Rapid host-plant adaptation in the herbivorous spider mite Tetranychus urticae occurs at low cost

Implications of Temperature and Prey Density on Predatory Mite Amblyseius swirskii (Acari: Phytoseiidae) Functional Responses

Amblyseius swirskii are predaceous mites that feed on phytophagous mites, pollens, and plant exudates and are known as one of the most potent biological pest management agents. Tetranychus urticae is a global mite that is difficult to manage because of its high population growth rates, necessitating alternative management measures like biological control. Regarding the functional response, the effects of temperature and prey density are some of the essential behaviors of natural enemies. This study investigates the effect of varying temperatures and prey densities on A. swirskii, a biological control agent for T. urticae. The present results demonstrated the change in the functional response estimates when A. swirskii was reared at various temperatures and different prey densities. The results of the estimates regarding the searching efficiency (a') showed the highest value (a' = 0.919) at 26 °C and the lowest value (a' = 0.751) at 14 °C. The handling time per prey item (Th) for the predatory mites changed with the temperature and prey density, showing the shortest handling time at 26 °C (Th = 0.005) and the highest value at 14 °C (Th = 0.015). The functional response curves matched the type II functional response model, demonstrating the inverse dependence of temperatures and prey density with a positive quadratic coefficient. The predation curves for A. swirskii showed a significant difference between the mean numbers of T. urticae consumed at various prey densities and temperatures, illustrating a relationship between A. swirskii and T. urticae. Therefore, the results of this research may be utilized to forecast the behavior of A. swirskii and its usefulness in controlling T. urticae populations.

Glutathione S-Transferase Genes Involved in Response to Short-Term Heat Stress in Tetranychus urticae (Koch)

Tetranychus urticae, a globally ubiquitous mite, poses a significant threat to agriculture. Elevated temperatures exacerbate the growth, development, and reproduction of T. urticae, leading to substantial crop damage. In this study, we employed comparative transcriptomic approaches with whole-genome information of T. urticae to identify six Glutathione S-transferase genes (GSTs) implicated in heat stress response. Through comprehensive bioinformatics analyses, we elucidated the tertiary structure and active sites of the corresponding proteins, providing a thorough characterization of these GST genes. Furthermore, we investigated the expression patterns of these six GST genes under short-term heat shock conditions. Our findings unveiled the involvement of T. urticae GST genes in combating oxidative stress induced by heat, underscoring their role in antioxidant defense mechanisms. This study contributes valuable insights into the molecular mechanisms underlying the response of T. urticae to heat stress, laying a foundation for the development of strategies aimed at mitigating its impact in high-temperature environments.

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.

Novel Jasmonic Acid-Coumarin Pathway in the Eggplant That Inhibits Vitellogenin Gene Expression To Prevent Mite Reproduction

Plants activate direct and indirect defense mechanisms in response to perceived herbivore invasion, which results in negative consequences for herbivores. Tetranychus cinnabarinus is a polyphagous generalist herbivore that inflicts substantial agricultural and horticultural damage. Our study revealed that mite feeding significantly increased jasmonic acid (JA) in the eggplant. The damage inflicted by the mites decreased considerably following the artificial application of JA, thereby indicating that JA initiated the defense response of the eggplant against mites. The transcriptomic and metabolomic analyses demonstrated the activation of the JA-coumarin pathway in response to mite feeding. This pathway protects the eggplant by suppressing the reproductive capacity and population size of the mites. The JA and coumarin treatments suppressed the vitellogenin gene (TcVg6) expression level. Additionally, RNA interference with TcVg6 significantly reduced the egg production and hatching rate of mites. In conclusion, the JA-coumarin pathway in the eggplant decreases the egg-hatching rate of mites through suppression of TcVg6.

Hybridisation between host races broadens the host range of offspring in Eotetranychus asiaticus (Acari: Tetranychidae)

Host adaptation in herbivorous arthropods is one of the first steps to sympatric speciation, and spider mites (Acari: Tetranychidae) are useful model organisms for studying this phenomenon. Many researchers have studied the process of host adaptation via artificial selection experiments. Recent analyses suggest that hybridisation has diversified host ranges, although empirical evidence is scarce. We explored the host exploitation of two strains of Eotetranychus asiaticus established from Ternstroemia gymnanthera (Pentaphylacaceae) and Japanese cinnamon, Cinnamomum yabunikkei (Lauraceae), and evaluated the effect of hybridisation on offspring host use. Transplant experiments showed that females oviposited and immature mites developed only on their native hosts, suggesting specialisation to the secondary metabolites of each host plant. However, F1 hybrids from reciprocal crosses developed on both host plants (survival rate: 92-100%) with normal female-biased sex ratios. Furthermore, all backcrosses to the parental strains yielded B1 offspring that were also viable on both host plants with normal sex ratios (69-87% and 39-92% females on T. gymnanthera and C. yabunikkei, respectively). B1 survival rates in interstrain crosses were varied (11-63%) and lower than those in intrastrain crosses (88-93%). We could not detect any reproductive barriers in these experiments, and host preference may be the sole factor determining pre-mating isolation. The survival rates and sex ratios we observed suggest cytochromosome interactions. In conclusion, hybridisation, which results in heterozygotes and recombination, is an underexplored way to provide spider mites with a novel host plant.

Wolbachia manipulates reproduction of spider mites by influencing herbivore salivary proteins

BACKGROUND

The endosymbiont Wolbachia is known for manipulating host reproduction. Wolbachia also can affect host fitness by mediating interactions between plant and herbivores. However, it remains unclear whether saliva proteins are involved in this process.

RESULTS

We found that Wolbachia infection decreased the number of deposited eggs but increased the egg hatching rate in the spider mite Tetranychus urticae Koch (Acari: Tetranychidae), a cosmopolitan pest that infects >1000 species of plants. Transcriptomic and proteomic analyses revealed that Wolbachia-infected mites upregulated the gene expression levels of many T. urticae salivary proteins including a cluster of Tetranychidae-specific, functionally uncharacterized SHOT1s (secreted host-responsive proteins of Tetranychidae). The SHOT1 genes were expressed more in the feeding stages (nymphs and adults) of mites than in eggs and highly enriched in the proterosomas. RNA interference experiments showed that knockdown of SHOT1s significantly decreased Wolbachia density, increased the number of deposited eggs and decreased the egg hatching rate.

CONCLUSION

Together, these results indicate that SHOT1s are positively correlated with Wolbachia density and account for Wolbachia-mediated phenotypes. Our results provide new evidence that herbivore salivary proteins are related to Wolbachia-mediated manipulations of host performance on plants. © 2022 Society of Chemical Industry.

Specific sequence of arrival promotes coexistence via spatial niche pre-emption by the weak competitor

Historical contingency, such as the order of species arrival, can modify competitive outcomes via niche modification or pre-emption. However, how these mechanisms ultimately modify stabilising niche and average fitness differences remains largely unknown. By experimentally assembling two congeneric spider mite species feeding on tomato plants during two generations, we show that order of arrival affects species' competitive ability and changes the outcome of competition. Contrary to expectations, order of arrival did not cause positive frequency dependent priority effects. Instead, coexistence was predicted when the inferior competitor (Tetranychus urticae) arrived first. In that case, T. urticae colonised the preferred feeding stratum (leaves) of T. evansi leading to spatial niche pre-emption, which equalised fitness and reduced niche differences, driving community assembly to a close-to-neutrality scenario. Our study demonstrates how the order of species arrival and the spatial context of competitive interactions may jointly determine whether species can coexist.

Functional response of Amblyseius eharai (Acari: Phytoseiidae) on Tetranychus urticae (Acari: Tetranychidae)

Amblyseius eharai is a generalist predatory mite that consumes spider mites, rust mites, thrips, and pollen, with a high adaptability to various plants. To better understand ecological and behavioral aspects of this species, we investigated its functional response to different stages of two-spotted spider mite, Tetranychus urticae. Furthermore, we compared its environmental adaptability with that of other referenced phytoseiids using a temperature-dependent model of the intrinsic rate of increase. We were able to calculate the functional response parameters of both sexes of A. eharai when preying on eggs or larvae of T. urticae and, for females only, when preying on the deutonymph of T. urticae. Among the various combinations tested herein, A. eharai females preying on T. urticae larvae had the highest attack rate and shortest handling time. For eggs of T. urticae, A. eharai showed a lower attack rate; however, its handling time for eggs was significantly shorter compared to other phytoseiids. Using T. urticae larva as a prey, the attack rate of female A. eharai was higher and the handling time of both sexes of this species was shorter than those of other phytoseiid mites. Amblyseius eharai populations can show maximum performance quickly due to this species’ lower optimal temperature for population growth (28.1°C) compared to other phytoseiid mites. Thus, we provided evidence that this predatory mite has the potential to be a new, effective biological control agent of greenhouse pests such as T. urticae due to its high predation capacity and low optimal temperature.

A sink host allows a specialist herbivore to persist in a seasonal source

In seasonal environments, sinks that are more persistent than sources may serve as temporal stepping stones for specialists. However, this possibility has to our knowledge, not been demonstrated to date, as such environments are thought to select for generalists, and the role of sinks, both in the field and in the laboratory, is difficult to document. Here, we used laboratory experiments to show that herbivorous arthropods associated with seasonally absent main (source) habitats can endure on a suboptimal (sink) host for several generations, albeit with a negative growth rate. Additionally, they dispersed towards this host less often than towards the main host and accepted it less often than the main host. Finally, repeated experimental evolution attempts revealed no adaptation to the suboptimal host. Nevertheless, field observations showed that arthropods are found in suboptimal habitats when the main habitat is unavailable. Together, these results show that evolutionary rescue in the suboptimal habitat is not possible. Instead, the sink habitat functions as a temporal stepping stone, allowing for the persistence of a specialist when the source habitat is gone.

Maize Inbred Line B96 Is the Source of Large-Effect Loci for Resistance to Generalist but Not Specialist Spider Mites

Maize (Zea mays subsp. mays) yield loss from arthropod herbivory is substantial. While the basis of resistance to major insect herbivores has been comparatively well-studied in maize, less is known about resistance to spider mite herbivores, which are distantly related to insects and feed by a different mechanism. Two spider mites, the generalist Tetranychus urticae, and the grass-specialist Oligonychus pratensis, are notable pests of maize, especially during drought conditions. We assessed resistance (antibiosis) to both mites of 38 highly diverse maize lines, including several previously reported to be resistant to one or the other mite species. We found that line B96, as well as its derivatives B49 and B75, were highly resistant to T. urticae. In contrast, neither these three lines, nor any others included in our study, were notably resistant to the specialist O. pratensis. Quantitative trait locus (QTL) mapping with replicate populations from crosses of B49, B75, and B96 to susceptible B73 identified a QTL in the same genomic interval on chromosome 6 for T. urticae resistance in each of the three resistant lines, and an additional resistance QTL on chromosome 1 was unique to B96. Single-locus genotyping with a marker coincident with the chromosome 6 QTL in crosses of both B49 and B75 to B73 revealed that the respective QTL was large-effect; it explained ∼70% of the variance in resistance, and resistance alleles from B49 and B75 acted recessively as compared to B73. Finally, a genome-wide haplotype analysis using genome sequence data generated for B49, B75, and B96 identified an identical haplotype, likely of initial origin from B96, as the source of T. urticae resistance on chromosome 6 in each of the B49, B75, and B96 lines. Our findings uncover the relationship between intraspecific variation in maize defenses and resistance to its major generalist and specialist spider mite herbivores, and we identified loci for use in breeding programs and for genetic studies of resistance to T. urticae, the most widespread spider mite pest of maize.

Photoperiodic control of reproductive arrest in the oak-inhabiting spider mite Schizotetranychus brevisetosus (Acari: Tetranychidae)

Populations of Schizotetranychus brevisetosus Ehara (Acari: Tetranychidae), which live on the evergreen oak (Quercus glauca), survive the coldest months as either adult females or winter eggs. Adult females comprise the majority of the population in early November and oviposit from late November to early March. Most winter eggs hatch by late March, and adults of the next generation emerge in April. This species is considered an egg-diapausing species, but the environmental cues that regulate female reproductive arrest and resumption are mostly unknown. We investigated the photoperiodic responses of autumn reproductive arrest in 10 populations collected from different elevations in Shikoku, Japan. All populations showed long-day responses to critical daylength (CDL) around 12.2 h light (12.2L) at 20 °C, though there was no linear relationship between CDL and altitude. This result explains the steep decline in the proportion of summer eggs in November. Nonreproductive females developed under 10L at 20 °C commenced oviposition 14.3-20.6 days after transferring to 15L. This long pre-oviposition period explains the reduction in eggs before winter reproduction and suggests shallow adult diapause. Eggs thus obtained hatched in 12.9-15.3 days, similarly to summer eggs. Therefore, egg diapause in S. brevisetosus is much shallower than in species on deciduous hosts, which lay their winter eggs in early autumn to hatch at leaf flush in spring. The reproductive arrest and short hatching period may be an adaptation allowing egg-laying in midwinter, when predation pressure is low.

Behavioral response of Panonychus citri (McGregor) (Acari: Tetranychidae) to synthetic chemicals and oils

Background

Panonychus citri (McGregor) (Acari: Tetranychidae) population outbreaks after the citrus plantation's chemical application is a common observation. Dispersal behavior is an essential tool to understand the secondary outbreak of P. citri population. Therefore, in the current study, the dispersal activity of P. citri was observed on the leaf surfaces of Citrus reticulata (Rutaceae) treated with SYP-9625, abamectin, vegetable oil, and EnSpray 99.

Method

Mites were released on the first (apex) leaf of the plant (adaxial surface) and data were recorded after 24 h. The treated, untreated, and half-treated data were analyzed by combining the leaf surfaces (adaxial right, adaxial left, abaxial right, and abaxial left). All experiments were performed in open-air environmental conditions.

Results

The maximum number of mites was captured on the un-treated or half-treated surfaces due to chemicals repellency. Chemical bioassays of the free-choice test showed that all treatments significantly increased the mortality of P. citri depending on application method and concentration. A significant number of mites repelled away from treated surfaces and within treated surfaces except adaxial left and abaxial right surfaces at LC₃₀. In the no-choice test, SYP-9625 gave maximum mortality and dispersal by oils than others. No significant differences were observed within the adaxial and abaxial except abaxial surface at LC₃₀. Therefore, the presence of tested acaricides interferes with P. citri dispersal within leaf surfaces of plantations depending on the mites released point and a preferred site for feeding.

Transcription response of Tetranychus cinnabarinus to plant-mediated short-term and long -term selenium treatment

Selenium is a trace element necessary for living organisms. It exists mainly in the form of selenite in nature. In plants, selenium can enhance defenses against pests. In this study, transcriptome sequencing technology was used to analyze the response mechanism of Tetranychus cinnabarinus to plant-mediated selenium treatment. We tested four sodium selenite treatments (5, 20, 50, and 200 μM) that were the same for short (2 d) and long (30 d) treatment durations. The results showed that the number of differentially expressed genes (DEGs) in the short-term treatment was greater than in the long-term treatment. This indicated that the gene expression of spider mites gradually stabilized during the selenium treatment. Regardless of the long-term and short-term conditions, spider mites had the largest response to the 20 μM sodium selenite treatment. The functional annotation classification of DEGs showed no significant difference under different concentrations and treatment durations. A total of 25 genes were differentially expressed in all eight treatments, including four down-regulated cytochrome P450 genes and one up-regulated chitinase gene. We speculate that selenium may have the potential to enhance the activity of chemical acaricides. Transcriptome sequencing of sodium selenite treatment at different concentrations and different times revealed the response mechanism of spider mites under plant-mediated selenium treatment. At the same time, it also provides new clues for the development of methods for preventing and controlling spider mites with selenium.

Genome-Wide Identification and Expression Analysis of Udp-Glucuronosyltransferases in the Whitefly Bemisia Tabaci (Gennadius) (HemipterA: Aleyrodidae)

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is an important agricultural pest worldwide. Uridine diphosphate (UDP)-glucuronosyltransferases (UGTs) are one of the largest and most ubiquitous groups of proteins. Because of their role in detoxification, insect UGTs are attracting increasing attention. In this study, we identified and analyzed UGT genes in B. tabaci MEAM1 to investigate their potential roles in host adaptation and reproductive capacity. Based on phylogenetic and structural analyses, we identified 76 UGT genes in the B. tabaci MEAM1 genome. RNA-seq and real-time quantitative PCR (RT-qPCR) revealed differential expression patterns of these genes at different developmental stages and in association with four host plants (cabbage, cucumber, cotton and tomato). RNA interference results of selected UGTs showed that, when UGT352A1, UGT352B1, and UGT354A1 were respectively silenced by feeding on dsRNA, the fecundity of B. tabaci MEAM1 was reduced, suggesting that the expressions of these three UGT genes in this species may be associated with host-related fecundity. Together, our results provide detailed UGTs data in B.tabaci and help guide future studies on the mechanisms of host adaptation by B.tabaci.

Creating outbred and inbred populations in haplodiploids to measure adaptive responses in the laboratory

Laboratory studies are often criticized for not being representative of processes occurring in natural populations. One reason for this is the fact that laboratory populations generally do not capture enough of the genetic variation of natural populations. This can be mitigated by mixing the genetic background of several field populations when creating laboratory populations. From these outbred populations, it is possible to generate inbred lines, thereby freezing and partitioning part of their variability, allowing each genotype to be characterized independently. Many studies addressing adaptation of organisms to their environment, such as those involving quantitative genetics or experimental evolution, rely on inbred or outbred populations, but the methodology underlying the generation of such biological resources is usually not explicitly documented. Here, we developed different procedures to circumvent common pitfalls of laboratory studies, and illustrate their application using two haplodiploid species, the spider mites Tetranychus urticae and Tetranychus evansi. First, we present a method that increases the chance of capturing high amounts of variability when creating outbred populations, by performing controlled crosses between individuals from different field-collected populations. Second, we depict the creation of inbred lines derived from such outbred populations, by performing several generations of sib-mating. Third, we outline an experimental evolution protocol that allows the maintenance of a constant population size at the beginning of each generation, thereby preventing bottlenecks and diminishing extinction risks. Finally, we discuss the advantages of these procedures and emphasize that sharing such biological resources and combining them with available genetic tools will allow consistent and comparable studies that greatly contribute to our understanding of ecological and evolutionary processes.

Endosymbiont diversity in natural populations of Tetranychus mites is rapidly lost under laboratory conditions

Although the diversity of bacterial endosymbionts in arthropods is well documented, whether and how such diversity is maintained remains an open question. We investigated the temporal changes occurring in the prevalence and composition of endosymbionts after transferring natural populations of Tetranychus spider mites from the field to the laboratory. These populations, belonging to three different Tetranychus species (T. urticae, T. ludeni and T. evansi) carried variable infection frequencies of Wolbachia, Cardinium, and Rickettsia. We report a rapid change of the infection status of these populations after only 6 months of laboratory rearing, with an apparent loss of Rickettsia and Cardinium, while Wolbachia apparently either reached fixation or was lost. We show that Wolbachia had variable effects on host longevity and fecundity, and induced variable levels of cytoplasmic incompatibility (CI) in each fully infected population, despite no sequence divergence in the markers used and full CI rescue between all populations. This suggests that such effects are largely dependent upon the host genotype. Subsequently, we used these data to parameterize a theoretical model for the invasion of CI-inducing symbionts in haplodiploids, which shows that symbiont effects are sufficient to explain their dynamics in the laboratory. This further suggests that symbiont diversity and prevalence in the field are likely maintained by environmental heterogeneity, which is reduced in the laboratory. Overall, this study highlights the lability of endosymbiont infections and draws attention to the limitations of laboratory studies to understand host-symbiont interactions in natural populations.