Higher plasticity in feeding preference of a generalist than a specialist: experiments with two closely related Helicoverpa species

Considerable genetic diversity and structure despite narrow endemism and limited ecological specialization in the Hayden's ringlet, Coenonympha haydenii

Understanding the processes that underlie the development of population genetic structure is central to the study of evolution. Patterns of genetic structure, in turn, can reveal signatures of isolation by distance (IBD), barriers to gene flow, or even the genesis of speciation. However, it is unclear how severe range restriction might impact the processes that dominate the development of genetic structure. In narrow endemic species, is population structure likely to be adaptive in nature, or rather the result of genetic drift? In this study, we investigated patterns of genetic diversity and structure in the narrow endemic Hayden's ringlet butterfly. Specifically, we asked to what degree genetic structure in the Hayden's ringlet can be explained by IBD, isolation by resistance (IBR) (in the form of geographic or ecological barriers to migration between populations), and isolation by environment (in the form of differences in host plant availability and preference). We employed a genotyping-by-sequencing (GBS) approach coupled with host preference assays, Bayesian modelling, and population genomic analyses to answer these questions. Our results suggest that despite their restricted range, levels of genetic diversity in the Hayden's ringlet are comparable to those seen in more widespread butterfly species. Hayden's ringlets showed a strong preference for feeding on grasses relative to sedges, but neither larval preference nor potential host availability at sampling sites correlated with genetic structure. We conclude that geography, in the form of IBR and simple IBD, was the major driver of contemporary patterns of differentiation in this narrow endemic species.

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.

Caterpillar Responses to Gustatory Stimuli in Potato Tuber Moths: Electrophysiological and Behavioral Insights

This research investigates how fourth-instar larvae of the potato tuber moth, Phthorimaea operculella, respond to plant secondary metabolites (sucrose, glucose, nicotine, and tannic acid) both in terms of gustatory electrophysiology and feeding behavior. The objective is to establish a theoretical foundation for employing plant-derived compounds in potato tuber moth control. We employed single-sensillum recording techniques and dual-choice leaf disk assays to assess the gustatory electrophysiological responses and feeding preferences of these larvae towards the mentioned compounds. Sensory neurons responsive to sucrose, glucose, nicotine, and tannic acid were identified in the larvae's medial and lateral sensilla styloconica. Neuronal activity was influenced by stimulus type and concentration. Notably, the two types of sensilla styloconica displayed distinct response patterns for sucrose and glucose while they had similar firing patterns towards nicotine and tannic acid. Sucrose and glucose significantly promoted larval feeding, while nicotine and tannic acid had significant inhibitory effects. These findings demonstrate that the medial and lateral sensilla styloconica house sensory neurons sensitive to both feeding stimulants and inhibitors, albeit with differing response profiles and sensitivities. This study suggests that sucrose and glucose are promising candidates for feeding stimulants, while nicotine and tannic acid show potential as effective feeding inhibitors of P. operculella larvae.

Midgut serine proteinases participate in dietary adaptations of the castor (Eri) silkworm Samia ricini Anderson transferred from Ricinus communis to an ancestral host, Ailanthus excelsa Roxb

Dietary change influenced the life-history traits, nutritional utilization, and midgut serine proteinases in the larvae of the domesticated polyphagous S. ricini, transferred from R. communis (common name: castor; family Euphorbiaceae; the host plant implicated in its domestication) to A. excelsa (common name: Indian tree of heaven; family Simaroubaceae; an ancestral host of wild Samia species). Significantly higher values for fecundity and body weight were observed in larvae feeding on R. communis (Scr diet), and they took less time to reach pupation than insects feeding on A. excelsa (Scai diet). Nevertheless, the nutritional index for efficiency of conversion of digested matter (ECD) was similar for larvae feeding on the two plant species, suggesting the physiological adaptation of S. ricini (especially older instars) to an A. excelsa diet. In vitro protease assays and gelatinolytic zymograms using diagnostic substrates and protease inhibitors revealed significantly elevated levels (p ≤ 0.05) of digestive trypsins, which may be associated with the metabolic costs influencing slow growth in larvae feeding on A. excelsa. RT-PCR with semidegenerate serine proteinase gene-specific primers, and cloning and sequencing of 3' cDNA ends identified a large gene family comprising at least two groups of putative chymotrypsins (i.e., Sr I and Sr II) resembling invertebrate brachyurins/collagenases with wide substrate specificities, and five groups of putative trypsins (i.e., Sr III, Sr IV, Sr V, Sr VII, and Sr VIII). Quantitative RT-PCR indicated that transcripts belonging to the Sr I, Sr III, Sr IV, and Sr V groups, especially the Sr IV group (resembling achelase I from Lonomia achelous), were expressed differentially in the midguts of fourth instars reared on the two plant species. Sequence similarity indicated shared lineages with lepidopteran orthologs associated with expression in the gut, protein digestion, and phytophagy. The results obtained are discussed in the context of larval serine proteinases in dietary adaptations, domestication, and exploration of new host plant species for commercial rearing of S. ricini.

Invasive Drosophila suzukii outnumbers native controphics and causes substantial damage to fruits of forest plants

Impacts of biological invasions are diverse and can have far-reaching consequences for ecosystems. The spotted wing drosophila, Drosophila suzukii, is a major invasive pest of fruits, which negatively affects fruit and wine production. However, little is known about the ecological impact of this fly species on more natural ecosystems it has invaded, such as forests. In this study, we investigated the use of potential host plants by D. suzukii at 64 sites in different forest communities in Switzerland from mid-June to mid-October 2020. We examined more than 12,000 fruits for egg deposits of D. suzukii to assess its direct impact on the plants. We recorded symptoms of fruit decay after egg deposition to determine if D. suzukii attacks trigger fruit decay. In addition, we monitored the drosophilid fauna with cup traps baited with apple cider vinegar, as we expected that D. suzukii would outnumber and potentially outcompete native controphics, especially other drosophilids. Egg deposits of D. suzukii were found on the fruits of 31 of the 39 potential host plant species studied, with 18 species showing an attack rate > 50%. Overall, fruits of Cotoneaster divaricatus (96%), Atropa bella-donna (91%), Rubus fruticosus corylifolius aggr. (91%), Frangula alnus (85%) and Sambucus nigra (83%) were attacked particularly frequently, resulting also in high predicted attack probabilities that varied among forest communities. Later and longer fruiting, black fruit colour, larger fruit size and higher pulp pH all positively affected attack rates. More than 50% of the plant species showed severe symptoms of decay after egg deposition, with higher pulp sugar content leading to more severe symptoms. The high fruit attack rate observed was reflected in a high abundance and dominance of D. suzukii in trap catches, independent of forest community and elevation. Drosophila suzukii was by far the most abundant species, accounting for 86% (81,395 individuals) of all drosophilids. The abundance of D. suzukii was negatively associated with the abundance of the native drosophilids. Our results indicate that the invasive D. suzukii competes strongly with other frugivorous species and that its presence might have far-reaching ecosystem-level consequences. The rapid decay of fruits attacked by D. suzukii leads to a loss of resources and may disrupt seed-dispersal mutualisms through the reduced consumption of fruits by dispersers such as birds.

Effects of Color Attributes on Trap Capture Rates of Chrysobothris femorata (Coleoptera: Buprestidae) and Related Species

Chrysobothris spp. (Coleoptera: Buprestidae) and other closely related buprestids are common pests of fruit, shade, and nut trees in the United States. Many Chrysobothris spp., including Chrysobothris femorata, are polyphagous herbivores. Their wide host range leads to the destruction of numerous tree species in nurseries and orchards. Although problems caused by Chrysobothris are well known, there are no reliable monitoring methods to estimate local populations before substantial damage occurs. Other buprestid populations have been effectively estimated using colored sticky traps to capture beetles. However, the attraction of Chrysobothris to specific color attributes has not been directly assessed. A multi-color trapping system was utilized to determine color attraction of Chrysobothris spp. Specific color attributes (lightness [L*], red to green [a*], blue to yellow [b*], chroma [C*], hue [h*], and peak reflectance [PR]) were then evaluated to determine beetle responses. In initial experiments with mostly primary colors, Chrysobothris were most attracted to traps with red coloration. Thus, additional experiments were performed using a range of trap colors with red reflectance values. Among these red reflectance colors, it was determined that the violet range of the electromagnetic spectrum had greater attractance to Chrysobothris. Additionally, Chrysobothris attraction correlated with hue and b*, suggesting a preference for traps with hues between red to blue. However, males and females of some Chrysobothris species showed differentiated responses. These findings provide information on visual stimulants that can be used in Chrysobothris trapping and management. Furthermore, this information can be used in conjunction with ecological theory to understand host-location methods of Chrysobothris.

Neuronal architecture and functional mapping of the taste center of larval Helicoverpa armigera (Lepidoptera: Noctuidae)

The sense of taste plays a crucial role in herbivorous insects by discriminating nutrients from complex plant metabolic compounds. The peripheral coding of taste has been thoroughly studied in many insect species, but the central gustatory pathways are poorly described. In the present study, we characterized single neurons in the gnathal ganglion of Helicoverpa armigera larvae using the intracellular recording/staining technique. We identified different types of neurons, including sensory neurons, interneurons, and motor neurons. The morphologies of these neurons were largely diverse and their arborizations seemingly covered the whole gnathal ganglion. The representation of the single neurons responding to the relevant stimuli of sweet and bitter cues showed no distinct patterns in the gnathal ganglion. We postulate that taste signals may be processed in a manner consistent with the principle of population coding in the gnathal ganglion of H. armigera larvae.

Review: Trait plasticity during plant-insect interactions: From molecular mechanisms to impact on community dynamics

Phenotypic plasticity, prevalent in all domains of life, enables organisms to cope with unpredictable or novel changes in their growing environment. Plants represent an interesting example of phenotypic plasticity which also directly represents and affects the dynamics of biological interactions occurring in a community. Insects, which interact with plants, manifest phenotypic plasticity in their developmental, physiological, morphological or behavioral traits in response to the various host plant defenses induced upon herbivory. However, plant-insect interactions are generally more complex and multidimensional because of their dynamic association with their respective microbiomes and macrobiomes. Moreover, these associations can alter plant and insect responses towards each other by modulating the degree of phenotypic plasticity in their various traits and studying them will provide insights into how plants and insects reciprocally affect each other's evolutionary trajectory. Further, we explore the consequences of phenotypic plasticity on relationships and interactions between plants and insects and its impact on their development, evolution, speciation and ecological organization. This overview, obtained after exploring and comparing data obtained from several inter-disciplinary studies, reveals how genetic and molecular mechanisms, underlying plasticity in traits, impact species interactions at the community level and also identifies mechanisms that could be exploited in breeding programs.

Habituation to a Deterrent Plant Alkaloid Develops Faster in the Specialist Herbivore Helicoverpa assulta Than in Its Generalist Congener Helicoverpa armigera and Coincides with Taste Neuron Desensitisation

The two closely related moth species, Helicoverpa armigera and H. assulta differ strongly in their degree of host-plant specialism. In dual-choice leaf disk assays, caterpillars of the two species that had been reared on standard artificial diet were strongly deterred by the plant-derived alkaloid strychnine. However, caterpillars of both species reared on artificial diet containing strychnine from neonate to the 5th instar were insensitive to this compound. Fifth instar caterpillars of H. assulta and 4th or 5th instars of H. armigera not exposed to strychnine before were subjected to strychnine-containing diet for 24 h, 36 h, 48 h, or 72 h. Whereas H. assulta displayed habituation to strychnine after 48 h, it took until 72 h for H. armigera to become habituated. Electrophysiological tests revealed that a deterrent-sensitive neuron in the medial sensillum styloconicum of both species displayed significantly reduced sensitivity to strychnine that correlated with the onset of habituation. We conclude that the specialist H. assulta habituated faster to strychnine than the generalist H. armigera and hypothesis that desensitization of deterrent-sensitive neurons contributed to habituation.

Elucidating the Fitness of a Dead-End Trap Crop Strategy against the Tomato Fruitworm, Helicoverpa armigera

Simple Summary

The tomato fruitworm, Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), is a destructive polyphagous insect pest of tomatoes and its control largely depends on chemical pesticides. However, indiscriminate use of chemical pesticides has resulted in the development of resistance and posed serious environmental problems. Alternatively, in search of environmentally friendly pest management techniques, the use of trap crops has recently gained more attention. In this study, we compared different accessions of S. viarum to investigate which of them have the highest potential as a dead-end trap crop for H. armigera. Results from the laboratory experiment showed a significant variation among the S. viarum accessions in terms of H. armigera oviposition, and exhibited a higher level of resistance against H. armigera larvae compared to the tomato plants. Under the semi-field condition, trap cropping of S. viarum significantly reduces the total egg-laying of H. armigera on tomato. This study provides important information about the abilities of S. viarum plants to influence the life parameter of H. armigera and highlighted the possibilities to use S. viarum as a dead-end trap crop for the management of H. armigera. However, the efficacy of S. viarum needs to be tested under large, open-field conditions.

Abstract

Solanum viarum has been proposed as a potential dead-end trap crop for the management of Helicoverpa armigera because of its unsuitability for larval growth and survival despite being overwhelmingly preferred for oviposition. This study delved into the different S. viarum accessions for ovipositional preference and non-suitability for larval growth and survival of H. armigera. Besides, foliage trichomes, acylsugars, and phenolic content of S. viarum plants were assessed and compared with tomato. Since there is no significant variation in the ovipositional preference and larval performance of H. armigera, our result revealed that all those evaluated accessions of S. viarum have the potential to be used as a dead-end trap crop for the management of H. armigera. However, significant variation among the S. viarum accessions in terms of H. armigera oviposition was also evident in a no-choice experiment. Because of high-density glandular trichomes, acylsugars, and phenolic content, S. viarum significantly impaired H. armigera larval growth and survival compared to the tomato. Hence, our study elucidated that the S. viarum plant fits with the criteria for dead-end trap crop, and has the potential as a dead-end trap crop for the H. armigera, which needs to be tested under large, open-field conditions.

Plant Metabolites Drive Different Responses in Caterpillars of Two Closely Related Helicoverpa Species

The host acceptances of insects can be determined largely by detecting plant metabolites using insect taste. In the present study, we investigated the gustatory sensitivity and feeding behaviors of two closely related caterpillars, the generalist Helicoverpa armigera (Hübner) and the specialist H. assulta (Guenée), to different plant metabolites by using the single sensillum recording technique and the dual-choice assay, aiming to explore the contribution of plant metabolites to the difference of diet breadth between the two species. The results depicted that the feeding patterns of caterpillars for both plant primary and secondary metabolites were significantly different between the two Helicoverpa species. Fructose, glucose, and proline stimulated feedings of the specialist H. assulta, while glucose and proline had no significant effect on the generalist H. armigera. Gossypol and tomatine, the secondary metabolites of host plants of the generalist H. armigera, elicited appetitive feedings of this insect species but drove aversive feedings of H. assulta. Nicotine and capsaicin elicited appetitive feedings of H. assulta, but drove aversive feedings of H. armigera. For the response of gustatory receptor neurons (GRNs) in the maxillary styloconic sensilla of caterpillars, each of the investigated primary metabolites induced similar responding patterns between the two Helicoverpa species. However, four secondary metabolites elicited different responding patterns of GRNs in the two species, which is consistent with the difference of feeding preferences to these compounds. In summary, our results of caterpillars’ performance to the plant metabolites could reflect the difference of diet breadth between the two Helicoverpa species. To our knowledge, this is the first report showing that plant secondary metabolites could drive appetitive feedings in a generalist insect species, which gives new insights of underscoring the adaptation mechanism of herbivores to host plants.

The "sequential cues hypothesis": a conceptual model to explain host location and ranking by polyphagous herbivores

Successfully locating a host plant is crucial for an insect herbivore to feed and/or oviposit. However, locating a host within a complex environment that may contain an array of different plant species is a difficult task. This is particularly the case for polyphagous herbivores, which must locate a host within environments that may simultaneously contain multiple suitable and unsuitable hosts. Here we review the mechanisms of host selection used by polyphagous herbivores, as well as exploring how prior experience may modify a generalist's response to host cues. We show that recent research demonstrates that polyphagous herbivores have the capacity to detect both common cues from multiple host species, as well as specific cues from individual host species. This creates a paradox in that generalists invariably rank hosts when given a choice, a finding at odds with the "neural limitations" hypothesis that says generalist insect herbivores should not have the neural capacity to identify cues specific to every possible host. To explain this paradox we propose a model, akin to parasitoid host location, that postulates that generalist herbivores use different cues sequentially in host location. We propose that initially common host cues, associated with all potential hosts, are used to place the herbivore within the host habitat and that, in the absence of any other host cues, these cues are sufficient in themselves to lead to host location. As such they are true "generalist" cues. However, once within the host habitat, we propose that the presence of a smaller group of cues may lead to further host searching and the location of preferred hosts: these are "specialist" cues. This model explains the current conflict in the literature where generalists can respond to both common and specific host-plant cues, while also exhibiting specialist and generalist host use behavior under different conditions.

Feeding Preference of Crapemyrtle Bark Scale (Acanthococcus lagerstroemiae) on Different Species

Crapemyrtle bark scale (CMBS; Acanthococcus lagerstroemiae) is an exotic pest species that causes aesthetic and economic damage to crapemyrtles and poses potential threats to other horticultural crops in the United States. Although previous studies reported the infestation of CMBS on several alternative hosts across multiple families in Asia, its potential threats to other documented alternative hosts remain elusive and yet to be confirmed. In this study, feeding preference studies of CMBS were conducted on forty-nine plant species and cultivars in 2016 and 2019, in order to gain insight into the expansion of CMBS distribution in the United States, as well as other regions of the world. The infestations of CMBS were confirmed on apple (Malus domestica), Chaenomeles speciosa, Disopyros rhombifolia, Heimia salicifolia, Lagerstroemia 'Spiced Plum', M. angustifolia, and twelve out of thirty-five pomegranate cultivars. However, the levels of CMBS infestation on these test plant hosts in this study is very low compared to Lagerstroemia, and may not cause significant damage. No sign of CMBS infestation was observed on Rubus 'Arapaho', R. 'Navaho', R. idaeus 'Dorman Red', R. fruticosus, B. microphylla var. koreana × B. sempervirens, B. harlandii, or D. virginiana.

Distribution of Serotonin-Immunoreactive Neurons in the Brain and Gnathal Ganglion of Caterpillar Helicoverpa armigera

Serotonin (5-hydroxytryptamine, 5-HT) is an important biogenic amine that acts as a neural circuit modulator. It is widespread in the central nervous system of insects. However, little is known about the distribution of serotonin in the nervous system of the cotton bollworm Helicoverpa armigera. In the present study, we performed immunohistochemical experiments with anti-serotonin serum to examine the distribution of serotonin in the central nervous system of H. armigera larvae. We found about 40 serotonin-immunoreactive neurons in the brain and about 20 in the gnathal ganglion. Most of these neurons are wide-field neurons giving rise to processes throughout the neuropils of the brain and the gnathal ganglion. In the central brain, serotonin-immunoreactive processes are present bilaterally in the tritocerebrum, the deutocerebrum, and major regions of the protocerebrum, including the central body (CB), lateral accessory lobes (LALs), clamps, crepine, superior protocerebrum, and lateral protocerebrum. The CB, anterior ventrolateral protocerebrum (AVLP), and posterior optic tubercle (POTU) contain extensive serotonin-immunoreactive process terminals. However, the regions of mushroom bodies, the lateral horn, and protocerebral bridges (PBs) are devoid of serotonin-immunoreactivity. In the gnathal ganglion, the serotonin-immunoreactive processes are also widespread throughout the neuropil, and some process projections extend to the tritocerebrum. Our results provide the first comprehensive description of the serotonergic neuronal network in H. armigera larvae, and they reveal the neural architecture and the distribution of neural substances, allowing us to explore the neural mechanisms of behaviors by using electrophysiological and pharmacological approaches on the target regions.