Temperature-dependent, behavioural, and transcriptional variability of a tritrophic interaction consisting of bean, herbivorous mite, and predator

Influence of temperature on the population size of Aceria litchii (Acari: Eriophyidae) and the development of its galls

The lychee erinose mite, Aceria litchii (Keifer), is a tiny eriophyid mite known to induce the formation of open galls (erinea) on lychee plants, Litchi chinensis Sonn. In lychee infested by A. litchii, four stages of erineum are observed, based on erineum color: hyaline, white, amber and dark brown. The cause of the color change in erinea is unknown, but it might be linked to the extent of A. litchii infestation. Artificially infested lychee seedlings were used to determine the developmental time, trichome density, and mite population size in each of the four erineum stages. The effect of temperature (15, 25, and 35 °C) on A. litchii population size and erineum developmental time was also investigated. Overall, each erineum stage lasted approximately 50 days and the population size of A. litchii increased gradually through the hyaline, white and amber stages, and a strong decline was observed in dark brown erinea. Visual comparisons of the low-temperature scanning electron microscope (LT-SEM) images revealed that the trichome density was lower in the hyaline stage than in the other stages. Temperature influenced mite population size and amber erinea developmental time. Mite population was lower at 35 °C than at 15 and 25 °C, suggesting that high temperatures promote detrimental effects on A. litchii. Amber erinea developed slower at 15 °C than at 25 and 35 °C. Possible effects of the temperature on A. litchii population in lychee plants are discussed.

Parental exposure to heat waves improves offspring reproductive investment in Tetranychus urticae (Acari: Tetranychidae), but not in its predator, Phytoseiulus persimilis (Acari: Phytoseiidae)

The more frequent and intense occurrence of heat waves is a challenge for arthropods because their unpredictable incidence requires fast adaptations by the exposed individuals. Phenotypic plasticity within and across generations might be a solution to cope with the detrimental effects of heat waves, especially for fast-developing, small arthropods with limited dispersal abilities. Therefore, we studied whether severe heat may affect the reproduction of a pest species, the spider mite Tetranychus urticae, and its counterpart, the predatory mite Phytoseiulus persimilis. Single offspring females with different parental thermal origins (reared under mild or extreme heat waves) of both species were exposed to mild or extreme heat waves on bean leaves over 10 days, and the oviposition, egg sizes, survival, and escape behavior of the females were evaluated daily. The total losses of predators mainly via escapers were very high compared to prey, which makes a separation between selective and plastic effects on shifted reproductive traits impossible. Predator females laid smaller eggs, while their consumption and oviposition rates were unaffected during extreme heat waves. In comparison, larger prey females fed more and produced more, but smaller, eggs due to within- and trans-generational effects. These advantages for the prey in comparison to its predator when exposed to extreme heat waves during the reproductive phase support the trophic sensitivity hypothesis: higher trophic levels (i.e., the predator) are more sensitive to thermal stress than lower trophic levels (i.e., the prey). Furthermore, the species-specific responses may reflect their lifestyles. The proactive and mobile predator should be selected for behavioral thermoregulation under heat waves via spatiotemporal avoidance of heat-exposed locations rather than relying on physiological adaptations in contrast to the more sessile prey. Whether these findings also influence predator-prey interactions and their population dynamics under heat waves remains an open question.

Environmental RNAi-based reverse genetics in the predatory mite Neoseiulus californicus: Towards improved methods of biological control

The predatory mite Neoseiulus californicus (McGregor) (Mesostigmata: Phytoseiidae) has been commercialized by manufacturers in the pest control industry and is used worldwide as a natural enemy of spider mites. However, because its genome has not been sequenced, reverse genetics techniques that could be used to analyze gene function have not been established. Here we partially sequenced the gene that encodes the vacuolar-type H⁺-ATPase (V-ATPase), an ATP-dependent proton pump, in N. californicus (NcVATPase) and then conducted a functional analysis using environmental RNA interference (eRNAi) by orally administering sequence-specific exogenous dsRNA (dsRNA-NcVATPase) to larvae and adult females. The larvae treated with dsRNA-NcVATPase took longer to develop and had lower survivorship, fecundity, and offspring viability at the adult stage than those treated with a control dsRNA. Adult females treated with dsRNA-NcVATPase showed significant reductions in survival, fecundity, and prey consumption, and their endogenous gene expression level of NcVATPase was reduced by approximately 65% compared with the control. Our findings suggest that the NcVATPase gene, silencing of which inhibits feeding and reproduction, is an excellent biomarker for investigating the eRNAi mechanism in N. californicus. The highly efficient experimental system of eRNAi established in this study paves the way for applied research using eRNAi to enhance the predatory ability of N. californicus.

Sustaining induced heat shock protein 70 confers biological thermotolerance in a high-temperature adapted predatory mite Neoseiulus barkeri (Hughes)

BACKGROUND

In fluctuating climatic environments, heat acclimation in predatory mites is a superior adaptation strategy for effective agricultural pest management and can be used to enhance the abilities critical in biological control efficiency. We investigated the regulatory mechanism governing the remarkable plastic response of thermotolerance in a high-temperature adapted strain (HTAS) and discerned the differences in the defensive reactions between the HTAS and the conventional strain (CS) in the predatory mite Neoseiulus barkeri.

RESULTS

At 42 °C, the relative expression levels of four identified HSP70 genes increased rapidly in both N. barkeri strains; meanwhile the expression of NbHSP70-1 and NbHSP70-2 in CS sharply decreased after 4 h, displaying a distinct contrast with the remaining elevated expression in HTAS. Western blot analysis showed that the protein level of NbHSP70-1 in CS was dramatically elevated at 0.5 h and decreased at 6 h at 42 °C. Conversely, in HTAS, NbHSP70-1 was constantly induced and peaked at 6 h at 42 °C. Furthermore, HSP70 suppression by RNAi knockdown had a greater influence on the survival of HTAS, causing a higher mortality under high temperature than CS. Finally, the recombinant exogenous NbHSP70-1 protein enhanced the viability of E. coli BL21 under a lethal temperature of 50 °C.

CONCLUSION

Sustained accumulation of HSP70 proteins results in predatory phytoseiid mites with the thermotolerance advantage that could promote their biological control function to pests. The divergent constitutive regulation of HSP70 to a thermal environment is conducive to the flexible adaptability of predators in the higher trophic level to trade off under extremely adversity stress.

Temperature stress response: A novel important function of Dermatophagoides farinae allergens

Dermatophagoides farinae, an important pathogen, has multiple allergens. However, their expression under physiological conditions are not understood. Our previous RNA-seq showed that allergens of D. farinae were up-regulated under temperature stress, implying that they may be involved in stress response. Here, we performed a comprehensive study. qRT-PCR detection indicated that 26 of the 34 allergens showed differential expression. Der f1 had the most abundant basic expression quantity. Der f 28.0201 (HSP70) and Der f3 had the same regulation pattern in 9 highly expressed transcripts, which only up-regulated at 41 °C and 43 °C, but Der f 28.0201 showed stronger regulation than Der f 3 (19.88-fold vs 6.02-fold). Whereas Der f 1, 2, 7, 21, 22, 27, and 30 were up-regulated under both heat and cold stress, and Der f 27 showed the strongest regulation ability among them. Der f 27 showed more significant up-regulation than Der f 28.0201 under heat stress (23.59-fold vs 19.88-fold), and Der f27 had more obvious up-regulation under cold than heat stress (30.70-fold vs 23.59-fold). The expression of Der f 27, 28.0201 and 1, and D. farinae survival rates significantly decreased following RNAi, indicating the upregulation of these allergens under temperature stress conferred thermo-tolerance or cold-tolerance to D. farinae. In this study, we described for the first time that these allergens have temperature-stress response functions. This new scientific discovery has important clinical value for revealing the more frequent and serious allergic diseases caused by D. farinae during the change of seasons.

Oral delivery of water-soluble compounds to the phytoseiid mite Neoseiulus californicus (Acari: Phytoseiidae)

Phytoseiids are predatory mites that prey on other mites and small arthropods, and several species are used in commercial agriculture for biological control of pests. To optimize phytoseiid mites’ use in biocontrol, an efficient method for oral delivery of test compounds is required to assess their sensitivities to pesticides, RNAi for gene functional analysis and artificial diets. Here we developed four methods for oral delivery of a solution of xenobiotics to different life stages of the commercially available generalist predatory mite Neoseiulus californicus: (i) soaking mites in the solution, or allowing them to feed on (ii) spider mites soaked in the solution, (iii) a solution droplet, or (iv) solution-saturated filter paper. As measured by ingestion of a tracer dye, the droplet-based feeding system was most efficient; the dye was observed in the alimentary canal of >90% test mites of all life stages, with no mortality. The droplet-based feeding system was also effective for the commercially available specialist predatory mite Phytoseiulus persimilis, with >80% delivery efficiency. This study paves the way for development of methods for high-throughput RNAi and for toxicological or nutritional assays in phytoseiid mites.

Spermatophore producing process and sperm transfer in Phytoseiulus persimilis

In phytoseiid mites, the spermatophore is an intermediate, isolated structure where sperms are stored during mating. In the present study, the producing process of Phytoseiulus persimilis spermatophore is described in details. Its shape, and the number and shape of sperms inside, are also analyzed, each as affected by coupling time. Based on our results, the spermatophore of P. persimilis is pyriform, and is produced from the male genital opening within 3 min since mating started. When produced, the spermatophore is transferred along the capitular groove to the cheliceral base, where it is adhered to one of the two small holes at the bases of the chelicerae. Seminal fluid passes through the hole, the hollow spermatodactyl, the opening on the spermatodactyl tip, and enters the insemination pore to reach the spermatheca of the female. No sperm was observed in spermatophores obtained 5 min after mating started. The number of sperms increased in the next 10 min, reached its maximum (ca. 52 sperms per spermatophore), and then decreased. All sperms were released in 75 min after mating started. Sperms are slender in shape and on average 3.9 ± 0.3 µm long.

Beyond insects: current status and achievements of RNA interference in mite pests and future perspectives

Mites comprise a group of key agricultural pests on a wide range of crops. They cause harm through feeding on the plant and transferring dangerous pathogens, and the rapid evolution of pesticide resistance in mites highlights the need for novel control methods. Currently, RNA interference (RNAi) shows great potential for insect pest control. Here, we review the literature regarding RNAi in mite pests. We discuss different target genes and RNAi efficiency in various mite species, a promising Varroa control program using RNAi, the synergy of RNAi with plant defense mechanisms and microorganisms, and current understanding of systemic movement of double-stranded RNA (dsRNA). On the basis of this evidence, we can conclude that there is clear potential for application of RNAi-based mite control, but further research on several aspects of RNAi in mites is needed, including: (i) the factors influencing RNAi efficiency, (ii) the mechanism of environmental RNAi and cross-kingdom dsRNA trafficking, (iii) the mechanism of possible systemic and parental RNAi, and (iv) non-target effects, specifically in predatory mites, which should be considered during RNAi target selection. © 2018 Society of Chemical Industry.

Intraspecific variation among Tetranychid mites for ability to detoxify and to induce plant defenses

Two genotypes coexist among Kanzawa spider mites, one of which causes red scars and the other of which causes white scars on leaves, and they elicit different defense responses in host plants. Based on RNA-Seq analysis, we revealed here that the expression levels of genes involved in the detoxification system were higher in Red strains than White strains. The corresponding enzyme activities as well as performances for acaricide resistance and host adaptation toward Laminaceae were also higher in Red strains than White strains, indicating that Red strains were superior in trait(s) of the detox system. In subsequent generations of strains that had survived exposure to fenpyroximate, both strains showed similar resistance to this acaricide, as well as similar detoxification activities. The endogenous levels of salicylic acid and jasmonic acid were increased similarly in bean leaves damaged by original Red strains and their subsequent generations that inherited high detox activity. Jasmonic acid levels were increased in leaves damaged by original White strains, but not by their subsequent generations that inherited high detox activity. Together, these data suggest the existence of intraspecific variation - at least within White strains - with respect to their capacity to withstand acaricides and host plant defenses.

Transcriptome Analysis of the Carmine Spider Mite, Tetranychus cinnabarinus (Boisduval, 1867) (Acari: Tetranychidae), and Its Response to β-Sitosterol

Tetranychus cinnabarinus (Acari: Tetranychidae) is a worldwide polyphagous agricultural pest that has the title of resistance champion among arthropods. We reported previously the identification of the acaricidal compound β-sitosterol from Mentha piperita and Inula japonica. However, the acaricidal mechanism of β-sitosterol is unclear. Due to the limited genetic research carried out, we de novo assembled the transcriptome of T. cinnabarinus using Illumina sequencing and conducted a differential expression analysis of control and β-sitosterol-treated mites. In total, we obtained >5.4 G high-quality bases for each sample with unprecedented sequencing depth and assembled them into 22,941 unigenes. We identified 617 xenobiotic metabolism-related genes involved in detoxification, binding, and transporting of xenobiotics. A highly expanded xenobiotic metabolic system was found in mites. T. cinnabarinus detoxification genes-including carboxyl/cholinesterase and ABC transporter class C-were upregulated after β-sitosterol treatment. Defense-related proteins, such as Toll-like receptor, legumain, and serine proteases, were also activated. Furthermore, other important genes-such as the chloride channel protein, cytochrome b, carboxypeptidase, peritrophic membrane chitin binding protein, and calphostin-may also play important roles in mites' response to β-sitosterol. Our results demonstrate that high-throughput-omics tool facilitates identification of xenobiotic metabolism-related genes and illustration of the acaricidal mechanisms of β-sitosterol.

Oral delivery of double-stranded RNA induces prolonged and systemic gene knockdown in Metaseiulus occidentalis only after feeding on Tetranychus urticae

Metaseiulus (=Typhlodromus or Galendromus) occidentalis is an important biological control agent. Functional genomic studies on this predator have been hampered by the lack of reverse genetic tools such as RNA interference (RNAi). In the current study, we evaluated possible RNAi responses in M. occidentalis females by feeding double-stranded RNA (dsRNA) of RpL11, RpS2, RpL8, or Pros26.4 genes in 20 % sucrose solution. Females needed to subsequently feed on two-spotted spider mites (Tetranychus urticae) to elicit a nearly complete loss of egg production. The corresponding gene knockdown was robust, long-term, and was observed in the very few eggs produced (systemic or parental RNAi). Interestingly, dsRNA-mediated gene knockdown could not be induced if these predators were provided only the sucrose diet after ingesting dsRNAs; T. urticae had to be provided to elicit the RNAi response. However, the spider mite diet was not needed for sustaining the dsRNA-mediated gene knockdown once it commenced. Oral delivery of dsRNA will be a valuable tool for efficient genome-wide functional screens in this important predatory mite.

Herbivore-induced indirect defense across bean cultivars is independent of their degree of direct resistance

We tested the extent to which resistance of common bean (Phaseolus vulgaris) cultivars to the spider mite Tetranychus urticae parallels the extent to which these plants display indirect defenses via the induced attraction of the predatory mite Phytoseiulus persimilis. First, via field and greenhouse trials on 19 commercial bean cultivars, we selected two spider mite-resistant (Naz and Ks41128) and two susceptible (Akthar and G11867) cultivars and measured the spider mite-induced volatiles and the subsequently induced attraction of predatory mites via olfactory choice assays. The two major volatiles, 4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) and (Z)-3-hexenyl-acetate, were induced in the resistant but not in the susceptible cultivars. However, uninfested susceptible cultivars emitted these volatiles at levels similar to those of mite-infested resistant cultivars. Significant induction of several minor components was observed for all four cultivars except for the infested-susceptible cultivar G11867. Both, the spider mite-resistant cultivar Naz and the susceptible cultivar G11867, attracted more predatory mites when they were infested. In contrast, spider mites induced increased emission of two major and five minor volatiles in Ks41128, but predatory mites did not discriminate between infested and uninfested plants. Overall, the attraction of predatory mites appeared to correlate positively with the presence of TMTT and (Z)-3-hexenyl acetate and negatively with β-caryophyllene and α-pinene in the bean headspace. Taken together, our data suggest that resistance and attraction of natural enemies via induced volatiles are independent traits. We argue that it should be possible to cross predator-attraction promoting traits into resistant cultivars that lack sufficiently inducible indirect defenses.

Effects of thermal stress on lipid peroxidation and antioxidant enzyme activities of the predatory mite, Neoseiulus cucumeris (Acari: Phytoseiidae)

Changes in temperature are known to cause a variety of physiological stress responses in insects and mites. Thermal stress responses are usually associated with the increased generation of reactive oxygen species (ROS), resulting in oxidative damage. In this study, we examined the time-related effect (durations for 1, 2, 3, and 5 h) of thermal stress conditions-i.e., relatively low (0, 5, 10, and 15 °C) or high (35, 38, 41, and 44 °C) temperatures-on the activities of antioxidant enzymes including catalase (CAT), superoxide dismutase (SOD), peroxidase (POX), glutathione S-transferases (GSTs), and total antioxidant capacity (T-AOC) of the predatory mite Neoseiulus cucumeris. Also the lipid peroxidation (LPO) levels of the predatory mite were measured under thermal stress conditions. The results confirmed that thermal stress results in a condition of so-called oxidative stress and the four antioxidant enzymes play an important role in combating the accumulation of ROS in N. cucumeris. CAT and POX activity changed significantly when the mites were exposed to cold and heat shock, respectively. The elevated levels of SOD and GSTs activity, expressed in a time-dependent manner, may have an important role in the process of antioxidant response to thermal stress. However, the levels of LPO in N. cucumeris were high, serving as an important signal that these antioxidant enzyme-based defense mechanisms were not always adequate to counteract the surplus ROS. Thus, we hypothesize that thermal stress, especially extreme temperatures, may contribute much to the generation of ROS in N. cucumeris, and eventually to its death.