Specialist Herbivores Research Articles

Larval instar‐dependent hemocytes in specialist herbivore Cydalima perspectalis fed on Buxus hyrcana and Buxus microphylla

AbstractThe boxwood moth, Cydalima perspectalis Walker (Lepidoptera: Crambidae), as invasive specialist species in Iran has caused considerable damage in endemic forest stands of Buxus hyrcana Pojark since 2016. Host plant species can alter herbivore–plant interactions through the quantitative and qualitative changes of hemocytes even within a specialist herbivore. To determine the hemocyte variations on different host plants across larval development, the third and sixth instar larvae of C. perspectalis fed on B. hyrcana (native boxwood) and B. microphylla Sieb. and Zucc. (introduced nonnative boxwood) were compared. Total (THC) and differential (DHC) hemocyte count were determined using light, phase‐contrast, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on results, six types of hemocytes were recognized in hemolymph including prohemocytes (PRs), plasmatocytes (PLs), granulocytes (GRs), oenocytoides (OEs) and spherulocytes (SPs) as well as unknown elongated quadrangular cells (elongocytes [ELs], the term has been first employed here) with obvious and distinguishable nuclei which were observed by the SEM and TEM microscopy. The ELs were rectangular or trapezoidal in shape, and the largest cells in hemolymph—their length varied from 14.00 to 15.73 μm. Our results showed that the total number of hemocytes (THC) significantly increased across larval development. Moreover, host plant species significantly affected total hemocyte count where the THC of sixth instar larvae on B. microphylla (2561.00 ± 10.60 cell/mm3) was significantly higher than on B. hyrcana (2440.00 ± 51.50 cell/mm3). The differential hemocyte count (DHC) profile study showed that GRs along with PLs were the most abundant cells in the hemolymph irrespective of larval instar and host plant species. Furthermore, the GRs% and ELs% increased throughout the larval development on both host plants, while a significant reduction of PLs% was recorded from third instar to sixth instar during the larval stage on two Buxus species. Apart from larval instar, host plant species had a significant effect on DHC of C. perspectalis. Despite higher total hemocyte number when fed on B. microphylla, the PLs% and ELs% were significantly higher in sixth‐instar larvae fed on B. hyrcana compared with B. microphylla. Oppositely, the percentage of GRs was 17% less in larvae reared on B. hyrcana than on B. microphylla. As hemocyte types are responsible for different immune functions, these findings on instar‐ and host plant‐dependent variation in their relative abundance would be critical to understand the immune response of this specialist herbivore.

CYP6B6 mediated adaptation to capsaicinoids in the generalist Helicoverpa armigera and specialist H. assulta: Transcriptional response and metabolic detoxification

Capsaicin and dihydrocapsaicin are the principal pungent compounds in hot peppers. The generalist Helicoverpa armigera and the specialist H. assulta are two of the few insects that can feed on hot pepper fruits. To further understand the mechanisms behind physiological adaptation to capsaicinoids in the two agricultural pests, we investigated the transcriptional response and metabolism capacity of CYP6B6 orthologs from the two closely related species to the capsaicinoids. The results showed that the expression of CYP6B6 was either weakly or moderately inducible in a tissue- and dose-specific manner. CYP6B6 induction to greater extent was observed in the larval midguts of the cotton bollworms, indicating that inducible defense is more prominant in the generalist. HassCYP6B6 exhibited a significantly higher activity towards capsaicin than HarmCYP6B6, suggesting that the specialist herbivore has evolved a more efficient enzyme in detoxifying capsaicin compared to the generalist. Multiple known and yet to be elucidated metabolites were detected in the reactions catalyzed by CYP6B6s, with dehydrogenation at the terminal carbon followed by N-macrocyclization and aliphatic hydroxylation being the most dominant pathways. The data make us hypothesize that these two Helicoverpa species have well adapted to capsaicinoids via their high P450-mediated detoxification ability.

Plant chemical diversity enhances defense against herbivory

Multiple hypotheses have been put forth to understand why defense chemistry in individual plants is so diverse. A major challenge has been teasing apart the importance of concentration vs. composition of defense compounds and resolving the mechanisms of diversity effects that determine plant resistance against herbivores. Accordingly, we first outline nonexclusive mechanisms by which phytochemical diversity may increase toxicity of a mixture compared to the average effect of each compound alone. We then leveraged independent in vitro, in vivo transgenic, and organismal experiments to test the effect of equimolar concentrations of purified milkweed toxins in isolation vs. mixtures on the specialist and sequestering monarch butterfly. We show that cardenolide toxin mixtures from milkweed plants enhance resistance against this herbivore compared to equal concentrations of single compounds. In mixtures, highly potent toxins dominated the inhibition of the monarch's target enzyme (Na+/K+-ATPase) in vitro, revealing toxin-specific affinity for the adapted enzyme in the absence of other physiological adaptations of the monarch. Mixtures also caused increased mortality in CRISPR-edited adult Drosophila melanogaster with the monarch enzyme in vivo, whereas wild-type flies showed lower survival regardless of mixture type. Finally, although experimentally administered mixtures were not more toxic to monarch caterpillars than single compounds overall, increasing caterpillar sequestration from mixtures resulted in an increasing burden for growth compared to single compounds. Phytochemical diversity likely provides an economical plant defense by acting on multiple aspects of herbivore physiology and may be particularly effective against sequestering specialist herbivores.

Validation and application of stable isotope analysis of dugong tusks to determine long‐term shifts in foraging patterns

AbstractUnderstanding the capacity of animals to shift diet in times of food shortage is crucial in assessing their resilience to climate change. Dugongs are specialist herbivores that live in genetically discrete populations and forage locally. This study validated the use of stable isotope analysis of collagen from the permanent, continuously growing tusks of dugongs to assess potential trophic and/or spatial shifts in foraging. Tusks were accessed from museum collections and included five locations spanning the dugong's Australian subtropical to tropical distribution. Collagen from tusk growth layers deposited during three life stages (nursing calf, subadult, and adult) and two seasons (winter, summer) were analyzed for δ15N and δ13C. In tropical Australia, mean δ15N values decreased significantly from calves to adults reflecting the ontogenetic dietary shift from milk to seagrass. In contrast, δ15N was significantly enriched in adult dugongs in subtropical Moreton Bay suggesting omnivory. δ13C values in tusks did not change with geographic location. Season did not appear to have a significant impact on δ15N or δ13C values in either tropical or subtropical locations, but dietary variation was greatest in the subtropics. Stable isotope analysis of tusks appears to be a useful method of investigating dietary/trophic shifts over a dugong's lifetime.

Microbiome analysis of monarch butterflies reveals effects of development and diet.

Diet profoundly influences the composition of an animal's microbiome, especially in holometabolous insects, offering a valuable model to explore the impact of diet on gut microbiome dynamics throughout metamorphosis. Here, we use monarch butterflies (Danaus plexippus), specialist herbivores that feed as larvae on many species of chemically well-defined milkweed plants (Asclepias sp.), to investigate the impacts of development and diet on the composition of the gut microbial community. While a few microbial taxa are conserved across life stages of monarchs, the microbiome appears to be highly dynamic throughout the life cycle. Microbial diversity gradually diminishes throughout the larval instars, ultimately reaching its lowest point during the pupal stage and then recovering again in the adult stage. The microbial composition then undergoes a substantial shift upon the transition from pupa to adult, with female adults having significantly different microbial communities than the eggs that they lay, indicating limited evidence for vertical transmission of gut microbiota. While diet did not significantly impact overall microbial composition, our results suggest that fourth instar larvae exhibit higher microbial diversity when consuming milkweed with high concentrations of toxic cardenolide phytochemicals. This study underscores how diet and developmental stage collectively shape the monarch's gut microbiota.

Landscape use by prey: Bottom‐up regulation prevails under reduced predation risk

AbstractResource specialist herbivores may suppress antipredator strategies to access scarcely available key resources, while proactive antipredator behaviors should be favored by generalists or when resources are abundant. We quantified the relative effects of predation‐ (top‐down) and resource‐driven (bottom‐up) constraints on spatiotemporal patterns of landscape use by a prey community in a dynamic system under low predator abundance, and investigated how prey manage the risk posed by predators with different hunting strategies. We fitted Royle–Nichols co‐abundance models to camera‐trapping data collected between 2017 and 2019 in Bicuar National Park (Angola), to assess spatial association/segregation across predator–prey dyads, while accounting for the effects of water and food availability during dry and wet seasons. We further estimated pairwise seasonal differences in diel activity overlap between predator and prey. We failed to detect spatiotemporal proactive antipredator responses either toward the cursorial or ambush predator. The community‐wide predator–prey association patterns we found support that predation pressure is insufficient to displace prey from their preferred habitats or to adjust their endogenous clock, and support predominantly bottom‐up regulated behaviors. We suggest that, in landscapes where predator density is low, limited perception of risk may prevent the employment of proactive antipredator behavior.

Potent Nitrogen-containing Milkweed Toxins are Differentially Regulated by Soil Nitrogen and Herbivore-induced Defense.

Theories have been widely proposed and tested for impacts of soil nitrogen (N) on phytochemical defenses. Among the hundreds of distinct cardenolide toxins produced by milkweeds (Asclepias spp.), few contain N, yet these appear to be the most toxic against specialist herbivores. Because N- and non-N-cardenolides coexist in milkweed leaves and likely have distinct biosynthesis, they present an opportunity to address hypotheses about drivers of toxin expression. We tested effects of soil N and herbivore-damage on cardenolide profiles of two milkweed species differing in life-history strategies (Asclepias syriaca and A. curassavica), and the toxicity of their leaves. In particular leaf extracts were tested against the target enzymes (Na+/K+-ATPase extracted from neural tissue) from both monarch butterflies (Danaus plexippus) as well as less cardenolide-resistant queen butterflies, D. gilippus. Increasing soil N enhanced biomass of Asclepias syriaca but had weak effects on cardenolides, including causing a significant reduction in the N-cardenolide labriformin; feeding by monarch caterpillars strongly induced N-cardenolides (labriformin), its precursors, and total cardenolides. Conversely, soil N had little impact on A. curassavica biomass, but was the primary driver of increasing N-cardenolides (voruscharin, uscharin and their precursors); caterpillar induction was weak. Butterfly enzyme assays revealed damage-induced cardenolides substantially increased toxicity of both milkweeds to both butterflies, swamping out effects of soil N on cardenolide concentration and composition. Although these two milkweed species differentially responded to soil N with allocation to growth and specific cardenolides, leaf toxicity to butterfly Na+/K+-ATPases was primarily driven byherbivore-induced defense. Thus, both biotic and abiotic factors shape the composition of phytochemical defense expression, and their relative importance may be dictated by plant life-history differences.

Discreet but diverse and specific: Determining plant‐herbivore interactions across a species‐rich plant family in a tropical rain forest

AbstractStudying plant–herbivore interactions within tropical rain forests is fundamental to understanding their ecology and evolution. An important aspect of plant–herbivore dynamics is the role of temporal and taxonomic variables in determining associations between herbivores and their host. Using the diverse and chemically rich plant family Annonaceae (Magnoliales), we conducted a year‐long study in Ecuador's Yasuní National Park in lowland Amazonia. We focused on nine understory tree species across a broad phylogenetic range within Annonaceae. For these species, we investigated patterns of herbivory, identified herbivores through DNA barcoding, and documented unique ant–butterfly associations. In general, leaf damage ranged from 0.09% to 25%, with significant temporal fluctuations for three species. Notably, Anaxagorea brevipes and Unonopsis veneficiorum faced higher herbivore pressure when compared to the other studied species. We document a discreet but diverse herbivore community, with 40 larvae from 12 Lepidoptera families collected throughout the year. Our findings identify, for the first time across a phylogenetically diverse sampling of Annonaceae, the specialization of herbivores on our focal species. Overall, our data provide valuable information on herbivory patterns at the local scale for this important rain forest plant family. Furthermore, these findings contribute to our understanding of the ecological processes that influence plant species diversity in tropical rain forests.Abstract in Spanish is available with online material.

Short-term responses of spider mites inform mechanisms of maize resistance to a generalist herbivore

Plants are attacked by diverse herbivorous pests with different host specializations. While host plant resistance influences pest pressure, how resistance impacts the behaviors of generalist and specialist herbivores, and the relationship to resistance, is less well known. Here, we investigated the short-term (< 1 h) behavioral changes of a generalist herbivore, the two-spotted spider mite (TSM), and a specialist herbivore, the Banks grass mite (BGM), after introduction to no-choice Tanglefoot leaf-arenas (2 × 2 cm) of three maize inbred lines (B73, B75, and B96). The widely-used inbred line B73 is susceptible to spider mites, while B75 and B96 are known to be mite resistant, especially to TSM. Video tracking was used to record TSM and BGM walking, probing, feeding, resting, web-building and travel distance on arenas of each line. Mite oviposition was also recorded after 72 h. B75, a resistant line, decreased the feeding behavior (i.e., time) of both mite species compared to B73 (susceptible control) and B96. Moreover, TSM appeared to be sensitive to both resistant lines (B75 and B96) with reduced oviposition, and increased resting and web-building times compared to susceptible B73. In contrast, the specialist BGM showed no difference in oviposition, resting and web-building time across all maize inbred lines. Our findings of quite broad and short-term responses of TSM to B75 and B96 are consistent with a role for constitutive or rapidly induced plant defenses in maize in conferring TSM resistance. Other mechanisms of plant resistance may be needed, however, for defense against specialists like BGM.

Identification and in vivo functional analysis of furanocoumarin-responsive cytochrome P450s in a Rutaceae-feeding Papilio butterfly.

The Order Lepidoptera contains nearly 160,000 described species and most of them are specialist herbivores that use restricted plant species as hosts. Speciation that originated from host shift is one of the important factors for the diversification of Lepidoptera. Because plants prepare secondary metabolites for defense against herbivores, with varying profiles of the components among different plant taxa, the specialist herbivores need to be adapted to the toxic substances unique to their host plants. Swallowtail butterflies of the genus Papilio consist of over 200 species. Approximately 80% of them utilize Rutaceae plants, and among the remaining species, a specific subgroup uses phylogenetically distant Apiaceae plants as larval hosts. Rutaceae and Apiaceae commonly contain toxic secondary metabolites, furanocoumarins, and molecular phylogenetic studies support the concept that Apiaceae feeders were derived from Rutaceae feeders. Molecular mechanisms underlying furanocoumarin tolerance in Papilio butterflies have been investigated almost exclusively in an Apiaceae feeder by an in vitro assay. In contrast, there is little information regarding the Rutaceae feeders. Here, we focused on a Rutaceae feeder, Papilio xuthus, and identified two furanocoumarin-responsive cytochrome P450-6B (CYP6B) genes, of which one was an ortholog of a furanocoumarin-metabolizing enzyme identified in the Apiaceae-feeding Papilio while the other was previously unreported. We further conducted in vivo functional analysis using the CRISPR/Cas9 system, revealing a contribution of these CYP6Bs to furanocoumarin tolerance of P. xuthus larvae. Our findings suggest that co-option of furanocoumarin-metabolizing CYP6B enzymes at least partially contributed to the host shift from Rutaceae to Apiaceae in Papilio butterflies.

Plasticity in biomass allocation underlies tolerance to leaf damage in native and non-native populations of Datura stramonium

An introduction to a novel habitat represents a challenge to plants because they likely would face new interactions and possibly different physical context. When plant populations arrive to a new region free from herbivores, we can expect an evolutionary change in their defense level, although this may be contingent on the type of defense, resistance or tolerance, and cost of defense. Here, we addressed questions on the evolution of tolerance to damage in non-native Spanish populations of Datura stramonium by means of two comparative greenhouse experiments. We found differences in seed production, specific leaf area, and biomass allocation to stems and roots between ranges. Compared to the Mexican native populations of this species, non-native populations produced less seeds despite damage and allocate more biomass to roots and less to stems, and had higher specific leaf area values. Plasticity to leaf damage was similar between populations and no difference in tolerance to damage between native and non-native populations was detected. Costs for tolerance were detected in both regions. Two plasticity traits of leaves were associated with tolerance and were similar between regions. These results suggest that tolerance remains beneficial to plants in the non-native region despite it incurs in fitness costs and that damage by herbivores is low in the non-native region. The study of the underlying traits of tolerance can improve our understanding on the evolution of tolerance in novel environments, free from plants’ specialist herbivores.

Patterns of constitutive and induced herbivore defence are complex, but share a common genetic basis in annual and perennial monkeyflower

Abstract Despite multiple ecological and evolutionary hypotheses that predict patterns of phenotypic relationships between plant growth, reproduction and constitutive and/or induced resistance to herbivores, these hypotheses do not make any predictions about the underlying molecular genetic mechanisms that mediate these relationships. We investigated how divergent plant life‐history strategies in the yellow monkeyflower and a life‐history altering locus, DIV1, influence plasticity of phytochemical herbivory resistance traits in response to attack by two herbivore species with different diet breadth. Life‐history strategy (annual vs. perennial) and the DIV1 locus significantly influenced levels of constitutive herbivory resistance, as well as resistance induction following both generalist and specialist herbivory. Perennial plants had higher total levels of univariate constitutive and induced defence than annuals, regardless of herbivore type. Annuals induced less in response to generalist herbivory than did perennials, while induction response was equivalent across the ecotypes for specialist herbivory. The effects of the DIV1 locus on levels of constitutive and induced defence were dependent on genetic background, the annual versus perennial haplotype of DIV1 and herbivore identity. The patterns of univariate induction due to DIV1 were non‐additive and did not always match expectations based on patterns of divergence for annual/perennial parents. For example, perennial plants had higher levels of constitutive and induced defence than did annuals, but when the annual DIV1 was present in the perennial genetic background induction response to herbivory was higher than for the perennial parent lines. Patterns for multivariate defence arsenals generally echoed those of univariate, with annual and perennial monkeyflowers and those with alternative versions of DIV1 differing significantly in constitutive and induced resistance. Like univariate resistance, induced multivariate defence arsenals were affected by herbivore identity. Our results highlight the complexity of the genetic mechanisms underlying plastic response to herbivory. While a genetic locus underlying substantial phenotypic variation in life‐history strategy and constitutive defence also influences defence plasticity, the induction response also depends on genetic background. This result demonstrates the potential for some degree of evolutionary independence between constitutive and induced defence, or induced defence and life‐history strategy, in monkeyflowers. Read the free Plain Language Summary for this article on the Journal blog.

Primary Metabolic Response of Aristolochia contorta to Simulated Specialist Herbivory under Elevated CO2 Conditions.

This study explores how elevated carbon dioxide (CO2) levels affects the growth and defense mechanisms of plants. We focused on Aristolochia contorta Bunge (Aristolochiaceae), a wild plant that exhibits growth reduction under elevated CO2 in the previous study. The plant has Sericinus montela Gray (Papilionidae) as a specialist herbivore. By analyzing primary metabolites, understanding both the growth and defense response of plants to herbivory under elevated CO2 conditions is possible. The experiment was conducted across four groups, combining two CO2 concentration conditions (ambient CO2 and elevated CO2) with two herbivory conditions (herbivory treated and untreated). Although many plants exhibit increased growth under elevated CO2 levels, A. contorta exhibited reduced growth with lower height, dry weight, and total leaf area. Under herbivory, A. contorta triggered both localized and systemic responses. More primary metabolites exhibited significant differences due to herbivory treatment in systemic tissue than local leaves that herbivory was directly treated. Herbivory under elevated CO2 level triggered more significant responses in primary metabolites (17 metabolites) than herbivory under ambient CO2 conditions (five metabolites). Several defense-related metabolites exhibited higher concentrations in the roots and lower concentrations in the leaves in response to the herbivory treatment in the elevated CO2 group. This suggests a potential intensification of defensive responses in the underground parts of the plant under elevated CO2 levels. Our findings underscore the importance of considering both abiotic and biotic factors in understanding plant responses to environmental changes. The adaptive strategies of A. contorta suggest a complex response mechanism to elevated CO2 and herbivory pressures.

Exploring the potential of root-associated bacteria to control an outbreak weed

Abstract Aims The spread of invasive weeds threatens biodiversity and stability of ecosystems. Jacobaea vulgaris is an invasive weed in some countries and an outbreak species in its native European range. Although biological control using specialist herbivores is available, controlling with soil microorganisms remains far less explored. Methods Twenty bacteria strains isolated from roots of J. vulgaris were used to examine bacterial effects on seed germination, root morphology and early plant growth. Moreover, we tested direct effects of the bacteria on a specialist herbivore of J. vulgaris, the leaf chewing caterpillar (Tyria jacobaeae), commonly used in biocontrol. We also tested indirect effects of bacteria, via the plant, on the performance of T. jacobaeae and the aphid species Aphis jacobaeae. Lastly, we examined the host specificity of two tested bacteria on three other forbs. Results Two Gammaproteobacteria, Pseudomonas brassicacearum and Serratia plymuthica, significantly reduced root growth of seedlings in-vitro, while seed germination was unaffected. However, these negative effects were observed across other forb species as well. Bacillus spp. injection led to the highest T. jacobaeae caterpillar mortality, while ingestion had no effect. Inoculation of the plants with bacteria did not affect aphid performance, but significantly affected T. jacobaeae preference. Specifically, P. syringae and one Bacillus sp. strain significantly increased T. jacobaeae preference. Conclusions Our results show that two root-associated bacteria inhibit J. vulgaris growth, but their lack of host specificity restricts their potential for biocontrol. Our study also highlights that belowground microorganisms can hamper or enhance the performance of aboveground insects.

Host preferences of an invasive specialist herbivore and native predators on chemically defended plants

Abstract Herbivorous insects generally choose host plants that optimize their growth. However, many specialists choose chemically defended host plants that slow their growth, prioritizing protection from natural enemies. Invasive specialist herbivores may have an added advantage in their non‐native range where they lack co‐evolved specialist natural enemies, in addition to being unpalatable to generalist predators. Here, we explore the diet preferences of the invasive yellowmargined leaf beetle, Microtheca ochroloma and those of native natural enemies. We measured M. ochroloma preferences among six Brassica cultivars varying in glucosinolate concentrations. In both field and laboratory experiments, M. ochroloma preferred the relatively less defended host plants, mizuna and Chinese cabbage. M. ochroloma grew quickest on mizuna. Generalist predators also preferred to eat M. ochroloma larvae that had been fed mizuna in choice trials, yet in the field, predatory insects were least commonly found on mizuna, and most commonly found on plants dominated by native herbivores. M. ochroloma consumes plants that increase their susceptibility to predators, but predation pressure in the field appears to be generally low. Our results suggest that invasive specialist herbivores may prioritize growth over protection against natural enemies. Weak top‐down regulation of specialist herbivore invaders might enable them to exploit less‐defended host plants in their novel range that facilitate faster growth and exacerbate crop damage. Invasive pests may be more effectively managed by leveraging plant preferences in the novel host range in trap crop systems.

Altered biological control species interactions despite phenological synchrony along an urban–rural temperature gradient

Biological control programs provide simplified systems in which to investigate the influence of temperature on trophic interactions. We investigated the interaction between the invasive thistle Carduus nutans and its specialist herbivore, the biological control agent weevil Rhinocyllus conicus, along a temperature gradient spanning an urban-to-rural urbanization gradient in central Pennsylvania. As expected, more urbanized sites were warmer than rural sites. Nevertheless, C. nutans phenology was only slightly earlier at warmer sites, and the timing of observations of adult R. conicus on thistles did not shift earlier, leaving synchrony apparently unchanged. Despite unchanged phenological matching, seasonal patterns in weevil damage varied with temperature: early-appearing capitula at warmer sites were more heavily damaged than early capitula at cooler sites, yet overall a lower proportion of capitula was damaged at warmer sites. Rising temperatures may therefore reduce effectiveness of R. conicus for C. nutans control, as more capitula escape damage. Biological control systems and naturally occurring temperature gradients are useful systems in which to investigate climate change impacts, but a focus on the phenology of organisms’ presence or absence may not be sufficient; our study demonstrates that consideration of the interaction itself may be necessary.

A draft reference genome assembly of California Pipevine, Aristolochia californica Torr.

The California Pipevine, Aristolochia californica Torr., is the only endemic California species within the cosmopolitan birthwort family Aristolochiaceae. It occurs as an understory vine in riparian and chaparral areas and in forest edges and windrows. The geographic range of this plant species almost entirely overlaps with that of its major specialized herbivore, the California Pipevine Swallowtail Butterfly Battus philenor hirsuta. While this species pair is a useful, ecologically well-understood system to study co-evolution, until recently, genomic resources for both have been lacking. Here, we report a new, chromosome-level assembly of A. californica as part of the California Conservation Genomics Project (CCGP). Following the sequencing and assembly strategy of the CCGP, we used Pacific Biosciences HiFi long reads and Hi-C chromatin proximity sequencing technology to produce a de novo assembled genome. Our genome assembly, the first for any species in the genus, contains 531 scaffolds spanning 661 megabase (Mb) pairs, with a contig N50 of 6.53 Mb, a scaffold N50 of 42.2 Mb, and BUSCO complete score of 98%. In combination with the recently published B. philenor hirsuta reference genome assembly, the A. californica reference genome assembly will be a powerful tool for studying co-evolution in a rapidly changing California landscape.

Spatial and temporal variability in the structure of the multiple-herbivore community of horsenettle, and evidence for evolutionary responses in host-plant resistance

The geographic mosaic model of plant–herbivore coevolution asserts that interactions between a plant species and an herbivore species vary in intensity among populations across the plant’s geographic range. Despite this model’s intuitive appeal, data to investigate its implications for the type of complex, multiple-herbivore communities that occur in nature are scant. This paper reports on the results of 2 years of field surveys of damage by five leaf herbivores and one stem herbivore in four Solanum carolinense (horsenettle) populations, combined with results of a common-garden study quantifying the mean resistance levels of the plants from each field against each of the six herbivores. The relative amounts of damage caused by each species (representing the “herbivore-community structure”) differed significantly among the four fields. The plants were much more heavily damaged in the 2nd year than in the first, but the herbivore-community structure remained stable within each field between years. Overall, the amount of damage by species of herbivores in a field tended to be positively correlated with the plants’ levels of resistance that were measured in the common garden (r = 0.40, P = 0.05). Specifically, for five of the six herbivores, greater damage in the field was associated with greater plant resistance. This result suggests that horsenettle’s evolution of resistance against specific herbivores can occur rather quickly within fields, creating a local-scale mosaic of populations specifically adapted to the particular structure of the herbivore community that they are facing, but that herbivore-community structure is not strongly determined by plant resistance.

Intercropping organic broccoli with Rhododendron tomentosum and Fagopyrum esculentum: a test of bottom-up and top-down strategies for reducing herbivory

Brassicaceous plants are attacked by a wealth of specialist herbivores that include the Diamondback moth (DBM) Plutella xylostella L. (Lepidoptera: Plutellidae), control of which requires novel biocontrol strategies. DBM is a cosmopolitan pest causing damage that varies yearly in Finland depending on the timing and extent of their migration. Intercropping with companion plants can hamper host location by herbivores or attract their natural enemies. We tested two sustainable companion plant-based protection strategies on field-grown broccoli (Brassica oleracea var. italica), which comprised 1) aromatic and repellent-releasing Rhododendron tomentosum (RT) (bottom-up strategy), and 2) nectar-producing buckwheat Fagopyrum esculentum (FE) (top-down strategy) combined with an early-season floating row cover (mechanical control) (MC + FE). In addition, a control (no companion plant or cover) and mechanical control (MC) without FE were included. DBM adults on yellow sticky traps and larvae on plants were counted, and feeding damage on leaves was quantified. Volatile organic compounds emitted by broccoli plants in control and RT plots, and emitted by boundary RT plants in RT plots, were identified and quantified. There was a mass outbreak of DBM in early summer with a population peak in mid-July when the second adult generation emerged. DBM adult densities were significantly lower in RT and MC + FE than in control plots. Broccoli in RT plots had a lower larval density and lower damage intensity than in control plots in the early-season. Larval densities and damage intensities were the highest in the latter half of July. Control plots had the highest number of larvae followed by RT, MC, and MC + FE plots. Damage intensities in control plots were significantly higher than in all other treatments throughout the season. Damage intensity was lower in MC and MC + FE plots than in RT plots at the end-of-July. R. tomentosum emitted two characteristic sesquiterpene alcohols, palustrol and ledol, but no evidence of adsorption and re-release of these compounds from Brassica plants grown in RT plots was found. We conclude that incorporating RT as a boundary plant and using a mechanical row cover reduces DBM damage on broccoli, but further multi-year trials under varying degrees of pest pressure are needed.

High intraspecific variability and previous experience affect polyphenol metabolism in polyphagous Lymantria mathura caterpillars.

Polyphagous insect herbivores feed on multiple host-plant species and face a highly variable chemical landscape. Comparative studies of polyphagous herbivore metabolism across a range of plants is an ideal approach for exploring how intra- and interspecific chemical variation shapes species interactions. We used polyphagous caterpillars of Lymantria mathura (Erebidae, Lepidoptera) to explore mechanisms that may contribute to its ability to feed on various hosts. We focused on intraspecific variation in polyphenol metabolism, the fates of individual polyphenols, and the role of previous feeding experience on polyphenol metabolism and leaf consumption. We collected the caterpillars from Acer amoenum (Sapindaceae), Carpinus cordata (Betulaceae), and Quercus crispula (Fagaceae). We first fed the larvae with the leaves of their original host and characterized the polyphenol profiles in leaves and frass. We then transferred a subset of larvae to a different host species and quantified how host shifting affected their leaf consumption and polyphenol metabolism. There was high intraspecific variation in frass composition, even among caterpillars fed with one host. While polyphenols had various fates when ingested by the caterpillars, most of them were passively excreted. When we transferred the caterpillars to a new host, their previous experience influenced how they metabolized polyphenols. The one-host larvae metabolized a larger quantity of ingested polyphenols than two-host caterpillars. Some of these metabolites could have been sequestered, others were probably activated in the gut. One-host caterpillars retained more of the ingested leaf biomass than transferred caterpillars. The pronounced intraspecific variation in polyphenol metabolism, an ability to excrete ingested metabolites and potential dietary habituation are factors that may contribute to the ability of L. mathura to feed across multiple hosts. Further comparative studies can help identify if these mechanisms are related to differential host-choice and response to host-plant traits in specialist and generalist insect herbivores.