anti-bat tiger moth sounds: Form and function

The night sky is the venue of an ancient acoustic battle between echolocating bats and their insect prey. Many tiger moths (Lepidoptera: Arctiidae) answer the attack calls of bats with a barrage of high frequency clicks. Some moth species use these clicks for acoustic aposematism and mimicry, and others for sonar jamming, however, most of the work on these defensive functions has been done on individual moth species. We here analyze the diversity of structure in tiger moth sounds from 26 spe-cies collected at three locations in North and South America. A principal components analysis of the anti-bat tiger moth sounds reveals that they vary markedly along three axes: (1) frequency, (2) duty cycle (sound production per unit time) and frequency modulation, and (3) modulation cycle (clicks produced during flexion and relaxation of the sound producing tymbal) structure. Tiger moth species appear to cluster into two distinct groups: one with low duty cycle and few clicks per modulation cycle that supports an acoustic aposematism function, and a second with high duty cycle and many clicks per modulation cycle that is con-sistent with a sonar jamming function. This is the first evidence from a community-level analysis to support multiple functions for tiger moth sounds. We also provide evidence supporting an evolutionary history for the development of these strategies. Further-more, cross-correlation and spectrogram correlation measurements failed to support a "phantom echo" mechanism underlying sonar jamming, and instead point towards echo interference.     

Cabbage Seasonal Leaf Quality Mediating the Diamondback Moth Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) Performance

Seasonal variation in plant quality may be intense enough to generate predictable patterns in insect herbivore populations. In order to explain seasonal oscillations in neotropical populations of the diamondback moth Plutella xylostella (L.), we tested the following: (1) if nutritional quality of cabbage (Brassica oleraceae var. capitata), a primary host plant of diamondback moth, adversely affects the performance of this insect in late spring and early summer, when populations decline and go extinct, and (2) if nutritional features of cabbage change with the seasons. We measured the performance of diamondback moth reared on leaves of cabbages grown during the four seasons of the year. Summer plants proved to be worse for the survival of the immature stages and subsequent adult fecundity, but there were no significant differences between the remaining seasons. Our results support the hypothesis that short-lived plants, grown in different seasons of the year in the tropics, have different nutritional and defensive attributes. We analyzed nutritional quality of cabbage leaves from the four seasons, but only total lipids were reduced in summer plants. Neotropical populations of diamondback moth collapse before plant quality decay in the summer. If the diamondback moth is well adapted to the seasonal deterioration of the habitat, including the reduction in the quality of host plants, it is expected that emigration happens before the mortality increases and natality decreases during the summer.     

Neutral and non-neutral factors shape an emergent plant–antagonist interaction

Discerning the mechanisms responsible for emergent evolutionary radiations, community assembly, and the maintenance of diversity is necessary for understanding the evolutionary ecology of species interactions in changing landscapes. These processes can be driven by stochastic (neutral) factors, such as genetic drift, or deterministic (non-neutral) factors, such as the external environment and heritable phenotypic variation. Neutral and non-neutral factors can shape species interactions, but the relative influence of these different processes on antagonistic relationships is not well understood. We leveraged the recent discovery of a novel herbivore (Caloptilia triadicae) on invasive Chinese tallow (Triadica sebifera) to investigate the nature and relative importance of different factors influencing plant–antagonist interactions. We assessed measures of host attributes, herbivore demography and herbivory across the North American range of Triadica according to geography, environmental variation, and host genetic variation. We found that leaf toughness corresponded to genetic variation in Triadica, longitude, and mean temperature. Genetic variation in Triadica was the strongest predictor of herbivore abundance, especially for the early leaf mining stages, though herbivore abundance also corresponded to longitude. Model variables did not explain leaf damage, which was driven by interactions with late-stage larvae. Trends in herbivore demography were not consistent with previously reported geographic patterns of Triadica genetic variation related to tannin defense, but were consistent with patterns revealed by other studies of Triadica phenolic compounds and C:N, as well as low sensitivity of endophagous herbivores to tannins in the absence of parasitoids. Our findings suggest that even simple geographic mosaics of genetic and environmental variation, as well as distance-dependent dispersal, can influence the establishment and trajectory of novel species interactions.     

Integrating ancient patterns and current dynamics of insect–plant interactions: Taxonomic and geographic variation in herbivore specialization

Abstract The search for pattern in the ecology and evolutionary biology of insect–plant associations has fascinated biologists for centuries. High levels of tropical (low-latitude) plant and insect diversity relative to poleward latitudes and the disproportionate abundance of host-specialized insect herbivores have been noted. This review addresses several aspects of local insect specialization, host use abilities (and loss of these abilities with specialization), host-associated evolutionary divergence, and ecological (including “hybrid”) speciation, with special reference to the generation of biodiversity and the geographic and taxonomic identification of “species borders” for swallowtail butterflies (Papilionidae). From ancient phytochemically defined angiosperm affiliations that trace back millions of years to recent and very local specialized populations, the Papilionidae (swallowtail butterflies) have provided a model for enhanced understanding of localized ecological patterns and genetically based evolutionary processes. They have served as a useful group for evaluating the feeding specialization/physiological efficiency hypothesis. They have shown how the abiotic (thermal) environment interacts with host nutrirional suitability to generate “voltinism/suitability” gradients in specialization or preference latitudinally, and geographical mosaics locally. Several studies reviewed here suggest strongly that the oscillation hypothesis for speciation does have considerable merit, but at the same time, some species-level host specializations may lead to evolutionary dead-ends, especially with rapid environmental/habitat changes involving their host plants. Latitudinal gradients in species richness and degree of herbivore feeding specialization have been impacted by recent developments in ecological genetics and evolutionary ecology. Localized insect–plant associations that span the biospectrum from polyphenisms, polymorphisms, biotypes, demes, host races, to cryptic species, remain academically contentious, with simple definitions still debated. However, molecular analyses combined with ecological, ethological and physiological studies, have already begun to unveil some answers for many important ecological/evolutionary questions.