Invasion Biology, Ecology, and Management of Western Flower Thrips

Comparison and Functional Analysis of Odorant-Binding Proteins and Chemosensory Proteins in Two Closely Related Thrips Species, Frankliniella occidentalis and Frankliniella intonsa (Thysanoptera: Thripidae) Based on Antennal Transcriptome Analysis

Two closely related thrips species, Frankliniella occidentalis and Frankliniella intonsa, are important pests on agricultural and horticultural crops. They have several similarities, including occurrence patterns, host range, and aggregation pheromone compounds. However, there are very few reports about the chemosensory genes and olfactory mechanisms in these two species. To expand our knowledge of the thrips chemosensory system, we conducted antennal transcriptome analysis of two thrips species, and identified seven odorant-binding proteins (OBPs) and eight chemosensory proteins (CSPs) in F. occidentalis, as well as six OBPs and six CSPs in F. intonsa. OBPs and CSPs showed high sequence identity between the two thrips species. The RT-qPCR results showed that the orthologous genes FoccOBP1/3/4/5/6, FintOBP1/3/4/6, FoccCSP1/2/3, and FintCSP1/2 were highly expressed in male adults. Molecular docking results suggested that orthologous pairs FoccOBP4/FintOBP4, FoccOBP6/FintOBP6, and FoccCSP2/FintCSP2 might be involved in transporting the major aggregation pheromone compound neryl (S)-2-methylbutanoate, while orthologous pairs FoccOBP6/FintOBP6, FoccCSP2/FintCSP2, and FoccCSP3/FintCSP3 might be involved in transporting the minor aggregation pheromone compound (R)-lavandulyl acetate. These results will provide a fundamental basis for understanding the molecular mechanisms of pheromone reception in the two thrips species.

Comprehensive analysis of lysine lactylation in Frankliniella occidentalis

Western flower thrips (Frankliniella occidentalis) are among the most important pests globally that transmit destructive plant viruses and infest multiple commercial crops. Lysine lactylation (Klac) is a recently discovered novel post-translational modification (PTM). We used liquid chromatography-mass spectrometry to identify the global lactylated proteome of F. occidentalis, and further enriched the identified lactylated proteins using Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO). In the present study, we identified 1,458 Klac sites in 469 proteins from F. occidentalis. Bioinformatics analysis showed that Klac was widely distributed in F. occidentalis proteins, and these Klac modified proteins participated in multiple biological processes. GO and KEGG enrichment analysis revealed that Klac proteins were significantly enriched in multiple cellular compartments and metabolic pathways, such as the ribosome and carbon metabolism pathways. Two Klac proteins were found to be involved in the regulation of the TSWV (Tomato spotted wilt virus) transmission in F. occidentalis. This study provides a systematic report and a rich dataset of lactylation in F. occidentalis proteome for potential studies on the Klac protein of this notorious pest.

Colour vision in thrips (Thysanoptera)

Insects are an astonishingly successful and diverse group, occupying the gamut of habitats and lifestyle niches. They represent the vast majority of described species and total terrestrial animal biomass on the planet. Their success is in part owed to their sophisticated visual systems, including colour vision, which drive a variety of complex behaviours. However, the majority of research on insect vision has focused on only a few model organisms including flies, honeybees and butterflies. Especially understudied are phytophagous insects, such as diminutive thrips (Thysanoptera), in spite of their damage to agriculture. Thrips display robust yet variable colour-specific responses despite their miniaturized eyes, but little is known about the physiological and ecological basis of their visual systems. Here, we review the known visual behavioural information about thrips and the few physiological studies regarding their eyes. Eye structure, spectral sensitivity, opsin genes and the presence of putative colour filters in certain ommatidia strongly imply dynamic visual capabilities. Finally, we discuss the major gaps in knowledge that remain for a better understanding of the visual system of thrips and why bridging these gaps is important for expanding new possibilities for applied pest management strategies for these tiny insects. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Efficacy of hot water treatment for postharvest control of western flower thrips, Frankliniella occidentalis, in French beans

BACKGROUND

The western flower thrips, Frankliniella occidentalis, is a quarantine pest of French beans that requires phytosanitary treatment to meet quarantine requirements for strict lucrative markets. In this study, the efficacy of hot water treatment against F. occidentalis eggs and its effects on the postharvest physicochemical quality parameters of French beans was evaluated.

RESULTS

The immersion time of 8.01 min (95% critical limits CL 7.77-8.24) was predicted by the probit model as the minimum time required to achieve a 99.9968% control level. Confirmatory tests with a large number of F. occidentalis eggs were performed to validate the estimated time to achieve probit-9 control level, and there were no survivors from the 50 103 eggs treated. Likewise, none of the 55 364 eggs exposed to 45 ± 0.2 °C for 7 min (observational time) survived. The effect of the treatment schedule on French beans quality parameters was assessed and there were no differences in weight loss, moisture content, total soluble solids, titratable acidity, pH, and reducing sugars between treated and untreated samples.

CONCLUSION

Our results indicate that hot water treatment (at 45 ± 0.2 °C for a duration of 8.01 min is an effective phytosanitary treatment for the control of Frankliniella occidentalis on French beans, with no significant impact on pods quality. © 2022 Society of Chemical Industry.

Sex-biased proteomic response to tomato spotted wilt virus infection of the salivary glands of Frankliniella occidentalis, the western flower thrips

Successful transmission of tomato spotted wilt virus (TSWV) by Frankliniella occidentalis requires robust infection of the salivary glands (SGs) and virus delivery to plants during salivation. Feeding behavior and transmission efficiency are sexually-dimorphic traits of this thrips vector species. Proteins secreted from male and female SG tissues, and the effect of TSWV infection on the thrips SG proteome are unknown. To begin to discern thrips factors that facilitate virus infection of SGs and transmission by F. occidentalis, we used gel- and label-free quantitative and qualitative proteomics to address two hypotheses: (i) TSWV infection modifies the composition and/or abundance of SG-expressed proteins in adults; and (ii) TSWV has a differential effect on the male and female SG proteome and secreted saliva. Our study revealed a sex-biased SG proteome for F. occidentalis, and TSWV infection modulated the SG proteome in a sex-dependent manner as evident by the number, differential abundance, identities and generalized roles of the proteins. Male SGs exhibited a larger proteomic response to the virus than female SGs. Intracellular processes modulated by TSWV in males indicated perturbation of SG cytoskeletal networks and cell-cell interactions, i.e., basement membrane (BM) and extracellular matrix (ECM) proteins, and subcellular processes consistent with a metabolic slow-down under infection. Several differentially-abundant proteins in infected male SGs play critical roles in viral life cycles of other host-virus pathosystems. In females, TSWV modulated processes consistent with tissue integrity and active translational and transcriptional regulation. A core set of proteins known for their roles in plant cell-wall degradation and protein metabolism were identified in saliva of both sexes, regardless of virus infection status. Saliva proteins secreted by TSWV-infected adults indicated energy generation, consumption and protein turnover, with an enrichment of cytoskeletal/BM/ECM proteins and tricarboxylic acid cycle proteins in male and female saliva, respectively. The nonstructural TSWV protein NSs - a multifunctional viral effector protein reported to target plant defenses against TSWV and thrips - was identified in female saliva. This study represents the first description of the SG proteome and secretome of a thysanopteran and provides many candidate proteins to further unravel the complex interplay between the virus, insect vector, and plant host.

Induced Resistance Combined with RNA Interference Attenuates the Counteradaptation of the Western Flower Thrips

The western flower thrips, Frankliniella occidentalis Pergande, is an invasive pest that damages agricultural and horticultural crops. The induction of plant defenses and RNA interference (RNAi) technology are potent pest control strategies. This study investigated whether the anti-adaptive ability of F. occidentalis to jasmonic acid (JA)- and methyl jasmonate (MeJA)-induced defenses in kidney bean plants was attenuated after glutathione S-transferase (GST) gene knockdown. The expression of four GSTs in thrips fed JA- and MeJA-induced leaves was analyzed, and FoGSTd1 and FoGSTs1 were upregulated. Exogenous JA- and MeJA-induced defenses led to increases in defensive secondary metabolites (tannins, alkaloids, total phenols, flavonoids, and lignin) in leaves. Metabolome analysis indicated that the JA-induced treatment of leaves led to significant upregulation of defensive metabolites. The activity of GSTs increased in second-instar thrips larvae fed JA- and MeJA-induced leaves. Co-silencing with RNAi simultaneously knocked down FoGSTd1 and FoGSTs1 transcripts and GST activity, and the area damaged by second-instar larvae feeding on JA- and MeJA-induced leaves decreased by 62.22% and 55.24%, respectively. The pupation rate of second-instar larvae also decreased by 39.68% and 39.89%, respectively. Thus, RNAi downregulation of FoGSTd1 and FoGSTs1 reduced the anti-adaptive ability of F. occidentalis to JA- or MeJA-induced defenses in kidney bean plants.

Transcriptome analysis and screening of putative sex-determining genes in the invasive pest, Frankliniella occidentalis (Thysanoptera: Thripidae)

The invasive insect pest, Frankliniella occidentalis, is a well-known vector that transmits a variety of ornamental and vegetable viruses. The mechanistic basis of sex determination in F. occidentalis is not well understood, and this hinders our ability to deploy sterile insect technology as an integrated pest management strategy. In this study, six cDNA libraries from female and male adults of F. occidentalis (three biological replicates each) were constructed and transcriptomes were sequenced. A total of 6000 differentially-expressed genes were identified in the two sexes including 2355 up- and 3645 down-regulated genes. A total of 149 sex-related genes were identified based on GO enrichment data and included transformer-2 (tra2), fruitless (fru), male-specific lethal (msl) and sex lethal (sxl); several of these exhibited sex-specific and/or sex-biased expression in F. occidentalis. This study contributes to our understanding of the sex-determined cascade in F. occidentalis and other members of the Thysanoptera.

Copper Chaperone for Superoxide Dismutase FoCCS1 in Frankliniella occidentalis May Be Associated with Feeding Adaptation after Host Shifting

Western flower thrips (Frankliniella occidentalis) pose a serious threat to the global vegetable and flower crop production. The regulatory mechanism for superoxide dismutase (SOD) in the feeding adaptation of F. occidentalis after host shifting remains unclear. In this study, the copper chaperone for SOD (CCS) and manganese SOD (MnSOD) genes in F. occidentalis were cloned, and their expression levels at different developmental stages was determined. The mRNA expression of FoCCS1 and FoMnSOD2 in F. occidentalis second-instar larvae and adult females of F₁, F₂, and F₃ generations was analyzed after shifting the thrips to kidney bean and broad bean plants, respectively. The F₂ and F₃ second-instar larvae and F₂ adult females showed significantly upregulated FoCCS1 mRNA expression after shifting to kidney bean plants. The F₁ second-instar larvae and F₂ adult females showed significantly upregulated FoCCS1 mRNA expression after shifting to broad bean plants. The RNA interference significantly downregulated the FoCCS1 mRNA expression levels and adult females showed significantly inhibited SOD activity after shifting to kidney bean and broad bean plants. F. occidentalis adult females subjected to RNA interference and released on kidney bean and broad bean leaves for rearing, respectively, significantly reduced the survival rate and fecundity. These findings suggest that FoCCS1 plays an active role in regulating the feeding adaptation ability of F. occidentalis after host shifting.

Proteome-wide analysis of lysine 2-hydroxyisobutyrylation in Frankliniella occidentalis

BACKGROUND

Lysine 2-hydroxyisobutyrylation (Khib) is a novel and conserved post-translational modification (PTM). Frankliniella occidentalis are economically important agricultural pests globally and also notorious for vectoring destructive plant viruses. To better study the disease transmission mechanism of F. occidentalis, it is necessary to conduct in-depth analysis of it. So far, no Khib modification of insects has been reported.

RESULTS

In this study, a proteome-wide analysis of Khib modifications in F. occidentalis was analyzed for the first time through the combination of high performance liquid chromatography fractionation technology and 2-hydroxyisobutyrylated peptide enrichment and other advanced technologies, 4093 Khib sites were identified on 1125 modified proteins. Bioinformatics and functional enrichment analyses showed that Khib-modified proteins were significantly enriched in many cell compartments and pathways, especially related to various cellular components and biological processes, and were more concentrated in ribosomes and proteasome subunits, involved in energy metabolism, protein synthesis and degradation, compared to the other nine species including Japonica rice, Homo sapiens, P. patens, Botrytis, Ustilaginoidea virens, Saccharomyces cerevisiae, T. gondii, C. albicans, and F. oxysporum. And Khib sites on virus-interacting insect proteins were discovered for the first time, such as cyclophilin and endoCP-GN.

CONCLUSIONS

After three repeated experiments, we found a total of 4093 Khib sites on 1125 proteins. These modified proteins are mainly concentrated in ribosomes and proteasome subunits, and are widely involved in a variety of critical biological activities and metabolic processes of F. occidentalis. In addition, for the first time, Khib modification sites are found on the proteome of F. occidentalis, and these sites could be acted as for the virus interaction, including cyclophilin and endoCP-GN. The global map of 2-hydroxyisobutyrylation in thrips is an invaluable resource to better understand the biological processes of thrips and provide new means for disease control and mitigation of pest damage to crops.

Egg Predation by Phytoseiid Predatory Mites: Is There Intraguild Predation Towards Predatory Bug Eggs?

Phytoseiid predatory mites are efficient biocontrol agents of important thrips pests, such as the western flower thrips, Frankliniella occidentalis Pergande (Thysanoptera: Thripidae). Until recently, it was believed that first instars, and to a lesser extent second instars, were the most vulnerable developmental stages of thrips to be attacked by phytoseiids. However, recent evidence showed that some phytoseiids can detect and prey upon thrips eggs inserted in the leaf tissue. As phytoseiid predatory mites often co-occur with other beneficial insects, such as mirid and anthocorid predatory bugs which also insert their eggs inside leaf material, this raises the question whether phytoseiid predatory mites may also feed on predatory bug eggs. Here we first tested the potential of Amblyseius swirskii Athias-Henriot, Transeius montdorensis Schicha, and Amblydromalus limonicus Garman and McGregor (Acari: Phytoseiidae) to kill eggs of F. occidentalis in leaf tissue. Secondly, we tested whether those phytoseiids were capable of killing eggs of Orius laevigatus Fieber (Hemiptera: Anthocoridae), Macrolophus pygmaeus Rambur and Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae), three biocontrol agents that also insert their eggs inside plant tissue. Our results showed that A. swirskii and A. limonicus could kill thrips eggs, whereas T. montdorensis could not. Furthermore, we show that the presence of phytoseiid predatory mites does not affect the hatch rate of predatory bugs that insert their eggs inside leaves.

Transcriptome analysis of female western flower thrips, Frankliniella occidentalis, exhibiting neo-panoistic ovarian development

The western flower thrips, Frankliniella occidentalis, is one of the most devastating insect pests with explosive reproductive potential. However, its reproductive physiological processes are not well understood. This study reports the ovarian development and associated transcriptomes of F. occidentalis. Each ovary consisted of four ovarioles, each of which contained a maximum of nine follicles in the vitellarium. The germarium consisted of several dividing cells forming a germ cell cluster, presumably consisting of oocytes and nurse cells. The nurse cells were restricted to the germarium while the subsequent follicles did not possess nurse cells or a nutritive cord, supporting the neo-panoistic ovariole usually found in thysanopteran insects. Oocyte development was completed 72 h after adult emergence (AAE). Transcriptome analysis was performed at mid (36 h AAE) and late (60 h AAE) ovarian developmental stages using RNA sequencing (RNASeq) technology. More than 120 million reads per replication were matched to ≈ 15,000 F. occidentalis genes. Almost 500 genes were differentially expressed at each of the mid and late ovarian developmental stages. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that these differentially expressed genes (DEGs) were associated with metabolic pathways along with protein and nucleic acid biosynthesis. In both ovarian developmental stages, vitellogenin, mucin, and chorion genes were highly (> 8-fold) expressed. Endocrine signals associated with ovarian development were further investigated from the DEGs. Insulin and juvenile hormone signals were upregulated only at 36 h AAE, whereas the ecdysteroid signal was highly maintained at 60 h AAE. This study reports the transcriptome associated with the ovarian development of F. occidentalis, which possesses a neo-panoistic ovariole.

Bacterial Competition Influences the Ability of Symbiotic Bacteria to Colonize Western Flower Thrips

Symbiont mediated RNAi (SMR) is a promising method for precision control of pest insect species such as Western Flower Thrips (WFT). Two species of bacteria are known to be dominant symbiotic bacteria in WFT, namely BFo1 and BFo2 (Bacteria from Frankliniella occidentalis 1 and 2), as we here confirm by analysis of next-generation sequence data derived to obtain a reference WFT genome sequence. Our first demonstration of SMR in WFT used BFo2, related to Pantoea, isolated from a domesticated Dutch thrips population. However, for successful use of SMR as a thrips control measure, these bacteria need to successfully colonize different environmental thrips populations. Here, we describe a United Kingdom thrips population that does not harbour BFo2, but does contain BFo1, a species related to Erwinia. Attempts to introduce BFo2 indicate that this bacterium is unable to establish itself in the United Kingdom thrips, in contrast to successful colonization by a strain of BFo1 expressing green fluorescent protein. Fluorescence microscopy indicates that BFo1 occupies similar regions of the thrips posterior midgut and hindgut as BFo2. Bacterial competition assays revealed that a barrier to BFo2 establishing itself in thrips is the identity of the resident BFo1; BFo1 isolated from the United Kingdom thrips suppresses growth of BFo2 to a greater extent than BFo1 from the Dutch thrips that is permissive for BFo2 colonization. The ability of the latter strain of BFo1 to colonize the United Kingdom thrips is also likely attributable to its ability to out-compete the resident BFo1. Lastly, we observed that United Kingdom thrips pre-exposed to the Dutch BFo1 could then be successfully colonized by BFo2. These results indicate, for the first time, that microbial competition and strain differences can have a large influence on how symbiotic bacteria can colonize different populations of an insect species.

Spinetoram resistance drives interspecific competition between Megalurothrips usitatus and Frankliniella intonsa

BACKGROUND

Species displacement by the outcome of interspecific competition is of particular importance to pest management. Over the past decade, spinetoram has been extensively applied in control of the two closely related thrips Megalurothrips usitatus and Frankliniella intonsa worldwide, while whether its resistance is implicated in mediating interspecific interplay of the two thrips remains elusive to date.

RESULTS

Field population dynamics (from 2017 to 2019) demonstrated a trend toward displacement of F. intonsa by M. usitatus on cowpea crops, supporting an existing interspecific competition. Following exposure to spinetoram, M. usitatus became the predominate species, which suggests the use of spinetoram appears to be responsible for mediating interspecific interactions of the two thrips. Further annual and seasonal analysis (from 2016 to 2020) of field-evolved resistance dynamics revealed that M. usitatus developed remarkably higher resistance to spinetoram compared to that of F. intonsa, implying a close relationship between evolution of spinetoram resistance and their competitive interactions. After 12 generations of laboratory selection, resistance to spinetoram in M. usitatus and F. intonsa increased up to 64.50-fold and 28.33-fold, and the average realized heritability (h² ) of resistance was calculated as 0.2550 and 0.1602, respectively. Interestingly, two-sex life table analysis showed that the spinetoram-resistant strain of F. intonsa exhibited existing fitness costs, but not the M. usitatus. These indicate that a rapid development of spinetoram resistance and the lack of associated fitness costs may be the mechanism underlying recent dominance of M. usitatus over F. intonsa.

CONCLUSION

Collectively, our results uncover the involvement of insecticide resistance in conferring displacement mechanism behind interspecific competition, providing a framework for understanding the significance of the evolutionary relationships among insects under ongoing changing environments. These findings also can be invaluable in proposing the most appropriate strategies for sustainable thrips control programs. © 2022 Society of Chemical Industry.

Efficient control of western flower thrips by plastid-mediated RNA interference

Plastid-mediated RNA interference (PM-RNAi) has emerged as a promising strategy for pest control. Expression from the plastid genome of stable double-stranded RNAs (dsRNAs) targeted against essential insect genes can effectively control some herbivorous beetles, but little is known about the efficacy of the transplastomic approach in other groups of pest insects, especially nonchewing insects that do not consume large amounts of leaf material. Here we have investigated the susceptibility of the western flower thrip (WFT, Frankliniella occidentalis), a notorious pest in greenhouses and open fields, to PM-RNAi. We show that WFTs ingest chloroplasts and take up plastid-expressed dsRNAs. We generated a series of transplastomic tobacco plants expressing dsRNAs and hairpin RNAs (hpRNAs) targeted against four essential WFT genes. Unexpectedly, we discovered plastid genome instability in transplastomic plants expressing hpRNAs, suggesting that dsRNA cassettes are preferable over hpRNA cassettes when designing PM-RNAi strategies. Feeding studies revealed that, unlike nuclear transgenic plants, transplastomic plants induced a potent RNAi response in WFTs, causing efficient suppression of the targeted genes and high insect mortality. Our study extends the application range of PM-RNAi technology to an important group of nonchewing insects, reveals design principles for the construction of dsRNA-expressing transplastomic plants, and provides an efficient approach to control one of the toughest insect pests in agriculture and horticulture.

Increasing Frequency of G275E Mutation in the Nicotinic Acetylcholine Receptor α6 Subunit Conferring Spinetoram Resistance in Invading Populations of Western Flower Thrips in China

Simple Summary

The western flower thrips (WFT) Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is an important invasive pest in agriculture and forestry. It has developed resistance to a frequently used pesticide spinetoram world widely, including the invading area of China. However, the mechanism of resistance to spinetoram is unclear in China. In this study, we found the presence of the G275E mutation in the nicotinic acetylcholine receptor Foα6 in the early invading populations, which has now increased to a high frequency in China. There was a correlation between the frequency of the G275E mutation and resistance to spinetoram as characterized by median lethal concentration. Our results showed that G275E mutation is one of the mechanisms conferring spinetoram resistance in invading populations in China, as in many other countries. Our study highlights the rapid spread of the G275E mutation in China in the 2009–2021 period.

Abstract

The western flower thrips Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is an important invasive pest worldwide. Field-evolved resistance to the pesticide spinetoram is an increasing problem in the chemical control of this pest. Here, we examined changes in the frequency of a genetic mutation associated with spinetoram resistance, the G275E mutation in the acetylcholine receptor Foα6, in 62 field populations collected from 2009 to 2021 across areas of China invaded by this pest. We found a low frequency of the G275E mutation in populations collected at the early invasion stage, in contrast to a high frequency in native USA populations. However, the frequency of the G275E mutation has increased to a high level in recently collected populations, with the mutation becoming fixed in some populations. There was a correlation between the frequency of the G275E mutation and resistance to spinetoram as characterized by median lethal concentration, although two populations were outliers. These results showed that G275E mutation is one of the mechanisms conferring spinetoram resistance in many invading populations in China. Ongoing dispersal of the WFT may have facilitated a rapid increase in the G275E mutation across China. Our study highlights the rapid evolution of pesticide resistance in an invasive species and points to a useful marker for molecular diagnostics of spinetoram resistance.

Are Pesticides Used to Control Thrips Harmonious with Soil-Dwelling Predatory Mite Cosmolaelaps sabelis (Mesostigmata: Laelapidae)?

Edaphic predatory mites could be introduced in pest management programs of pests that live, or spend part of their life cycle, in the soil. Some mesostigmatic mites have been widely used for the management of different species of thrips (Thysanoptera), especially in protected cultivation. The edaphic predator Cosmolaelaps sabelis (Mesostigmata: Laelapidae) was a model species in this study, being exposed to the most applied insecticides for the control of thrips in Brazil. After lethal, sublethal and transgenerational effects were evaluated. The pesticides acephate, acetamiprid + etofenprox, azadirachtin, spinetoram, formetanate hydrochloride, and imidacloprid were classified according to the IOBC/WPRS (International Organization for Biological Control-West Paleartic Regional Section) recommendation, considering the acute toxicity and the effects on adult females' reproduction, in the maternal and first generation. The pesticides acetamiprid + etofenprox and azadirachtin were classified as slightly harmful (Class 2), while spinetoram was classified as moderately harmful (Class 3). Acephate and formetanate hydrochloride were classified as harmful (Class 4). Only imidacloprid didn't cause negative effects on the females. Regarding effects on the first generation, acetamiprid + etofenprox, azadirachtin, and spinetoram caused reduction in the oviposition rates. Therefore, we suggest that complimentary bioassays should be done under semi-field and field conditions using the pesticides that were considered harmful in this study, to assess their effects on this predator in other environments prior to recommending not to use them in integrated programs to manage soil-based pests using chemical and biological tools.

Problems with the Concept of "Pest" among the Diversity of Pestiferous Thrips

Almost all of the thrips species that are considered pests are members of a single subfamily of Thripidae, the Thripinae, a group that represents less than 30% of the species in the insect Order Thysanoptera. Three of the five major Families of Thysanoptera (Aeolothripidae, Heterothripidae, Melanthripidae) are not known to include any pest species. The Phlaeothripidae that includes more than 50% of the 6300 thrips species listed includes very few that are considered to be pests. Within the Thripidae, the members of the three smaller subfamilies, Panchaetothripinae, Dendrothripinae and Sericothripinae, include remarkably few species that result in serious crop losses. It is only in the subfamily Thripinae, and particularly among species of the Frankliniella genus-group and the Thrips genus-group that the major thrips species are found, including all but one of the vectors of Orthotospovirus infections. It is argued that the concept of pest is a socio-economic problem, with the pest status of any particular species being dependent on geographical area, cultivation practices, and market expectations as much as the intrinsic biology of any thrips species.

Effects of elevated CO2 on activities of protective and detoxifying enzymes in Frankliniella occidentalis and F. intonsa under spinetoram stress

BACKGROUND

Elevated CO₂ can directly affect the toxicity of insecticides to insects and the physiological response of insects to insecticides. Frankliniella occidentalis and F. intonsa are highly destructive pests that target horticultural crops. Spinetoram is an effective pesticide against thrips. This study sought to explore the effect of elevated CO₂ on efficacy of spinetoram against F. occidentalis and F. intonsa and effect of the spinetoram on activities of protective and detoxifying enzymes under elevated CO₂ . Notably, these enzymes can be exploited in further studies to develop interventions for thrips resistance management.

RESULTS

Toxicity bioassay showed that the LC₅₀ values of F. occidentalis and F. intonsa exposed to spinetoram at elevated CO₂ (800 μL L^-1 concentration) for 48 h was 0.08 and 0.006 mg L^-1 , respectively, which is 0.62 and 0.75 times of the values at ambient CO₂ (400 μL L^-1 concentration). The findings showed that elevated CO₂ decreased activities of the superoxide dismutase and acetylcholinesterase in thrips, while increasing the activities of carboxylesterase and glutathione S-transferase. However, spinetoram increased activities of protective and detoxifying enzymes in both thrips under the two CO₂ levels. Elevated CO₂ and spinetoram affect the physiological enzyme activity in thrips synergistically, and the activities of analyzed enzymes were generally higher in F. occidentalis than in F. intonsa.

CONCLUSION

Elevated CO₂ amplifies the efficacy of spinetoram on thrips, F. intonsa is more susceptibility to spinetoram than F. occidentalis and the latter showed better adaptation to adverse conditions than the former. © 2021 Society of Chemical Industry.

Combined Transcriptome and Metabolome Analysis of Alfalfa Response to Thrips Infection

Thrips (Thysanoptera: Thripidae) is a major insect pest for alfalfa which can result in decreased plant nutrients, low yields, and even plant death. To identify the differentially expressed genes and metabolites in response to thrips in alfalfa, a combination of metabolomics and transcriptomics was employed using alfalfa (Caoyuan No. 2) with and without thrips infestation. The results showed that the flavonoid biosynthesis and isoflavonoid biosynthesis pathways were the most significantly enriched pathways in response to thrips infection, as shown by the combined transcriptome and metabolome analysis. The transcriptome results showed that SA and JA signal transduction and PAPM-triggered immunity and the MAPK signaling pathway–plant pathways played a crucial role in thrips-induced plant resistance in alfalfa. In addition, we found that thrips infestation could also induce numerous changes in plant primary metabolism, such as carbohydrate and amino acid metabolism as compared to the control. Overall, our results described here should improve fundamental knowledge of molecular responses to herbivore-inducible plant defenses and contribute to the design of strategies against thrips in alfalfa.

Life Table and Preference Choice of Frankliniella occidentalis (Thysanoptera: Thripidae) for Kidney Bean Plants Treated by Exogenous Calcium

Simple Summary

Western flower thrips, Frankliniella occidentalis, is an invasive key pest that damages vegetables and ornamentals worldwide. The activation of induced resistance by chemicals may provide a simple and feasible way of achieving improvement of resistance to stress in crop plants, which is an important technology for the development of sustainable agriculture. Calcium (Ca) is an essential element for plants; numerous studies have shown that Ca can confer crop plants with resistance to abiotic and biotic stresses. For the first time, we report the negative effects of exogenous Ca on kidney bean plants in relation to the performance of F. occidentalis, including a reduced preference of thrips. Therefore, Ca could potentially be used to control F. occidentalis.

Abstract

Exogenous calcium (Ca) has been used to induce host plant resistance in response to abiotic and biotic stresses, including from thrips attack. The aim of this study was to determine whether exogenously applied Ca affects the performance of Frankliniella occidentalis. We assessed the development time, total longevity, reproduction, and population parameters of F. occidentalis, and its preference choice on Ca-treated or untreated control kidney bean plants under laboratory conditions. The results showed that F. occidentalis fed on Ca-treated leaves had a longer developmental time but lower longevity (female and male) and fecundity than F. occidentalis fed on control leaves. Population parameters, including the intrinsic rate of increase (r), finite rate of increase (λ), and net reproductive rate (R₀), were all found higher in control leaves than in Ca-treated leaves, and the mean generation time (T) was shorter. In preference choices, the number of thrips on control plants was higher than the number of thrips on Ca-treated kidney bean plants. Overall, our results indicated that exogenous Ca pretreatment on kidney bean plants affected the life history and preference choice of F. occidentalis, suggesting Ca might be used as a promising elicitor of inducible plant defense against thrips.

Complexity and local specificity of the virome associated with tospovirus-transmitting thrips species

Frankliniella occidentalis (western flower thrips=WFT) and Thrips tabaci (onion thrips=OT) are insect species that greatly impact horticultural crops through direct damage and their efficient vectoring of tomato spotted wilt virus and iris yellow spot virus. In this study we collected thrips of these species from 12 field populations in various regions in Italy. We also included one field population of Neohydatothrips variabilis (soybean thrips=ST) from the U.S.A. Total RNAseq from high-throughput sequencing (HTS) was used to assemble the virome and then we assigned putative viral contigs to each thrips sample by qRT-PCR. Excluding plant and fungal viruses, we were able to identify 61 viral segments, corresponding to 41 viruses: 14 were assigned to WFT, 17 to OT, one from ST and 9 viruses could not be assigned to any species based on our stringent criteria. All these viruses are putative representative of new species (with only the exception of a sobemo-like virus that is 100% identical to a virus recently characterized in ST) and some belong to new higher-ranking taxa. These additions to the viral phylogeny suggest previously undescribed evolutionary niches. Most of the Baltimore's classes of RNA viruses were present (positive- and minus- strand and dsRNA viruses), but only one DNA virus was identified in our collection. Repeated sampling in a subset of locations in 2019 and 2020 and further virus characterization in a subset of four thrips populations maintained in laboratory allowed us to provide evidence of a locally persistent thrips core virome that characterizes each population. IMPORTANCE Harnessing the insect microbiome can result in new approaches to contain their populations or the damage they cause vectoring viruses of medical, veterinary, or agricultural importance. Persistent insect viruses are a neglected component of their microbiota. Here for the first time, we characterize the virome associated with the two model systems for tospovirus-transmitting thrips species, of utmost importance for the direct and indirect damage they cause to a number of different crops. The thrips virome here characterized includes several novel viruses, that in some cases reveal previously undescribed clades. More importantly, some of the viruses we describe are part of a core virome that is specific and consistently present in distinct geographical locations monitored over the years, hinting at a possible mutualistic symbiotic relationship with their host.

Morphological and Chemical Factors Related to Western Flower Thrips Resistance in the Ornamental Gladiolus

Understanding the mechanisms involved in host plant resistance opens the way for improved resistance breeding programs by using the traits involved as markers. Pest management is a major problem in cultivation of ornamentals. Gladiolus (Gladiolus hybridus L.) is an economically important ornamental in the Netherlands. Gladiolus is especially sensitive to attack by western flower thrips (Frankliniella occidentalis (Pergande) (Thysanoptera:Thripidae)). The objective of this study was, therefore, to investigate morphological and chemical markers for resistance breeding to western flower thrips in Gladiolus varieties. We measured thrips damage of 14 Gladiolus varieties in a whole-plant thrips bioassay and related this to morphological traits with a focus on papillae density. Moreover, we studied chemical host plant resistance to using an eco-metabolomic approach comparing the 1H NMR profiles of thrips resistant and susceptible varieties representing a broad range of papillae densities. Thrips damage varied strongly among varieties: the most susceptible variety showed 130 times more damage than the most resistant one. Varieties with low thrips damage had shorter mesophylls and epidermal cells, as well as a higher density of epicuticular papillae. All three traits related to thrips damage were highly correlated with each other. We observed a number of metabolites related to resistance against thrips: two unidentified triterpenoid saponins and the amino acids alanine and threonine. All these compounds were highly correlated amongst each other as well as to the density of papillae. These correlations suggest that papillae are involved in resistance to thrips by producing and/or storing compounds causing thrips resistance. Although it is not possible to distinguish the individual effects of morphological and chemical traits statistically, our results show that papillae density is an easy marker in Gladiolus-breeding programs targeted at increased resistance to thrips.

Host plant resistance to thrips (Thysanoptera: Thripidae) - current state of art and future research avenues

Integrated Pest Management (IPM) is endorsed as the future standard for crop protection worldwide. This holistic concept integrates preventative and curative measures amongst which host plant resistance (HPR) plays an essential role. Up to now HPR has been a somewhat under-utilized tool in pest management due to widespread use of pesticides and technological hindrance. Thrips are key pests in agriculture and horticulture worldwide. Here we provide an overview on the current status of research on constitutive and induced HPR including thrips-host relationships and thrips as virus vectors. We stress modulation of plant defense responses by abiotic and biotic elicitors to increase HPR and provide an outlook on the increasing potential of HPR inspired by the fast advancement of -omics techniques.

Effect of Different Plants on the Growth and Reproduction of Thrips flavus (Thysanoptera: Thripidae)

Simple Summary

Thrips flavus Schrank (Thysanoptera: Thripidae) is a worldwide phytophagous pest in Palearctic Asian and European countries. T. flavus feeds on a wide spectrum of host plants. Thus, understanding its host plant preferences is important for pest control. We tested the development duration, population parameters, and population growth of T. flavus on five species of plants. The intrinsic rate of increase and fecundity was the highest on Cucumis sativus, followed by Glycine max and Capsicum annuum. However, Solanum melongena and Brassica rapa var. glabra were not suitable host plants. These results help to improve our understanding of the population dynamics of T. flavus and should lead to positive measures to control thrips in the field.

Abstract

Host plants play an important role in affecting insect development and reproduction. Understanding the host plant preferences is important for pest control. Thrips flavus Schrank (Thysanoptera: Thripidae) is a worldwide phytophagous pest in Palearctic Asian and European countries. We used a life table analysis to study the development duration, population parameters, and population growth of T. flavus on five plant species, including Solanum melongena (Solanaceae), Capsicum annuum (Solanaceae), Glycine max (Leguminosae), Brassica rapa var. glabra (Cruciferae), and Cucumis sativus (Cucurbitaceae). The results showed that T. flavus can survive and reproduce on Cu. sativus and G. max, which were two potentially suitable host plants. T. flavus preferred to oviposit on Cu. sativus with a shorter duration of development (17.8 days) at 25 °C. Therefore, the host plant was an important factor influencing the development and fecundity of T. flavus populations. These results will improve our understanding of the population dynamics of T. flavus and facilitate the development of more scientific and efficient measures to control thrips.

Orius similis (Hemiptera: Anthocoridae): A Promising Candidate Predator of Spodoptera frugiperda (Lepidoptera: Noctuidae)

The bug Orius similis Zheng is a native generalist predator of insect pests in southern China. The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), is a notorious defoliator that is now an economically important insect pest throughout China. To investigate the ability of O. similis to control FAW, we evaluated the predatory capacity, behavior, and functional response of O. similis with respect to FAW and their olfactory response. Both females and males successfully preyed on FAW eggs and first-instar larvae but not on second-instar or older larvae. Adult O. similis only attacked and killed one egg or one larva at a time before sucking the prey, and similar predatory behavior was also observed with regard to FAW egg masses. Both female and male O. similis exhibited type II functional response when preying on FAW eggs and first-instar larvae. Maximum estimated prey consumed per day was 23.7 eggs and 26.2 larvae for adult females and 22.5 eggs and 19.6 larvae for adult males. Moreover, in a Y-tube olfactometer experiment, both female and male O. similis exhibited a significant preference for maize (Zea mays L.) seedlings damaged by FAW over clean air, healthy seedlings, FAW feces, and FAW, suggesting that seedlings damaged by FAW may releases specific volatiles attracting the predator. Collectively, the results of the study suggest that O. similis is a promising candidate for the biological control of FAW eggs and first-instar larvae, particularly given its attraction to FAW-damaged leaves, which may enable it to locate the target prey rapidly.

Characterization of Frankliniella occidentalis and Frankliniella bispinosa (Thysanoptera: Thripidae) Injury to Strawberry

Frankliniella flower thrips are pests of numerous fruit and vegetable crops as they feed and reproduce in the flowers and fruits. The invasive Frankliniella occidentalis (Pergande) from the Southwestern United States, an economic pest in Florida since 2005, and the native Frankliniella bispinosa Morgan are both found in Florida strawberries. The objective of this research was to characterize injury to strawberry fruit from these species. Densities of 0, 2, 4, 8, 16, or 32 adult females of each species were caged on a strawberry plant with one flower. The cages were removed after 2, 5, or 10 d, and the number of surviving adults and the number of larvae determined. Injury was quantified on the mature fruit 21 d after thrips were introduced onto the experimental plant. Initial densities as low as two adult F. occidentalis females per flower decreased fruit set and increased cat-facing on the fruits that developed. There were no significant effects on fruit set and cat-facing by any initial density of F. bispinosa. Feeding by thrips on the small, green fruit affected the size and shape of the mature fruit: the diameters were decreased as a linear function of increased feeding by F. bispinosa, and the weights were decreased as a linear function of increased feeding by F. occidentalis. Overall, results showed that F. occidentalis was more damaging to strawberry than F. bispinosa. The differences in pest status between these common flower thrips species in Florida presents challenges to management programs.

Species Composition of Thrips (Thysanoptera: Thripidae) in Strawberry High Tunnels in Denmark

Thrips are a major pest in protected strawberry production. Knowledge of thrips species composition could be instrumental for improved thrips management, but very little is known about which species are present in strawberries grown in high-tunnels in Denmark. Thrips (adults and larvae) were sampled in two strawberry tunnels of the cultivars Murano and Furore from May to August 2018, in the middle and in the edges of the tunnels. The most abundant thrips species found in the tunnels were Frankliniella intonsa and Thrips tabaci adults. Frankliniella intonsa were also the most frequently found species of the immatures sampled, followed by T. tabaci larvae, and other species. The number of thrips differed between the two cultivars, sampling times and location in the tunnel. Frankliniella intonsa was more abundant in the middle of the tunnels, while T. tabaci was more abundant in the edge of the tunnels adjacent to the field margins. The number of thrips peaked by the end of July. Both chemical and biological control should consider species composition and occurrence; hence, a fundamental first step for thrips management is to identify the species present on the target crop.

Rosmarinus officinialis L. (Lamiales: Lamiaceae), a Promising Repellent Plant for Thrips Management

A number of thrips species are among the most significant agricultural pests globally. Use of repellent intercrop plants is one of the key components in plant-based 'push-pull' strategies to manage pest populations. In this study, the behavioral responses of three thrips species, Frankliniella occidentalis (Pergande), Frankliniella intonsa (Trybom), and Thrips palmi Karny (Thysanoptera: Thripidae) to Rosmarinus officinalis were investigated in Y-tube olfactometer bioassays and cage experiments. In addition, the major volatile compounds from rosemary were identified and the effect of the individual compounds on thrips behavior was evaluated. Females and males of the three thrips species were significantly repelled by the volatiles from cut rosemary leaves. The presence of rosemary plants significantly reduced settlement of females of the three thrips species and eggs laid by F. occidentalis females on target host plants. In total, 47 compounds were identified in the volatiles collected from the cut leaves of rosemary plants. The responses of the three thrips species to 10 major volatile compounds showed significant differences. However, α-pinene, the most abundant volatile, was repellent to F. occidentalis and F. intonsa. Eucalyptol, the second most abundant volatile, showed significant repellent activity to all the three thrips species. Our findings showed that rosemary is a promising repellent plant against the three thrips pests we tested, which could be a good candidate for 'push' plants in plant-based 'push-pull' strategies. The identified volatile compounds that accounted for the repellent activity could be developed as repellents for sustainable thrips management.

Association Between Susceptibility of Thrips palmi to Spinetoram and Frequency of G275E Mutation Provides Basis for Molecular Quantification of Field-Evolved Resistance

Putative mechanisms underlying spinosyn resistance have been identified in controlled studies on many species; however, mechanisms underlying field-evolved resistance and the development of a molecular diagnostic method for monitoring field resistance have lagged behind. Here, we examined levels of resistance of melon thrips, Thrips palmi Karny (Thysanoptera:Thripidae), to spinetoram as well as target site mutations in field populations across China to identify potential mechanisms and useful molecular markers for diagnostic and quantifying purposes. In resistant populations, we identified the G275E mutation, which has previously been linked to spinosyns resistance, and F314V mutation, both located in the α6 subunit of the nicotinic acetylcholine receptor. There was a strong correlation between levels of spinetoram resistance and allele frequency of G275E mutation in field-collected populations (r2 = 0.84) and those reared under laboratory conditions for two to five generations (r2 = 0.91). LC50 ranged from 0.12 to 0.66 mg/liter in populations without G275E mutation, whereas it ranged from 33.12 to 39.91 mg/liter in most populations with a G275E mutation frequency more than 90%. Our results indicate that the field-evolved resistance of T. palmi to spinetoram in China is mainly conferred by the G275E mutation. The frequency of the G275E mutation provides a useful diagnostic for quantifying resistance levels in field populations of T. palmi.

Intercropping Rosemary (Rosmarinus officinalis) with Sweet Pepper (Capsicum annum) Reduces Major Pest Population Densities without Impacting Natural Enemy Populations

Intercropping of aromatic plants provides an environmentally benign route to reducing pest damage in agroecosystems. However, the effect of intercropping on natural enemies, another element which may be vital to the success of an integrated pest management approach, varies in different intercropping systems. Rosemary, Rosmarinus officinalis L. (Lamiaceae), has been reported to be repellent to many insect species. In this study, the impact of sweet pepper/rosemary intercropping on pest population suppression was evaluated under greenhouse conditions and the effect of rosemary intercropping on natural enemy population dynamics was investigated. The results showed that intercropping rosemary with sweet pepper significantly reduced the population densities of three major pest species on sweet pepper, Frankliniella intonsa, Myzus persicae, and Bemisia tabaci, but did not affect the population densities of their natural enemies, the predatory bug, Orius sauteri, or parasitoid, Encarsia formosa. Significant pest population suppression with no adverse effect on released natural enemy populations in the sweet pepper/rosemary intercropping system suggests this could be an approach for integrated pest management of greenhouse-cultivated sweet pepper. Our results highlight the potential of the integration of alternative pest control strategies to optimize sustainable pest control.

Semiochemicals for Thrips and Their Use in Pest Management

Thrips (Thysanoptera) are small insects that can cause huge problems in agriculture, horticulture, and forestry through feeding and the transmission of plant viruses. They produce a rich chemical diversity of pheromones and allomones and also respond to a broad range of semiochemicals from plants. These semiochemicals offer many opportunities to develop new approaches to pest management. Aggregation pheromones and plant-derived semiochemicals are already available in commercial products. We review these semiochemicals and consider how we can move away from using them mainly for monitoring to using them for control. We still know very little about the behavioral responses of thrips to semiochemicals, and we show that research in this area is needed to improve the use of semiochemicals in pest management. We also propose that thrips should be used as a model system for semiochemically mediated behaviors of small insects that have limited ability to fly upwind.

Elicitor Application in Strawberry Results in Long-Term Increase of Plant Resilience Without Yield Loss

For a first step integrating elicitor applications into the current IPM strategy increasing plant resilience against pests, we investigated repeated elicitor treatments in a strawberry everbearer nursery and cropping cycle under glass. During nursery methyl-jasmonate (MeJA), testing induction of defenses with plant bioassays was applied every 3 weeks. Thrips damage and reproduction by spider mites, whitefly and aphids were strongly reduced upon elicitor treatment. Subsequently, we applied MeJA every 3 weeks or based on scouting pests during a whole cropping cycle. Thrips leaf bioassays and LC-MS leaf metabolomics were applied to investigate the induction of defenses. Leaf damage by thrips was lower for both MeJA application schemes compared to the control except for the last weeks. While elicitor treatments after scouting also reduced damage, its effect did not last. Thrips damage decreased from vegetative to mature plants during the cropping cycle. At the end of the nursery phase, plants in the elicitor treatment were smaller. Surprisingly, growth during production was not affected by MeJA application, as were fruit yield and quality. LC-MS leaf metabolomics showed strong induction of vegetative plants decreasing during the maturation of plants toward the end of cultivation. Concurrently, no increase in the JA-inducible marker PPO was observed when measured toward the end of cultivation. Mostly flavonoid and phenolic glycosides known as plant defense compounds were induced upon MeJA application. While induced defense decreased with the maturation of plants, constitutive defense increased as measured in the leaf metabolome of control plants. Our data propose that young, relatively small plant stages lack constitutive defense necessitating an active JA defense response. As plants, mature constitutive defense metabolites seem to accumulate, providing a higher level of basal resistance. Our results have important implications for but are not limited to strawberry cultivation. We demonstrated that repeated elicitor application could be deployed as part of an integrated approach for sustainable crop protection by vertical integration with other management tactics and horizontal integration to control multiple pests concurrently. This approach forms a promising potential for long-term crop protection in greenhouses.

Emulsions containing essential oils, their components or volatile semiochemicals as promising tools for insect pest and pathogen management

Most of the traditional strategies used for facing the management of insect pest and diseases have started to fail due to different toxicological issues such as the resistance of target organism and the impact on environment and human health. This has made mandatory to seek new effective strategies, which minimize the risks and hazards without compromising the effectiveness of the products. The use of essential oils, their components and semiochemicals (pheromones and allelochemicals) has become a promising safe and eco-sustainable alternative for controlling insect pest and pathogens. However, the practical applications of this type of molecules remain rather limited because their high volatility, poor solubility in water and low chemical stability. Therefore, it is required to design strategies enabling their use without any alteration of their biological and chemical properties. Oil-in-water nano/microemulsions are currently considered as promising tools for taking advantage of the bioactivity of essential oils and their components against insects and other pathogens. Furthermore, these colloidal systems also allows the encapsulation and controlled release of semiochemicals, which enables their use in traps for monitoring, trapping or mating disruption of insects, and in push-pull strategies for their behavioral manipulation. This has been possible because the use of nano/microemulsions allows combining the protection provided by the hydrophobic environment created within the droplets with the enhanced dispersion of the molecules in an aqueous environment, which favors the handling of the bioactive molecules, and limits their degradation, without any detrimental effect over their biological activity. This review analyzes some of the most recent advances on the use of emulsion-like dispersions as a tool for controlling insect pest and pathogens. It is worth noting that even though the current physico-chemical knowledge about these systems is relatively poor, a deeper study of the physico-chemical aspects of nanoemulsions/microemulsions containing essential oils, their components or semiochemicals, may help for developing most effective formulations, enabling the generalization of their use.

The Epidemiology of Plant Virus Disease: Towards a New Synthesis

Epidemiology is the science of how disease develops in populations, with applications in human, animal and plant diseases. For plant diseases, epidemiology has developed as a quantitative science with the aims of describing, understanding and predicting epidemics, and intervening to mitigate their consequences in plant populations. Although the central focus of epidemiology is at the population level, it is often necessary to recognise the system hierarchies present by scaling down to the individual plant/cellular level and scaling up to the community/landscape level. This is particularly important for diseases caused by plant viruses, which in most cases are transmitted by arthropod vectors. This leads to range of virus-plant, virus-vector and vector-plant interactions giving a distinctive character to plant virus epidemiology (whilst recognising that some fungal, oomycete and bacterial pathogens are also vector-borne). These interactions have epidemiological, ecological and evolutionary consequences with implications for agronomic practices, pest and disease management, host resistance deployment, and the health of wild plant communities. Over the last two decades, there have been attempts to bring together these differing standpoints into a new synthesis, although this is more apparent for evolutionary and ecological approaches, perhaps reflecting the greater emphasis on shorter often annual time scales in epidemiological studies. It is argued here that incorporating an epidemiological perspective, specifically quantitative, into this developing synthesis will lead to new directions in plant virus research and disease management. This synthesis can serve to further consolidate and transform epidemiology as a key element in plant virus research.

Fast Recognition of Lecanicillium spp., and Its Virulence Against Frankliniella occidentalis

BACKGROUND

Frankliniella occidentalis (Thysanoptera: Thripidae) is a highly rasping-sucking pest of numerous crops. The entomogenous fungi of Lecanicillium spp. are important pathogens of insect pests, and some species have been developed as commercial biopesticides. To explore Lecanicillium spp. resources in the development of more effective F. occidentalis controls, efficient barcode combinations for strain identification were screened from internal transcribed spacers (ITS), SSU, LSU, TEF, RPB1, and RPB2 genes.

RESULTS

Six genes were used to reconstruct Lecanicillium genus phylogeny. The results showed that ITS, TEF, RPB1, and RPB2 could be used to identify the strains. All phylogenetic trees reconstructed by free combination of these four genes exhibited almost the same topology. Bioassay studies of a purified conidial suspension further confirmed the infection of second-instar nymphs and adult female F. occidentalis by seven Lecanicillium strains. L. attenuatum strain GZUIFR-lun1405 was the most virulent, killing approximately 91.67% F. occidentalis adults and 76.67% nymphs after a 7-day exposure. L. attenuatum strain GZUIFR-lun1405 and L. cauligalbarum strain GZUIFR-ZHJ01 were selected to compare the fungal effects on the number of eggs laid by F. occidentalis. The number of F. occidentalis nymphs significantly decreased when F. occidentalis adults were treated with L. cauligalbarum strain GZUIFR-ZHJ01.

CONCLUSIONS

The combination of ITS and RPB1 could be used for fast recognition of Lecanicillium spp. This is the first report of the pathogenicity of L. attenuatum, L. cauligalbarum, L araneogenum, and L. aphanocladii against F. occidentalis. Additionally, L. cauligalbarum strain GZUIFR-ZHJ01 caused high F. occidentalis mortality and inhibited the fecundity of the pest.

Constitutive and Inducible Resistance to Thrips Do Not Correlate With Differences in Trichome Density or Enzymatic-Related Defenses in Chrysanthemum

Western flower thrips (WFT), Frankliniella occidentalis, is a serious insect pest of Chrysanthemum [Chrysanthemum × morifolium Ramat. (Asteraceae)]. Here we have investigated whether genotypic variation in constitutive and inducible resistance to WFT correlates with phenotypic differences in leaf trichome density and the activity of the defense-related enzyme polyphenol oxidase (PPO) in chrysanthemum. Non-glandular and glandular leaf trichome densities significantly varied among ninety-five chrysanthemum cultivars. Additional analyses in a subset of these cultivars, differing in leaf trichome density, revealed significant variation in PPO activities and resistance to WFT as well. Constitutive levels of trichome densities and PPO activity, however, did not correlate with chrysanthemum resistance to WFT. Further tests showed that exogenous application of the phytohormone jasmonic acid (JA) increased non-glandular trichome densities, PPO activity and chrysanthemum resistance to WFT, and that these effects were cultivar dependent. In addition, no tradeoff between constitutive and inducible resistance to WFT was observed. JA-mediated induction of WFT resistance, however, did not correlate with changes in leaf trichome densities nor PPO activity levels. Taken together, our results suggest that chrysanthemum can display both high levels of constitutive and inducible resistance to WFT, and that leaf trichome density and PPO activity may not play a relevant role in chrysanthemum defenses against WFT.

Genome-enabled insights into the biology of thrips as crop pests

BACKGROUND

The western flower thrips, Frankliniella occidentalis (Pergande), is a globally invasive pest and plant virus vector on a wide array of food, fiber, and ornamental crops. The underlying genetic mechanisms of the processes governing thrips pest and vector biology, feeding behaviors, ecology, and insecticide resistance are largely unknown. To address this gap, we present the F. occidentalis draft genome assembly and official gene set.

RESULTS

We report on the first genome sequence for any member of the insect order Thysanoptera. Benchmarking Universal Single-Copy Ortholog (BUSCO) assessments of the genome assembly (size = 415.8 Mb, scaffold N50 = 948.9 kb) revealed a relatively complete and well-annotated assembly in comparison to other insect genomes. The genome is unusually GC-rich (50%) compared to other insect genomes to date. The official gene set (OGS v1.0) contains 16,859 genes, of which ~ 10% were manually verified and corrected by our consortium. We focused on manual annotation, phylogenetic, and expression evidence analyses for gene sets centered on primary themes in the life histories and activities of plant-colonizing insects. Highlights include the following: (1) divergent clades and large expansions in genes associated with environmental sensing (chemosensory receptors) and detoxification (CYP4, CYP6, and CCE enzymes) of substances encountered in agricultural environments; (2) a comprehensive set of salivary gland genes supported by enriched expression; (3) apparent absence of members of the IMD innate immune defense pathway; and (4) developmental- and sex-specific expression analyses of genes associated with progression from larvae to adulthood through neometaboly, a distinct form of maturation differing from either incomplete or complete metamorphosis in the Insecta.

CONCLUSIONS

Analysis of the F. occidentalis genome offers insights into the polyphagous behavior of this insect pest that finds, colonizes, and survives on a widely diverse array of plants. The genomic resources presented here enable a more complete analysis of insect evolution and biology, providing a missing taxon for contemporary insect genomics-based analyses. Our study also offers a genomic benchmark for molecular and evolutionary investigations of other Thysanoptera species.

Response of Protective Enzymes in Western Flower Thrips (Thysanoptera: Thripidae) to Two Leguminous Plants

The western flower thrips, Frankliniella occidentalis, is a major invasive pest of commercially important crops worldwide. We compared the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) and the expressions of two putative SOD and two putative POD sequences in second instar larvae and adults after three generations of adaptation to kidney bean and broad bean plants. The results showed that the SOD, POD, and CAT activities in adults were significantly higher than those in the second instar larvae. The SOD activities were significantly higher in both the second instar larvae and the adults fed on kidney bean (Phaseolus vulgaris) plants versus broad bean (Vicia faba) plants, whereas the POD and CAT activities showed the opposite trend. The gene expression data showed that the FoPOD-2 expression levels were lower in the second instar larvae after three generations of feeding on broad bean plants versus kidney bean plants. The expression levels of FoSOD-1 and FoSOD-2, and FoPOD-1 under broad bean plant treatment were higher than those under kidney bean plant treatment. Additionally, gene expression fluctuated among the different generations. Our results indicated that western flower thrips demonstrated plasticity in gene expression and activity of protective enzymes, which is related to their adaptability to the host plants. Western flower thrips can change the expression of protective enzyme genes and enzyme activity in vivo to better adapt to kidney bean and broad bean plants.

Variable resistance to spinetoram in populations of Thrips palmi across a small area unconnected to genetic similarity

The melon thrips, Thrips palmi, is an increasingly important pest of vegetables in northern China. Some populations have developed resistance in the field to the insecticide spinetoram. Understanding the origin and dispersal of insecticide-resistant populations can shed light on resistance management strategies. In this study, we tested susceptibility of seven greenhouse populations of T. palmi to spinetoram collected from a small area of about 300 km2 in Shandong Province and examined population genetic structure across the area based on a segment of mitochondrial cox1 gene and 22 microsatellite loci to infer the possible origin and dispersal of insecticide resistance. Levels of resistance to spinetoram differed among seven populations, which included one population with high resistance (LC50 = 759.34 mg/L), three populations with medium resistance (LC50 ranged from 28.69 to 34.79 mg/L), and three populations with low resistance (LC50 ranged from 7.61 to 8.97 mg/L). The populations were genetically differentiated into two groups unrelated to both levels of resistance and geographic distance. The molecular data indicated high levels of gene flow between populations with different levels of resistance to spinetoram and low gene flow among populations with the same level of resistance, pointing to a likely separate history of resistance evolution. Resistance levels of two tested populations to spinetoram decreased 23 and 4.6 times after five generations without any exposure to the pesticide. We therefore suspect that resistance of T. palmi most likely evolved in response to local applications of the insecticide. Our study suggests that the development of resistance could be avoided or resistance even reversed by reducing usage of spinetoram.

Specific and Spillover Effects on Vectors Following Infection of Two RNA Viruses in Pepper Plants

Mixed infection of plant viruses is ubiquitous in nature and can affect virus-plant-vector interactions differently than single virus infection. While several studies have examined virus-virus interactions involving mixed virus infection, relatively few have examined effects of mixed virus infection on vector preference and fitness, especially when multiple vectors are involved. This study explored how single and mixed viral infection of a non-persistently transmitted cucumber mosaic virus (CMV) and propagative and persistently-transmitted tomato spotted wilt orthotospovirus (TSWV) in pepper, Capsicum annum L., influenced the preference and fitness of their vectors, the green peach aphid, Myzus persicae (Sulzer), and the tobacco thrips, Frankliniella fusca (Hinds), respectively. In general, mixed infected plants exhibited severe symptoms compared with individually infected plants. An antagonistic interaction between the two viruses was observed when CMV titer was reduced following mixed infection with TSWV in comparison with the single infection. TSWV titer did not differ between single and mixed infection. Myzus persicae settling preference and median developmental were not significantly different between CMV and/or TSWV-infected and non-infected plants. Moreover, M. persicae fecundity did not differ between CMV-infected and non-infected pepper plants. However, M. persicae fecundity was substantially greater on TSWV-infected plants than non-infected plants. Myzus persicae fecundity on mixed-infected plants was significantly lower than on singly-infected and non-infected plants. Frankliniella fusca fecundity was higher on CMV and/or TSWV-infected pepper plants than non-infected pepper plants. Furthermore, F. fusca-induced feeding damage was higher on TSWV-infected than on CMV-infected, mixed-infected, or non-infected pepper plants. Overall, our results indicate that the effects of mixed virus infection on vectors were not different from those observed following single virus infection. Virus-induced host phenotype-modulated effects were realized on both specific and non-specific vectors, suggesting crosstalk involving all vectors and viruses in this pathosystem. The driving forces of these interactions need to be further examined. The effects of interactions between two viruses and two vectors towards epidemics of one or both viruses also need to be examined.

Relative susceptibility to pesticides and environmental conditions of Frankliniella intonsa and F. occidentalis (Thysanoptera: Thripidae), an underlying reason for their asymmetrical occurrence

To explain the asymmetrical abundance of native Frankliniella intonsa (Trybom) (Thysanoptera: Thripidae) and invasive Frankliniella occidentalis (Pergande) in the fields, we examined differential susceptibility to pesticides and environmental conditions, i.e., nine combinations of temperatures and relative humidities (RHs). We found adult female F. intonsa to be more susceptible to most of the tested insecticides as compared to F. occidentalis. Chlorfenapyr was most toxic to both thrips’ species. In the evaluation of environment conditions in the adult stage, F. intonsa survived 2.5 and 2.4-fold longer as RH increased at 20 and 25 °C, respectively, whereas F. occidentalis survived 1.8 and 1.6-fold longer, respectively. In both pupal and larval stage, no significant effect of interaction of temperatures and RHs was found between the two species. In conclusion, the insecticides tested differed considerably in their species-specific toxicity, and F. intonsa was generally more susceptible to the insecticides, while at the same time survivorship was better at higher RH conditions than F. occidentalis. Thus, differences in the relative susceptibility to changing environmental conditions, especially humidity, may be an underlying mechanism for the recent dominance of F. intonsa over F. occidentalis in the strawberry plastic greenhouse in Korea.

Effects of melon yellow spot orthotospovirus infection on the preference and developmental traits of melon thrips, Thrips palmi, in cucumber

Melon yellow spot orthotospovirus (MYSV), a member of the genus Orthotospovirus, is an important virus in cucurbits. Thrips palmi is considered the most serious pest of cucurbits because it directly damages and indirectly transmits MYSV to the plant. The effects of MYSV-infected plants on the development time, fecundity, and preference of the thrips were analyzed in this study. Our results showed that the development time of male and female thrips did not differ significantly between MYSV-infected and non-infected cucumbers. The survival rate of thrips in non-infected and MYSV-infected cucumbers were not significantly different. In a non-choice assay, T. palmi adults were released on non-infected and MYSV-infected cucumbers and allowed to lay eggs. The number of hatched larvae did not significantly differ between non-infected and MYSV-infected cucumbers. In a choice assay, MYSV had no detectable effect on the number of adult thrips and preceding hatched larvae. In a pull assay, the settling rate of thrips on the released plant did not differ significantly when the adult thrips were released to non-infected or MYSV infected cucumbers for any cucumber cultivar. Based on our results, we propose that the effects of MYSV-infected cucumbers on the development time, fecundity, or preference of T. palmi may not be an important factor in MYSV spread between cucumbers.

Characterization of pyridalyl resistance in a laboratory-selected strain of Frankliniella occidentalis

Frankliniella occidentalis, the western flower thrips, is one of most notorious pests on a variety of crops worldwide and many populations have high resistance to different types of insecticides. In order to determine the susceptibility of F. occidentalis to the insecticide pyridalyl in the field and to understand the potential mechanism of resistance, we conducted field monitoring of resistance and investigated cross-resistance, fitness cost, and synergism for the 75.8-fold pyridalyl-resistant strain (PY-R) established with a long-term successive laboratory selection. The results showed that all field populations of F. occidentalis had high susceptibility to pyridalyl from 2016 to 2017. The PY-R strain showed no cross-resistance to spinosad, spinetoram, abamectin, imidacloprid, or pyriproxyfen. Tests of synergism demonstrated that piperonyl butoxide (PBO) and diethyl maleate (DEM) significantly inhibited pyridalyl resistance in the PY-R strain. Moreover, PY-R showed lower pupation and emergence rates than the susceptible strain (XY-S), and female fecundity was also significantly lower in PY-R than in XY-S. By calculating the net replacement rate (R₀) of XY-S, the fitness of PY-R was established as 0.52. In conclusion, the susceptibility of F. occidentalis to pyridalyl is still high in China, but resistance could be developed by successive-generation selections. Enhanced metabolic detoxification is possibly involved in resistance in PY-R, which caused significant fitness costs. Thus, pyridalyl will likely prove useful in the field for controlling F. occidentalis in China, and development of resistance could be delayed by rotating pyridalyl with other chemical agents lacking cross-resistance.

A thrips vector of tomato spotted wilt virus responds to tomato acylsugar chemical diversity with reduced oviposition and virus inoculation

There is increasing evidence that acylsugars deter insect pests and plant virus vectors, including the western flower thrips (WFT), Frankliniella occidentalis (Pergande), vector of tomato spotted wilt virus (TSWV). Acylsugars are sugar-polyesters composed of saturated, un-saturated, and variously branched short and long chain fatty acids (FAs) esterified to a glucose (acylglucose) or sucrose (acylsucrose) moiety. We sought to understand how acylsucrose amount and composition of associated FA profiles interacted to mediate resistance to WFT oviposition and TSWV inoculation on tomato leaves. Towards this goal, we examined WFT oviposition and TSWV inoculation behavior on tomato lines bred to exude varying amounts of acylsucrose in association with diverse FA profiles. Our data show that as acylsucrose amounts increased, WFT egg-laying (oviposition) decreased and TSWV inoculation was suppressed. Western flower thrips also responded to FA profiles that included iC4, iC11, nC12 and nC10 FA. These findings support improving acylsugar-mediated resistance against WFT by breeding tomatoes exuding greater amounts of acylsucrose associated with specific FA profiles. We show that increasing acylsucrose amount output by type IV trichomes and selecting for particular FA profiles through advanced breeding profoundly affects WFT behavior in ways that benefit management of WFT as direct pests and as TSWV vectors.