Deleterious effects of UV-B radiation on herbivorous spider mites: they can avoid it by remaining on lower leaf surfaces

Leaf trichome-mediated predator effects on the distribution of herbivorous mites within a kidney bean plant

Some predators prefer to settle on leaf patches with microstructures (e.g., trichomes and domatia), leaving traces on the patches. Herbivorous arthropods, in turn, select leaf patches in response to these traces left by predators. It remains unclear whether traces of predators on leaf patches affect the distribution of herbivorous prey within plants through plant microstructure. Therefore, we examined the distribution of herbivorous mite (Tetranychus urticae) and predatory mite (Phytoseiulus persimilis) by investigating their oviposition pattern. We used a kidney bean plant (Phaseolus vulgaris) with two expanded primary leaves and the first trifoliate leaf, focusing on leaf trichomes as the microstructure. The density of trichomes was higher on the first trifoliate leaf than on the primary leaves and on the abaxial surface of the leaves than on the adaxial surface. Adult female P. persimilis laid more eggs on the first trifoliate leaf to the primary leaves. Although adult female T. urticae preferred to oviposit on the abaxial surface of primary leaves, previous exposure of plants to predators diminished this preference. The altered egg distribution would be a response to the traces of P. persimilis rather than eggs of P. persimilis. Our findings indicate that T. urticae reproduces on leaf patches with traces of predators without altering their oviposition preference. Given that the presence of predator traces is known to reduce the reproduction of T. urticae, it may have a substantial effect on the population of T. urticae in the next generations on kidney bean plants.

Combined nighttime ultraviolet B irradiation and phytoseiid mite application provide optimal control of the spider mite Tetranychus urticae on greenhouse strawberry plants

BACKGROUND

Tetranychus urticae is a hard-to-control pest of greenhouse strawberry production. Nighttime ultraviolet B (UV-B) radiation using light reflection sheets (LRS) has been applied as a physical method to control T. urticae through direct ovicidal effects (the UV method). However, because strawberry leaves grow more densely, UV-B radiation fails to reach the lower leaf surfaces inhabited by spider mites; therefore, a complementary method is required. We propose the supplemental application of phytoseiid mites in greenhouse strawberry production. We evaluated the control effects of UV-B irradiation, phytoseiid mite application and their combined use. The effects of UV-B irradiation on the degree of overlap relative to the independent distributions (ω) between predators and prey were also analyzed.

RESULTS

The UV method alone maintained low T. urticae density levels from November to February; however, mite populations increased from March onward. Phytoseiid mite application in January and February without UV-B irradiation resulted in a temporary increase in spider mites in March and/or April. By contrast, combined application of the UV method and phytoseiid mites had a greater control effect during the strawberry growing season. The ω values were higher for the UV method compared with no UV-B irradiation, suggesting that UV-B irradiation increased phytoseiid mite foraging rates.

CONCLUSION

The release of phytoseiid mites compensated for the shortcomings of the UV method, and UV-B irradiation promoted predation by phytoseiid mites by increasing the behavioral numerical response. Consequently, combined application of UV-B irradiation and phytoseiid mites is optimal for T. urticae control in greenhouse strawberry production. © 2023 Society of Chemical Industry.

Modelling the interaction between a pest (Aculops lycopersici), two predators (Pronematus ubiquitus and Macrolophus pygmaeus) and climate variables: a 3-year greenhouse study in a tomato crop

BACKGROUND

The tomato russet mite, Aculops lycopersici, is a major pest in tomato crops, causing damage through leaf and stem browning, defoliation and russeting of fruit. Biological control of this mite on tomato plants is difficult. While several phytoseiid mites feed on the pest, they cannot survive, move, or reproduce on tomato plants due to the presence of glandular trichomes. Pronematus ubiquitus has recently been identified as a biocontrol agent of A. lycopersici in tomato crops, but the predator-prey interaction between these two species is not well studied. In this paper, we present a validated logistic regression predator-prey model based on a 3-year study supplemented with additional datasets. Besides the predator and the prey, this model takes into account an extra generalist predator, Macrolophus pygmaeus, and various climate parameters.

RESULTS

The population trend of A. lycopersici is best explained by the presence of the predator P. ubiquitus, the relative humidity and the fact that the crop was lit or unlit using artificial light. P. ubiquitus has proved to be an efficient biocontrol agent of A. lycopersici. For P. ubiquitus the presence of M. pygmaeus, the vapour pressure deficit, the number of light hours and radiation explained the population trend best. For both the predator and the prey density-dependent interactions were identified. Model outcomes are discussed in detail.

CONCLUSION

Our study provides insights into the potential use of P. ubiquitus as a biocontrol agent for A. lycopersici in tomato crops in combination with M. pygmaeus. However, we highlight the importance of considering the presence of other predators and environmental conditions when developing integrated pest management strategies. © 2023 Society of Chemical Industry.

Interactive effects of changes in UV radiation and climate on terrestrial ecosystems, biogeochemical cycles, and feedbacks to the climate system

Terrestrial organisms and ecosystems are being exposed to new and rapidly changing combinations of solar UV radiation and other environmental factors because of ongoing changes in stratospheric ozone and climate. In this Quadrennial Assessment, we examine the interactive effects of changes in stratospheric ozone, UV radiation and climate on terrestrial ecosystems and biogeochemical cycles in the context of the Montreal Protocol. We specifically assess effects on terrestrial organisms, agriculture and food supply, biodiversity, ecosystem services and feedbacks to the climate system. Emphasis is placed on the role of extreme climate events in altering the exposure to UV radiation of organisms and ecosystems and the potential effects on biodiversity. We also address the responses of plants to increased temporal variability in solar UV radiation, the interactive effects of UV radiation and other climate change factors (e.g. drought, temperature) on crops, and the role of UV radiation in driving the breakdown of organic matter from dead plant material (i.e. litter) and biocides (pesticides and herbicides). Our assessment indicates that UV radiation and climate interact in various ways to affect the structure and function of terrestrial ecosystems, and that by protecting the ozone layer, the Montreal Protocol continues to play a vital role in maintaining healthy, diverse ecosystems on land that sustain life on Earth. Furthermore, the Montreal Protocol and its Kigali Amendment are mitigating some of the negative environmental consequences of climate change by limiting the emissions of greenhouse gases and protecting the carbon sequestration potential of vegetation and the terrestrial carbon pool.

Ultraviolet-B radiation in relation to agriculture in the context of climate change: a review

Over the past few decades, the amount of ultraviolet-B radiation (UV-B) reaching the earth's surface has been altered due to climate change and stratospheric ozone dynamics. This narrow but highly biologically active spectrum of light (280-320 nm) can affect plant growth and development. Depletion of ozone and climate change are interlinked in a very complicated manner, i.e., significantly contributing to each other. The interaction of climate change, ozone depletion, and changes in UV-B radiation negatively affects the growth, development, and yield of plants. Furthermore, this interaction will become more complex in the coming years. The ozone layer reduction is paving a path for UV-B radiation to impact the surface of the earth and interfere with the plant's normal life by negatively affecting the plant's morphology and physiology. The nature and degree of the future response of the agricultural ecosystem to the decreasing or increasing UV-B radiation in the background of climate change and ozone dynamics are still unclear. In this regard, this review aims to elucidate the effects of enhanced UV-B radiation reaching the earth's surface due to the depletion of the ozone layer on plants' physiology and the performance of major cereals.

The flashy red color of the red velvet mite Balaustium murorum (Prostigmata: Erythraeidae) is caused by high abundance of the keto-carotenoids, astaxanthin and 3-hydroxyechinenone

The red velvet mite, Balaustium murorum (Hermann), is a pollenophagous free-living mite with a flashy red body. This mite occurs in early spring and lives on sunny surfaces of human-made structures, such as concrete. Hence, it is inevitably exposed to a harsh environment due to solar ultraviolet-B (UV-B) radiation and radiant heat, which cause oxidative stress via the production of reactive oxygen species. The spider mite Panonychus citri that resides on upper leaf surfaces accumulates synthesized keto-carotenoids to protect against oxidative stress. Therefore, we evaluated carotenoid composition in the red pigment of B. murorum. To identify major carotenoids, we performed a high-performance liquid chromatography analysis of intact and de-esterified pigments of B. murorum females. The flashy red pigments of B. murorum consisted of the highly abundant keto-carotenoids astaxanthin and 3-hydroxyechinenone (60 and 38% of major carotenoids, respectively), and a small amount of β-carotene (2%). Although P. citri is an astaxanthin-rich species, the astaxanthin concentration (per protein) in B. murorum is 127-fold that in P. citri. Due to their high antioxidant activities, those keto-carotenoids probably contribute to the survival of B. murorum in the harsh environment caused by solar UV-B radiation and radiant heat in inorganic habitats.

Mechanisms underlying the impact and interaction of temperature and UV-B on the hatching of spider mite and phytoseiid mite eggs

BACKGROUND

A spider mite control method using night-time ultraviolet (UV)-B irradiation was recently developed for strawberry greenhouses (UV method). The control effect of this UV method is negatively affected by increasing temperature. Tetranychus urticae eggs are more resistant to a single dose of UV-B irradiation than Neoseiulus californicus eggs. By contrast, N. californicus can better survive nightly UV-B irradiation with the UV method compared with T. urticae. To elucidate the mechanism underlying these phenomena, we explored the hypotheses that higher temperature promotes photoenzymatic repair (PER) and that mortality is determined by UV-B susceptibility in the embryonic stage exposed to UV-B.

RESULTS

PER efficacy was not promoted by increasing temperature. The lowest hatchability (around zero) of T. urticae eggs after a single dose of UV-B irradiation (0.288 and 0.432 kJ m^-2 ) without photoreactivation was seen in the morphogenesis stages between "cleavage ended" and "eye points became colored". Based on these results, we developed a linear function of daily UV-B irradiance and deviation of cumulative irradiance during vulnerable embryonic developmental phases from 50% lethal dose (LD₅₀ ) after a single dose of UV-B irradiation. The difference between T. urticae and N. californicus and changes in UV-B vulnerability due to temperature could be explained by this simple relationship.

CONCLUSION

Slower development in T. urticae than N. californicus in nature and developmental delay under low temperatures increase the ovicidal effects of the UV method. This shows the advantage of the simultaneous use of the UV method and biological control, contributing to the development of integrated pest management. © 2022 Society of Chemical Industry.

Within-plant Distributions and Density of Amblyseius swirskii (Acari: Phytoseiidae) as Influenced by Interactions Between Plastic Mulch and Vegetable Crop Species

Plastic mulch of different colors and ultraviolet (UV) reflectivity individually or combined with released arthropod predators is an important component of an integrated pest management strategy. In 2015 and 2016, we evaluated the density and within-plant distribution of a released predatory mite, Amblyseius swirskii Athius-Henriot (Acari: Phytoseiidae) in snap bean (Phaseolus vulgaris L.), cucumber (Cucumis sativus L.), yellow squash (Cucurbita pepo L.), eggplant (Solanum melongena L.), Jalapeno pepper (Capsicum annuum L.), and tomato (Solanum lycopersicum L.) grown on different plastic mulches. The mulch treatments evaluated were: metalized top and black bottom, metalized top and white bottom, black-on-black, black-on-white, white-on-black, and bare soil with no mulch. Crop species had a significant effect on the density of A. swirskii. Eggplant and cucumber had higher numbers of A. swirskii than the other crops tested in 2015. In 2016, the density of A. swirskii was higher on eggplant than on cucumber. There was a variation in the distribution of A. swirskii in different strata of the plant canopies with the highest number in the bottom stratum of each crop, which was positively correlated with the population of Thrips palmi Karny (Thysanoptera: Thripidae). Mulch type had no effect on the density or distribution of A. swirskii in any strata of any of the crops tested. The results of this study indicate that releasing A. swirskii is compatible with the use of UV-reflective mulch. This information about host preference and within-plant distribution of A. swirskii should be of value in pest management programs for the crops studied.

Flexible habitat choice by aphids exposed to multiple cues reflecting present and future benefits

Mothers choose suitable habitats for laying offspring to maximize fitness. Because habitat quality varies in space and time, mothers gather information to choose among available habitats through multiple cues reflecting different aspects of habitat quality at present and in the future. However, it is unclear how females assess and integrate different cues associated with current rewards and future safety to optimize oviposition/larviposition decisions, especially across small spatial scales. Here, we tested the individual and interactive effects of leaf surface, leaf orientation, and leaf bending direction on larviposition site choice and fitness benefits of wheat aphids (Metopolophium dirhodum) within individual leaves. We found that females preferred upper over lower surfaces for gaining current food-related rewards, downward- over upward-facing surfaces for avoiding potential abiotic risks, and sunken over protruding surfaces for avoiding potential biotic risks. When facing conflicting cues during larviposition, females preferred downward-facing/sunken surfaces over upper surfaces, suggesting that females prioritize potential safety at the cost of current rewards during decision making. Most importantly, our combined-cue experiments showed females still assessed secondary cues (i.e., the upper surface) when first-ranked cues (i.e., the downward-facing/sunken surface) are available, even though females only gained relatively small fitness rewards through secondary cues, and females can integrate different cues associated with current rewards and potential safety in a multiplicative way to make flexible and complex larviposition decisions. Overall, our findings provide new insights into how animals collect and process multi-cue information associated with current rewards and potential safety to maximize fitness at small spatial scales.

New understanding of the direct effects of spectral balance on behaviour in Myzus persicae

The study of insect responses to colour has mainly focused on flying species and morphs, however colour cues are likely to be important for insect positioning within the canopy. We examine the role of illumination colour in canopy positioning of apterous Myzus persicae (Sulzer) using both a field experiment, utilising various UV-manipulating optical filters, and a laboratory experiment using video tracking of individuals illuminated by a variable intensity UVA-Blue-Green LED-array. In the field experiment, approximately twice as many aphids were located on exposed leaf surfaces under UV-deficient environments compared to UV-rich environments. The lab experiment showed all three M. persicae photoreceptors were involved in a visually-mediated feeding/avoidance behaviour. Highly UV-rich, green-deficient environments were up to 3 times as likely to trigger an avoidance behaviour compared to UV-absent, green-rich environments such as those found below the leaf surface. We show that apterous M. persicae use this, in addition to other cues, in order to locate feeding positions that minimise exposure to direct sunlight. This has relevance to both the fundamental understanding of photoprotective behaviour in Hemiptera as well as to applied research of crop production environments that disrupt pest behaviour.

Effects of Growth Phase and Ultraviolet-B Pretreatment in Perilla Leaves on the Two-Spotted Spider Mite

Perilla, Perilla frutescens (L.) Britton var. crispa (Thunb.) H. Deane, is traditionally cultivated as an edible/medicinal crop in East Asia. Its essential oil contains many bioactive compounds that are expected to have high pharmacological functionality, as well as antimicrobial and insecticidal activity. Spider mites are a major pest group for perilla cultivation. The two-spotted spider mite, Tetranychus urticae Koch, possesses divergent detoxification enzymes and has developed resistance against most acaricides. The essential oil content of perilla halves from the pre-flowering phase to the flowering phase, and ultraviolet (UV)-B radiation generally increases defense compounds. To clarify the effects of this change in essential oil content and the effects of UV-B pretreatment, we investigated the developmental success and egg production of T. urticae on leaves from the preflowering and flowering phases cultivated with and without nighttime UV-B irradiation. Both the parameters significantly increased on leaves from the flowering phase in comparison with that from the preflowering phase, suggesting that constitutively produced essential oil provided protection against mite pests in a growth phase-specific manner. The defense system also extended the developmental period of mites on red perilla leaves, but not on green perilla leaves, in preflowering phase. Although egg production was lower on red perilla leaves pretreated with UV-B, no negative effects were caused on the developmental success and duration on red and green perilla and the egg production on green perilla by UV-B pretreatment. Our findings reveal a significant impact of investment allocation of perilla plants and a small contribution of UV-B irradiation to the plant defense system.

Spider Mites Cause More Damage to Tomato in the Dark When Induced Defenses Are Lower

Plants have evolved robust mechanisms to cope with incidental variation (e.g. herbivory) and periodical variation (e.g. light/darkness during the day-night cycle) in their environment. It has been shown that a plant's susceptibility to pathogens can vary during its day-night cycle. We demonstrated earlier that the spider mite Tetranychus urticae induces jasmonate- and salicylate-mediated defenses in tomato plants while the spider mite T. evansi suppresses these defenses probably by secreting salivary effector proteins. Here we compared induction/suppression of plant defenses; the expression of mite-effector genes and the amount of damage due to mite feeding during the day and during the night. T. urticae feeding upregulated the expression of jasmonate and salicylate marker-genes albeit significantly higher under light than under darkness. Some of these marker-genes were also upregulated by T. evansi-feeding albeit to much lower levels than by T. urticae-feeding. The expression of effector 28 was not affected by light or darkness in either mite species. However, the expression of effector 84 was considerably higher under light, especially for T. evansi. Finally, while T. evansi produced overall more feeding damage than T. urticae both mites produced consistently more damage during the dark phase than under light. Our results suggest that induced defenses are subject to diurnal variation possibly causing tomatoes to incur more damage due to mite-feeding during the dark phase. We speculate that mites, but especially T. evansi, may relax effector production during the dark phase because under these conditions the plant's ability to upregulate defenses is reduced.

Light intensity influences feeding and fecundity of Tetranychus urticae (Acari: Tetranychidae) through the responses of host Cucumis sativus leaves

We investigated feeding and fecundity of the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae), on leaves of cucumber (Cucumis sativus) seedlings that had been acclimatized to different light intensities. Based on these data, we analyzed the relationships between mite performance (feeding and fecundity) and leaf properties. The cucumber seedlings were grown in controlled-environment chambers under different light intensities at a photosynthetic photon flux density of 50, 100, 150, 300, or 450 µmol m^- 2 s^- 1 until the first true leaves had expanded. Adult females were released on the adaxial surfaces of excised leaf samples from the seedlings of each treatment group and held under standardized light intensity (200 µmol m^- 2 s^- 1). Fecundity and leaf damage area increased and decreased, respectively, as the acclimatization light intensity increased, indicating indirect effects of light intensity on feeding and fecundity through changes in the host leaf properties. Leaf mass per area (LMA) and photosynthetic capacity, which increased as the acclimatization light intensity increased, was positively related to the fecundity, but was negatively related to the leaf damage area. The higher LMA and photosynthetic capacity results in an increased amount of mesophyll per unit leaf area. This would allow the mites to feed efficiently from a limited area, which may explain the increased fecundity on these leaves.

Dose-Response and Temperature Dependence of the Mortality of Spider Mite and Predatory Mite Eggs Caused by Daily Nighttime Ultraviolet-B Irradiation

The two-spotted spider mite, Tetranychus urticae, is an economically important agricultural pest. A novel physical control method involving daily nighttime UV-B irradiation was recently developed for use in strawberry greenhouses. However, the overlapping of leaves after March prevents direct irradiation to T. urticae on the lower leaf surface, decreasing control effect. Excessive UV-B irradiation causes leaf sunscald in winter. Therefore, optimization of UV-B irradiance and a compensatory control agent are desired. Temperature may affect the survival of organisms exposed to UV-B, although the temperature dependence of UV-B damage is controversial. A phytoseiid mite, Neoseiulus californicus, is a prominent predator but vulnerable to a single UV-B irradiation. We compared dose-response and temperature dependence of UV-B damage between T. urticae and N. californicus eggs under daily nighttime UV-B irradiation. Unexpectedly, N. californicus showed greater resistance to UV-B than T. urticae, and the mortality was increased and decreased at low and high temperatures, respectively. This makes possible the application of UV-B doses that are lethal for spider mites but safe for phytoseiid mites. Overall, we concluded that combined use of phytoseiid mites with UV-B lamps is advantageous to spider mite management in strawberry greenhouses.

The Effects of Weather on the Flight of an Invasive Bark Beetle, Pityophthorus juglandis

The walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), vectors the fungus Geosmithia morbida, which has been implicated in thousand cankers disease of walnut. Little is known about the flight behavior of the insect across seasons, or about the variability in its flight patterns with weekly fluctuations in weather. We sampled flying adults weekly over a 142-week period (from 29 August, 2011 to 2 June, 2014) with 12-unit black plastic multiple funnel traps baited with a male-produced aggregation pheromone in California, USA. Up to 5000 beetles were captured per trap per week, although catches in most weeks were less than 100 insects. Trap catches were regressed against terms for precipitation, solar radiation, vapor pressure, air temperature, relative humidity, wind speed, and trap catches in preceding weeks. The number of beetles captured in each of the preceding two weeks explained most variation in a current week's catch. This strong temporal autocorrelation was present in regression models developed for males, females, and both sexes pooled. These models were improved by including two environmental variables. Captures of P. juglandis increased with mean weekly air temperature and decreased with increasing mean minimum relative humidity. The percentage of variation in male, female, or total trap catch explained by the temporal variables and the two environmental variables in these multiple regression models ranged from 72% to 76%. While the flight of this invasive insect will likely be affected by site-specific factors as it spreads to new areas, the strong temporal correlation present in this system may provide a useful starting point for developing flight models for newly invaded areas.

Anti-parasite behaviour of birds

Birds have many kinds of internal and external parasites, including viruses, bacteria and fungi, as well as protozoa, helminths and arthropods. Because parasites have negative effects on host fitness, selection favours the evolution of anti-parasite defences, many of which involve behaviour. We provide a brief review of anti-parasite behaviours in birds, divided into five major categories: (i) body maintenance, (ii) nest maintenance, (iii) avoidance of parasitized prey, (iv) migration and (v) tolerance. We evaluate the adaptive significance of the different behaviours and note cases in which additional research is particularly needed. We briefly consider the interaction of different behaviours, such as sunning and preening, and how behavioural defences may interact with other forms of defence, such as immune responses. We conclude by suggesting some general questions that need to be addressed concerning the nature of anti-parasite behaviour in birds.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

Effects of UV-B radiation on the survival, egg hatchability and transcript expression of antioxidant enzymes in a high-temperature adapted strain of Neoseiulus barkeri

Biological control of spider mites in hot and dry weather is a serious technical issue. A high-temperature adapted strain (HTAS) of the predatory mite Neoseiulus barkeri Hughes was selected from its conventional strain (CS), via long-term heat acclimation and frequent heat hardenings in our previous studies. However, the environment of high temperature is usually associated with enhanced ultraviolet (UV) radiation. In the present study, the physiological effects of UV-B radiation on survival rate and egg damage of N. barkeri were investigated, as well as the activities and expression profiles of antioxidant enzymes to UV-B radiation stress. UV-B radiation had deleterious effects on egg hatchability and survival of N. barkeri. Adults of the HTAS strain were less UV-B resistant than those of the CS strain; they also had lower levels of enzymatic activity of superoxide dismutase (SOD) and catalase against oxidative damage and weaker upregulation of SOD genes. The mRNA expression of three SOD genes of CS adult females immediately increased whereas that of HTAS showed almost no difference under UV-B stress for 1 h. The results showed the HTAS of N. barkeri had lower fitness under UV-B stress compared with the CS of N. barkeri. These results suggested that long-term heat acclimation may exert a profound impact on the developmental physiology of N. barkeri.

Physiological Response of Basil Plants to Twospotted Spider Mite (Acari: Tetranychidae) Infestation

The induction of plant resistance against pests is considered a potential method of controlling mite infestation as it restricts the use of chemical pesticides in herbal crops. Our goal was to investigate whether plant physiological response to mite feeding varied depending on basil cultivar and/or duration of mite infestation. The effect of plant acceptance, mite mortality rate, and changes in physiological parameters: malondialdehyde content (MDA), hydrogen peroxide (H2O2) concentration, and antioxidant enzyme activities, including guaiacol peroxidase (GPX) and catalase (CAT) were examined in this study. Tetranychus urticae Koch (Acari: Tetranychidae) infestation induced oxidative stress in three Ocimum basilicum L. cultivars: 'Sweet basil,' 'Purpurascens,' and 'Fino Verde.' The analysis of mite behavior and alteration in metabolic plant profiles showed different sensitivities of basil cultivars to biotic stress that were dependent on the cultivar and duration of infestation. All basil plants were suitable as host plants for T. urticae, but they varied in the level of susceptibility to mite feeding. O. basilicum 'Fino Verde' was the most suitable host for the twospotted spider mite. In turn, O. basilicum 'Purpurascens' was characterized by the lowest level of susceptibility to T. urticae feeding. The lowest acceptance, the highest mortality of twospotted spider mite individuals as well as decreased levels of H2O2 and MDA, significantly increased GPX activity and low level of CAT activity were recorded in O. basilicum 'Purpurascens' leaves. Research on plant responses to biotic stress can inform breeding cultivars resistant to arthropod attack.

UV-B susceptibility and photoreactivation in embryonic development of the two-spotted spider mite, Tetranychus urticae

Developmental errors are often induced in the embryos of many organisms by environmental stress. Ultraviolet-B radiation (UV-B) is one of the most serious environmental stressors in embryonic development. Here, we investigated susceptibility to UV-B (0.5 kJ m^-2) in embryos of the two-spotted spider mite, Tetranychus urticae, to examine the potential use of UV-B in control of this important agricultural pest worldwide. Peak susceptibility to UV-B (0% hatchability) was found in T. urticae eggs 36-48 h after oviposition at 25 °C, which coincides with the stages of morphogenesis forming the germ band and initial limb primordia. However, hatchability recovered to ~ 80% when eggs irradiated with UV-B were subsequently exposed to visible radiation (VIS) at 10.2 kJ m^-2, driving photoreactivation (the photoenzymatic repair of DNA damage). The recovery effect decreased to 40-70% hatchability, depending on the embryonic developmental stage, when VIS irradiation was delayed for 4 h after the end of exposure to UV-B. Thus UV-B damage to T. urticae embryos is critical, particularly in the early stages of morphogenesis, and photoreactivation functions to mitigate UV-B damage, even in the susceptible stages, but immediate VIS irradiation is needed after exposure to UV-B. These findings suggest that nighttime irradiation with UV-B can effectively kill T. urticae eggs without subsequent photoreactivation and may be useful in the physical control of this species.

Behavioral thermoregulation in a small herbivore avoids direct UVB damage

Direct damage of increased solar ultraviolet-B (UVB) on organism fitness has attracted attention due to stratospheric ozone depletion. Although most ectotherms are not capable of detecting and avoiding solar UVB, they may avoid direct exposure to solar UVB via thermoregulation behavior. However, it is still not clear whether organisms are harmed by ambient UVB radiation before escaping to shaded microhabitats. In this study we used the English grain aphid, Sitobion avenae (Hemiptera: Aphididae), to test whether sunlight-avoidance behavior was caused by heat stress rather than UVB, and whether behavioral thermoregulation in shaded microhabitats contributes to avoidance or reduction of direct UVB damage. Our results showed that S. avenae tended to inhabit exposed adaxial leaf surfaces in mid-May in Mongolia, but inhabited shaded leaf surfaces in mid-June, thereby avoiding strong sunlight. Heat exposure rather than solar UVB was the primary reason for such avoidance behavior. The average and extreme temperatures of shaded leaf surfaces were several degrees lower than sunlight-exposed surfaces at midday, suggesting that movement to shaded leaf surfaces represents a form of behavioral thermoregulation. Such responses occurred before UVB radiation reached harmful levels, and contributed to avoiding direct UVB damage. As future climate warming is expected to lead to harmful UVB radiation as well as increasing temperatures, this may represent a case where responses to one stressor inadvertently protect against the harmful effects of a different stressor.

Can Plant Defence Mechanisms Provide New Approaches for the Sustainable Control of the Two-Spotted Spider Mite Tetranychus urticae?

Tetranychus urticae (T. urticae) Koch is a cosmopolitan, polyphagous mite which causes economic losses in both agricultural and ornamental plants. Some traits of T. urticae hamper its management, including a short life cycle, arrhenotokous parthenogenesis, its haplodiploid sex determination system, and its extraordinary ability to adapt to different hosts and environmental conditions. Currently, the use of chemical and biological control are the major control methods used against this mite. In recent years, some studies have focused on plant defence mechanisms against herbivores. Various families of plant compounds (such as flavonoids, glucosinolates, or acyl sugars) have been shown to behave as acaricides. Plants can be induced upon appropriate stimuli to increase their resistance against spider mites. This knowledge, together with the understanding of mechanisms by which T. urticae detoxifies and adapts to pesticides, may complement the control of this pest. Herein, we describe plant volatile compounds (VOCs) with repellent activity, and new findings about defence priming against spider mites, which interfere with the T. urticae performance. The use of VOCs and defence priming can be integrated into current management practices and reduce the damage caused by T. urticae in the field by implementing new, more sustainable crop management tools.

Effects of Low Temperature on Spider Mite Control by Intermittent Ultraviolet-B Irradiation for Practical Use in Greenhouse Strawberries

The application of ultraviolet-B (UVB) radiation to control spider mites is challenging as a key technology for integrated pest management (IPM) in greenhouse strawberries in Japan. To address this, concurrent use of phytoseiid mites and reduced UVB irradiance is desirable to ensure control effects in areas shaded from UVB radiation and to minimize the sunscald in winter, respectively. We designed experiments reproducing the UVB dose on the lower leaf surfaces in strawberry and evaluated the effects of intermittent UVB irradiation at midnight for practical application in the greenhouse and low temperature on the survival of the spider mite Tetranychus urticae Koch (Acari: Tetranychidae) and damage to the phytoseiid mite Neoseiulus californicus (McGregor) (Acari: Phytoseiidae). The midnight intermittent UVB irradiation effectively suppressed egg hatching and development of larvae of T. urticae, and the control effect was reinforced at 20°C (no eggs hatched at 0.13 kJ m-2 d-1) rather than, at 25°C (70.8% eggs hatched). In contrast, the hatchability of N. californicus eggs was unaffected by intermittent UVB irradiation at 0.27 kJ m-2 d-1 at 25°C and 20°C. However, residual effects of UVB irradiation to N. californicus eggs on survival of hatched larvae were seen, so that reducing the UVB dose is also advantageous for this phytoseiid mite. N. californicus showed a photoreactivation capacity, whereas their UVB tolerance was improved by prey species, suggesting the possibility of the improvement of phytoseiid mites by diet. The reduction of UVB dose and concurrent use of phytoseiid mites increase reliability of the UVB method in IPM strategies in strawberry greenhouse.

Developmental Phase-Specific Mortality After Ultraviolet-B Radiation Exposure in the Two-Spotted Spider Mite

Exposure to ambient ultraviolet-B (UVB) radiation generates DNA lesions, such as cyclobutane pyrimidine dimers and 6-4 pyrimidine-pyrimidine photoproducts in Tetranychus urticae Koch (Acari: Tetranychidae). Larvae appeared normal and healthy after UVB irradiation. Conversely, many mites were trapped in their old epidermis or experienced retarded development and shrunk, thus failing to molt from protochrysalises to protonymphs and died. This suggested that DNA lesions per se were not causing lethality in mites unless damaged genes were expressed. UVB-induced DNA lesions may have interfered with DNA replication and gene expression during the physiological changes of morphogenesis in the chrysalis stage. Comprehensive gene expression analysis by RNA sequencing revealed that gene expression involving epidermal tissue (characteristically cuticular protein genes) and myosin heavy chain muscle-like genes were downregulated in protochrysalises irradiated with UVB at the larval stage. We conclude that the success of protochrysalis molting is determined by whether the DNA lesions of genes, particularly those connected with morphogenesis, are repaired before expression at the protochrysalis stage.

Tolerance of the eriophyid mite Aceria salsolae to UV-A light and implications for biological control of Russian thistle

Aceria salsolae (Acari: Eriophyidae) is being evaluated as a candidate biological control agent of Russian thistle (Salsola tragus, Chenopodiaceae), a major invasive weed of rangelands and dryland crops in the western USA. Prior laboratory host range testing under artificial lighting indicated reproduction on non-native Bassia hyssopifolia and on a native plant, Suaeda calceoliformis. However, in field tests in the native range, mite populations released on these 'nontarget' plants remained low. We hypothesized that UV-A light, which can affect behavior of tetranychid mites, would affect populations of the eriophyid A. salsolae differently on the target and nontarget plant species, decreasing the mite's realized host range. Plants were infested with A. salsolae under lamps that emitted UV-A, along with broad-spectrum lighting, and the size of mite populations and plant growth was compared to infested plants exposed only to broad-spectrum light. Russian thistle supported 3- to 55-fold larger mite populations than nontarget plants regardless of UV-A treatment. UV-A exposure did not affect mite populations on Russian thistle or S. calceoliformis, whereas it increased populations 7-fold on B. hyssopifolia. Main stems on nontarget plants grew 2- to 6-fold faster than did Russian thistle under either light treatment. The two nontarget plants attained greater volume under the control light regime than UV-A, but Russian thistle was unaffected. Although Russian thistle was always the superior host, addition of UV-A light to the artificial lighting regime did not reduce the ability of A. salsolae to reproduce on the two nontarget species, suggesting that UV-B or other environmental factors may be more important in limiting mite populations in the field.

Antioxidant Protection by Astaxanthin in the Citrus Red Mite (Acari: Tetranychidae)

Solar ultraviolet-B (UVB) radiation and radiant heat have lethal effects on plant-dwelling mites, including spider mites, and their natural enemies, such as phytoseiid mites, leading them to reside on lower leaf surfaces. Panonychus spider mites are outcompeted by Tetranychus spider mites and thus exploit upper leaf surfaces, where they are exposed to both UVB radiation and radiant heat. Panonychus spider mites are thought to produce astaxanthin constitutionally. In this study, we compared carotenoid components, antioxidant capacity, lipid peroxidation, survival, and egg production in wild-type (WTS) and albino-type strains (ATS) of Panonychus citri (McGregor). Four carotenoids (neoxanthin, violaxanthin, lutein, and carotene) and their isomers and esters were identified in both strains, but astaxanthin and its esters were present only in WTS. The singlet oxygen scavenging capacity of lipid-soluble ingredients was greater in WTS than in ATS, whereas the oxygen radical absorbance capacities of hydrophilic ingredients were equivalent between them. Lipid peroxide accumulation was clearly higher in ATS than in WTS under both UVB irradiation (25 °C) and high temperature (35 °C) conditions. The findings are consistent with an antioxidant protective function of astaxanthin in this mite. Survival periods at 38 °C were longer in WTS than in ATS, although no difference was shown at 35 °C or under UVB irradiation. Therefore, astaxanthin accumulation was shown to be a major mechanism for survival under radiant heat, although other mechanisms, such as photoreactivation, might play a major role in survival under UVB radiation.

Cross-talk between high light stress and plant defence to the two-spotted spider mite in Arabidopsis thaliana

Little is known about how plants deal with arthropod herbivores under the fluctuating light intensity and spectra which occur in natural environments. Moreover, the role of simultaneous stress such as excess light (EL) in the regulation of plant responses to herbivores is poorly characterized. In the current study, we focused on a mite-herbivore, specifically, the two-spotted spider mite (TSSM), which is one of the major agricultural pests worldwide. Our results showed that TSSM-induced leaf damage (visualized by trypan blue staining) and oviposition rate (measured as daily female fecundity) decreased after EL pre-treatment in wild-type Arabidopsis plants, but the observed responses were not wavelength specific. Thus, we established that EL pre-treatment reduced Arabidopsis susceptibility to TSSM infestation. Due to the fact that a portion of EL energy is dissipated by plants as heat in the mechanism known as non-photochemical quenching (NPQ) of chlorophyll fluorescence, we tested an Arabidopsis npq4-1 mutant impaired in NPQ. We showed that npq4-1 plants are significantly less susceptible to TSSM feeding activity, and this result was not dependent on light pre-treatment. Therefore, our findings strongly support the role of light in plant defence against TSSM, pointing to a key role for a photo-protective mechanism such as NPQ in this regulation. We hypothesize that plants impaired in NPQ are constantly primed to mite attack, as this seems to be a universal evolutionarily conserved mechanism for herbivores.

Remote Sensing Evaluation of Two-spotted Spider Mite Damage on Greenhouse Cotton

The objective of this study was to evaluate a ground-based multispectral optical sensor as a remote sensing tool to assess foliar damage caused by the two-spotted spider mite (TSSM), Tetranychus urticae Koch, on greenhouse grown cotton. TSSM is a polyphagous pest which occurs on a variety of field and horticultural crops. It often becomes an early season pest of cotton in damaging proportions as opposed to being a late season innocuous pest in the mid-southern United States. Evaluation of acaricides is important for maintaining the efficacy of and preventing resistance to the currently available arsenal of chemicals and newly developed control agents. Enumeration of spider mites for efficacy evaluations is laborious and time consuming. Therefore, subjective visual damage rating is commonly used to assess density of spider mites. The NDVI (Normalized Difference Vegetation Index) is the most widely used statistic to describe the spectral reflectance characteristics of vegetation canopy to assess plant stress and health consequent to spider mite infestations. Results demonstrated that a multispectral optical sensor is an effective tool in distinguishing varying levels of infestation caused by T. urticae on early season cotton. This remote sensing technique may be used in lieu of a visual rating to evaluate insecticide treatments.

Photo-enzymatic repair of UVB-induced DNA damage in the two-spotted spider mite Tetranychus urticae

Ambient ultraviolet-B (UVB) radiation induces lethal effects in the two-spotted spider mite Tetranychus urticae, whereas photoreactivation by irradiation with ultraviolet-A and visible light (VIS) plays an important role to increase survival of mites irradiated by UVB. The physiological mechanisms and ecological significance of photoreactivation in terrestrial arthropods have not been shown clearly. We verified the biological impact and accumulation of DNA lesions by UVB irradiation and the repair of them by photoreactivation in T. urticae larvae. Survival of UVB-irradiated larvae decreased with increasing UVB dose, but recovered remarkably with VIS exposure after UVB irradiation (photoreactivation). The DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 pyrimidine-pyrimidine photoproducts (6-4PPs) linearly increased with the UVB dose. The CPDs were repaired after exposure to VIS, whereas the frequency of 6-4PPs was unaffected by VIS; CPD photolyase genes, but not (6-4) photolyase genes, have been found in the T. urticae genome. Therefore, DNA damage and CPD photo enzymatic repair (PER) is significant for survival in this mite under ambient UVB radiation. Unexpectedly, gene expression of CPD photolyase was unaffected by irradiation with UVB and VIS. Instead, expression of xeroderma pigmentosum A (XPA) was increased by irradiation. XPA is a core factor in nucleotide excision repair (NER), which is a repair system unrelated to photo energy. The relationship between gene expression and enzymatic repair remains unclear. To elucidate the PER process in T. urticae, further study will be necessary on the gene expression patterns and molecular functions of CPD photolyase in PER and of XPA in NER.

Egg hatching response to a range of ultraviolet-B (UV-B) radiation doses for four predatory mites and the herbivorous spider mite Tetranychus urticae

Egg hatchability of four predatory mites-Phytoseiulus persimilis Athias-Henriot, Iphiseius [Amblyseius] degenerans Berlese, Amblyseius swirskii Athias-Henriot, and Euseius finlandicus Oudemans (Acari: Phytoseiidae)-and the spider mite Tetranychus urticae Koch (Acari: Tetranychidae) was determined under various UV-B doses either in constant darkness (DD) or with simultaneous irradiation using white light. Under UV-B irradiation and DD or simultaneous irradiation with white light, the predator's eggs hatched in significantly lower percentages than in the control non-exposed eggs, which indicates deleterious effects of UV-B on embryonic development. In addition, higher hatchability percentages were observed under UV-B irradiation and DD in eggs of the predatory mites than in eggs of T. urticae. This might be caused by a higher involvement of an antioxidant system, shield effects by pigments or a mere shorter duration of embryonic development in predatory mites than in T. urticae, thus avoiding accumulative effects of UV-B. Although no eggs of T. urticae hatched under UV-B irradiation and DD, variable hatchability percentages were observed under simultaneous irradiation with white light, which suggests the involvement of a photoreactivation system that reduces UV-B damages. Under the same doses with simultaneous irradiation with white light, eggs of T. urticae displayed higher photoreactivation and were more tolerant to UV-B than eggs of the predatory mites. Among predators variation regarding the tolerance to UV-B effects was observed, with eggs of P. persimilis and I. degenerans being more tolerant to UV-B radiation than eggs of A. swirskii and E. finlandicus.

Influence of mowing on dynamics of native phytoseiid mites and Tetranychus urticae in apple orchards in northern Japan

To support practical integrated pest management in commercial apple orchards, I investigated the influence of mowing on the occurrence of Tetranychus urticae and native phytoseiid mites in apple orchards sprayed with selective insecticides in Akita Prefecture, northern Japan, from 2013 to 2015. The orchards were not mown in 2013, and unmown and mown plots were compared in 2014 and 2015. There were significantly fewer Typhlodromus vulgaris on apple leaves and Amblyseius tsugawai in the undergrowth in mown plots than in unmown plots in both years. Conversely, there were significantly more T. urticae on leaves and undergrowth in mown plots than in unmown plots. The reason for the decreased populations of these phytoseiid mites may be a lack of food (pollen) needed for reproduction on apple trees and in the undergrowth due to mowing. These results indicate that mowing strongly influences generalist phytoseiid mites in apple orchards. Moreover, mowing might increase the density of T. urticae in apple trees because increased nitrogen in the leaves increases fecundity; in addition, drought might promote the increase of mite numbers. Thus, retention of undergrowth suppresses T. urticae in apple orchards.

Physical Control of Spider Mites Using Ultraviolet-B With Light Reflection Sheets in Greenhouse Strawberries

Development of spider mite management technology other than chemical control is desired because of the serious development of acaricide resistance worldwide. Recent studies have evidenced the lethal effects of ultraviolet-B (UVB) radiation on spider mites. To develop the technology on how to use UVB irradiation for spider mite control, we tested whether UVB lamp-light reflection sheet (LRS) combinations suppressed the population size of Tetranychus urticae Koch on strawberry in a greenhouse from December to May (2012-2013, 2013-2014) in Japan. We designed four combinations of UVB lamps and LRSs: 1) neither UVB lamps nor LRSs (UV-LRS-); 2) a UVB lamp without an LRS (UV+LRS-; 2012-2013 only); 3) a UVB lamp and a mulch-type LRS (UV+LRSm); and 4) a UVB lamp and a wing-type LRS (UV+LRSw). The number of adult females peaked at 438.0 and 222.0 per plant in UV-LRS- of 2012-2013 and 2013-2014, respectively, and peaked at 191.6 females in UV+LRS- of 2012-2013. In contrast, the peak abundance was 20.9-98.0 females in UV+LRSm, and fewer than 15 females were noted in UV+LRSw over either experimental period. UVB irradiance on lower leaf surfaces was higher in UV+LRSm and UV+LRSw than UV-LRS- and UV+LRS-, and the mite densities were significantly correlated with UVB irradiance on lower leaf surfaces. Consequently, we conclude that the combinations of UVB lamp-LRS have an excellent capacity to control T. urticae on greenhouse strawberry, and that the LRS was an essential component in this technological approach.

A scenario for the evolution of selective egg coloration: the roles of enemy-free space, camouflage, thermoregulation and pigment limitation

Behavioural plasticity can drive the evolution of new traits in animals. In oviparous species, plasticity in oviposition behaviour could promote the evolution of new egg traits by exposing them to different selective pressures in novel oviposition sites. Individual females of the predatory stink bug Podisus maculiventris are able to selectively colour their eggs depending on leaf side, laying lightly pigmented eggs on leaf undersides and more pigmented eggs, which are more resistant to ultraviolet (UV) radiation damage, on leaf tops. Here, we propose an evolutionary scenario for P. maculiventris egg pigmentation and its selective application. We experimentally tested the influence of several ecological factors that: (i) could have favoured a behavioural shift towards laying eggs on leaf tops and thus the evolution of a UV-protective egg pigment (i.e. exploitation of enemy-reduced space or a thermoregulatory benefit) and (ii) could have subsequently led to the evolution of selective pigment application (i.e. camouflage or costly pigment production). We found evidence that a higher predation pressure on leaf undersides could have caused a shift in oviposition effort towards leaf tops. We also found the first evidence of an insect egg pigment providing a thermoregulatory advantage. Our study contributes to an understanding of how plasticity in oviposition behaviour could shape the responses of organisms to ecological factors affecting their reproductive success, spurring the evolution of new morphological traits.

Joint Effect of Solar UVB and Heat Stress on the Seasonal Change of Egg Hatching Success in the Herbivorous False Spider Mite (Acari: Tenuipalpidae)

Seasonal population dynamics of an herbivorous mite has been documented in terms of the relationship between thermoresponses and temporal biological factors such as resource availability or predation risk. Although recent studies emphasize the deleterious effects of solar ultraviolet-B (UVB; 280-320 nm wavelengths) radiation on plant-dwelling mites, how UVB affects mite population remains largely unknown. On a wild shrub Viburnum erosum var. punctatum in Kyoto, an herbivorous false spider mite, Brevipalpus obovatus Donnadieu, occurs only in autumn. Females of this species lay one-third of their eggs on upper leaf surfaces. Oviposition on upper surfaces is beneficial for avoiding predation by phytoseiids, but exposes eggs to solar UVB and heat stress. To test the hypothesis that the seasonal occurrence of this mite is determined by interactions between solar UVB radiation and temperature, we examined variation in egg hatching success under near-ambient and UV-attenuated sunlight conditions from spring to autumn. The UV-attenuation significantly improved hatching success. However, most eggs died under heat stress regardless of UV treatments in July and August. We established a deterministic heat stress-cumulative UVB dose-egg hatching success response model, which we applied to meteorological data. The model analyses illustrated lower and higher survivability peaks in late May and October, respectively, which partly corresponded to data for annual field occurrence, indicating the importance of solar UVB radiation and heat stress as determinants of the seasonal occurrence of this mite.

Regulation of three isoforms of SOD gene by environmental stresses in citrus red mite, Panonychus citri

Superoxide dismutase (SOD) is a family of enzymes with multiple isoforms that possess antioxidative abilities in response to environmental stresses. Panonychus citri is one of the most important pest mites and has a global distribution. In this study, three distinct isoforms of SOD were cloned from P. citri and identified as cytoplasmic Cu-ZnSOD (PcSOD1), extracellular Cu-ZnSOD (PcSOD2), and mitochondrial MnSOD (PcSOD3). mRNA expression level analysis showed that all three isoforms were up-regulated significantly after exposure to the acaricide abamectin and to UV-B ultraviolet irradiation. In particular, PcSOD3 was up-regulated under almost all environmental stresses tested. The fold change of PcSOD3 expression was significantly higher than those of the two Cu-ZnSOD isoforms. Taken together, the results indicate that abamectin and UV-B can induce transcripts of all three SOD isoforms in P. citri. Furthermore, PcSOD3 seems to play a more important role in P. citri tolerance to oxidative stress.

Deoxidant-induced anoxia as a physical measure for controlling spider mites (Acari: Tetranychidae)

Tiny agricultural pests such as spider mites (Acari: Tetranychidae) attached to seedlings grown outdoors often invade greenhouses, thereby triggering pest outbreaks. To solve the problem, we examined whether differences in anoxia tolerance between animals and plants would permit the application of an anoxic environment to control spider mites without the aid of acaricides. Under an anoxic environment created by using a commercial deoxidant at 25 °C, the time for 50 % mortality of eggs, non-diapausing adults (summer form), and diapausing adults (winter form) were 6.1, 5.5, and 23.6 h, respectively, for Tetranychus urticae Koch and 5.4, 3.9, and 23.2 h, respectively, for Tetranychus kanzawai Kishida. With anoxia for 12 h, no eggs and non-diapausing adults survived in either species, whereas most diapausing adults (98 % for T. urticae and 88 % for T. kanzawai) survived. Under this treatment, host Phaseolus vulgaris L. seedlings showed serious physiological disorders in their primary leaves and apical buds, and unusual lateral buds developed in the cotyledon axils. The spider mites acquire anoxia tolerance during diapause, but anoxia can potentially control them during the summer if no negative effects are observed in the treated seedlings.

Management of apple orchards to conserve generalist phytoseiid mites suppresses two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae)

To improve the success of integrated pest management (IPM) in apple orchards, we investigated whether generalist phytoseiid mites have suppressed the occurrence of Tetranychus urticae. In Akita Prefecture, northern Japan, in 2012 and 2013, two types of experimental plot were compared. Conservation plots had been managed for the conservation of generalist phytoseiid mites by selective chemical spraying without mowing since 2009. Conventional plots were managed by non-selective chemical spraying with regular mowing. The conservation plots had significantly fewer T. urticae adult females per tree in both years. Two species of generalist phytoseiid mites-Typhlodromus vulgaris and Amblyseius tsugawai-were continuously present in the conservation plots, with only a few T. urticae. The conservation plots had significantly more A. tsugawai adult females in the undergrowth in both years, and significantly more T. vulgaris adult females on apple leaves in 2012. Typhlodromus vulgaris was continuously present in the conservation plots but was scarce from late May to early August in the conventional plots. In the presence of T. vulgaris, low numbers of T. urticae did not increase on apple leaves. These results indicate that the generalist phytoseiid mites serve as important biological control agents in IPM in apple orchards.

Beyond xeroderma pigmentosum: DNA damage and repair in an ecological context. A tribute to James E. Cleaver

The ability to repair DNA is a ubiquitous characteristic of life on Earth and all organisms possess similar mechanisms for dealing with DNA damage, an indication of a very early evolutionary origin for repair processes. James E. Cleaver's career (initiated in the early 1960s) has been devoted to the study of mammalian ultraviolet radiation (UVR) photobiology, specifically the molecular genetics of xeroderma pigmentosum and other human diseases caused by defects in DNA damage recognition and repair. This work by Jim and others has influenced the study of DNA damage and repair in a variety of taxa. Today, the field of DNA repair is enhancing our understanding of not only how to treat and prevent human disease, but is providing insights on the evolutionary history of life on Earth and how natural populations are coping with UVR-induced DNA damage from anthropogenic changes in the environment such as ozone depletion.

Lethal effects of short-wavelength visible light on insects

We investigated the lethal effects of visible light on insects by using light-emitting diodes (LEDs). The toxic effects of ultraviolet (UV) light, particularly shortwave (i.e., UVB and UVC) light, on organisms are well known. However, the effects of irradiation with visible light remain unclear, although shorter wavelengths are known to be more lethal. Irradiation with visible light is not thought to cause mortality in complex animals including insects. Here, however, we found that irradiation with short-wavelength visible (blue) light killed eggs, larvae, pupae, and adults of Drosophila melanogaster. Blue light was also lethal to mosquitoes and flour beetles, but the effective wavelength at which mortality occurred differed among the insect species. Our findings suggest that highly toxic wavelengths of visible light are species-specific in insects, and that shorter wavelengths are not always more toxic. For some animals, such as insects, blue light is more harmful than UV light.

Factors affecting photoreactivation in UVB-irradiated herbivorous spider mite (Tetranychus urticae)

Ambient ultraviolet-B (UVB) radiation causes lethal damage to spider mites, and the extent of photochemical effects is determined by cumulative irradiance in the two-spotted spider mite, Tetranychus urticae. However, the LD50 values obtained using a UVB lamp were much lower than those elicited by solar UVB radiation. As solar radiation includes intense visible light and UVA, we assumed that a photoreactivation mechanism would play a role to survive under solar radiation. We assessed the capacity for photoreactivation in T. urticae eggs and larvae, and found that the efficacy of photoreactivation was determined by the cumulative irradiance of visible light (VIS) after exposure to UVB radiation. The wavelength range effective for photoreactivation went from UVA to green. Next, we found that an increased time lag between UVB and VIS radiation reduced the photoreactivation efficacy in eggs. In contrast, a time lag ≤4 h did not affect that in larvae. We discussed the possibility that the timing of photoreactivation occurs related with phase-specific UVB vulnerability and outbreak symptoms due to UVB-induced DNA damage. Our results suggest that T. urticae depends on a photoreactivation mechanism, and that the photoreactivation efficiency probably caused the divergence in UVB impact between UV lamp and solar radiation.

Spectrum-specific UV egg damage and dispersal responses in the phytoseiid predatory mite Neoseiulus californicus (Acari: Phytoseiidae)

Solar ultraviolet-B (UVB) radiation is deleterious to plant-dwelling mites. Neoseiulus californicus (McGregor) is a predominant predator of agriculturally important pest species of spider mite. However, phytoseiid mites are more vulnerable to UVB radiation than spider mites. Thus, the UVB radiation may influence decision making in foraging phytoseiid mites whether disperse or not. We tested the difference in impact and behavioral response among wavelengths of monochromatic UV radiation using a spectroscopic light source in N. californicus in the laboratory. We also examined whether the behavioral responses of N. californicus females to UV radiation varied based on the presence of prey (Tetranychus urticae Koch) eggs and residues (webs and excreta of T. urticae: foraging cue). The impact of UV radiation on the N. californicus egg hatchability varied drastically between wavelengths of ≤300 nm (0%) and ≥310 nm (100%). The N. californicus females escaped from UV radiation more quickly when they were irradiated with UV at shorter wavelength. Presence of T. urticae eggs had no effects arresting the escape of phytoseiid mites. In contrast, prey residues (including eggs) markedly detained N. californicus females from escaping under UV irradiation at ≥310 nm. However, N. californicus females quickly escaped when irradiated with UV at harmful 300 nm wavelength, regardless of prey cues. This indicates that the eyeless phytoseiid mite is capable of perceiving UV radiation, and whether escape or not is determined on the basis of harmful/harmless UV wavelength and presence/absence of foraging cues.

An LED-based UV-B irradiation system for tiny organisms: System description and demonstration experiment to determine the hatchability of eggs from four Tetranychus spider mite species from Okinawa

We developed a computer-based system for controlling the photoperiod and irradiance of UV-B and white light from a 5×5 light-emitting diode (LED) matrix (100×100mm). In this system, the LED matrix was installed in each of four irradiation boxes and controlled by pulse-width modulators so that each box can independently emit UV-B and white light at irradiances of up to 1.5 and 4.0Wm(-2), respectively, or a combination of both light types. We used this system to examine the hatchabilities of the eggs of four Tetranychus spider mite species (T. urticae, T. kanzawai, T. piercei and T. okinawanus) collected from Okinawa Island under UV-B irradiation alone or simultaneous irradiation with white light for 12hd(-1) at 25°C. Although no eggs of any species hatched under the UV-B irradiation, even when the irradiance was as low as 0.02Wm(-2), the hatchabilities increased to >90% under simultaneous irradiation with 4.0Wm(-2) white light. At 0.06Wm(-2) UV-B, T. okinawanus eggs hatched (15% hatchability) under simultaneous irradiation with white light, whereas other species showed hatchabilities <1%. These results suggest that photolyases activated by white light may reduce UV-B-induced DNA damage in spider mite eggs and that the greater UV-B tolerance of T. okinawanus may explain its dominance on plants in seashore environments, which have a higher risk of exposure to reflected UV-B even on the undersurface of leaves. Our system will be useful for further examination of photophysiological responses of tiny organisms because of its ability to precisely control radiation conditions.

Stellate hairs on leaves of a deciduous shrub Viburnum erosum var. punctatum (Adoxaceae) effectively protect Brevipalpus obovatus (Acari: Tenuipalpidae) eggs from the predator Phytoseius nipponicus (Acari: Phytoseiidae)

The eggs of the herbivorous false spider mite Brevipalpus obovatus Donnadieu have a longer incubation period than those of spider mites and are not protected by webs. Brevipalpus obovatus often lays its eggs in the gaps among the hairs on host leaves. We examined the effects of stellate hairs of Viburnum erosum var. punctatum (VEP) leaves on the survival of B. obovatus eggs. Adult B. obovatus and Phytoseius nipponicus Ehara, a generalist predator, were introduced to VEP leaf disks; each B. obovatus egg was inspected daily until hatching. More eggs (63 vs. 42 %) survived on the abaxial surfaces of VEP leaves, where the stellate hairs are more complicated, than on the adaxial surfaces. Predation hazard decreased rapidly with increasing egg age and a substantial portion of the eggs hatched. Phytoseius nipponicus preyed on eggs regardless of egg age when mixed-age eggs were provided. Manipulative experiments with bent stellate hairs showed that the normal hairs reduced the predation risk of B. obovatus eggs by P. nipponicus. Therefore, the predation hazard was considered to decrease since the stellate hairs hindered the search for B. obovatus eggs by the phytoseiid mite.

The effects of enhanced ultraviolet-B radiation on the biology of green and brown morphs of Sitobion avenae (Hemiptera: Aphididae)

The effects of enhanced UV-B radiation on the biology of green and brown morphs of Sitobion avenae (F.) (Hemiptera: Aphididae) were tested under laboratory conditions. The two S. avenae morphs were exposed directly to different doses (0, 216, 432, and 864 kJ/m(2)) of artificial UV-B. Under low doses of UV-B (216 kJ/m(2)), the nymphal development period was significantly shorter, whereas mean relative growth, total fecundity, and gross reproductive rate were significantly higher for both aphid morphs compared with those morphs under control treatments (0 kJ/m(2)). Under high doses of UV-B (432 kJ/m(2) and/or 864 kJ/m(2)), the nymphal development period was significantly longer, whereas the reproductive period, postreproductive period, difference in weight, mean relative growth, and life table parameters were significantly reduced for both aphid morphs. Moreover, the nymphal development period was significantly shorter and the differences in weight, mean relative growth, total fecundity, and life table parameters were significantly higher for the brown morph under high doses of UV-B compared with the green morph. The results showed that enhanced UV-B affects the performance of the green and brown S. avenae morphs from positively to negatively. The brown morph exhibited stronger adaptability than the green morph at high doses of UV-B.

Geotaxis and leaf-surface preferences mitigate negative effects of a predatory mite on an herbivorous mite

Reproductive success and population growth of an herbivorous mite are limited by activities of phytoseiid predators. However, occurrences on upper versus lower leaf surfaces are sometimes mismatched between these prey and predators. The mismatch potentially mitigates predation risk for the prey species. We assessed factors that affect mite distributions on leaf surfaces, testing whether the presence of the phytoseiid mite Phytoseius nipponicus alters the leaf-surface distribution and reproductive success of the herbivorous false spider mite Brevipalpus obovatus. The host plant was Viburnum erosum var. punctatum (Adoxaceae). Leaves were set in natural (TRUE) and reversed (upside down; INVERTED) orientations using experimental devices. Both surfaces were accessible to mites. We detected lower and abaxial leaf-surface preferences in P. nipponicus. In contrast, upper and adaxial surfaces were preferred by B. obovatus. Thus, prey and predatory mites accumulated on different sides of leaves. Presence of the predator also indirectly decreased egg production in B. obovatus. Brevipalpus obovatus females actively avoided leaf surfaces with elevated predator numbers; these females shifted their distributions and changed oviposition sites to leaf surfaces with fewer predators. In consequence, B. obovatus eggs on the upper sides of leaves were less frequently preyed upon than were those on lower sides. We suggest that upper leaf-surface exploitation in this particular herbivorous mite species mitigates predation risk from phytoseiid mites, which prefer lower leaf surfaces.

The Bunsen-Roscoe reciprocity law in ultraviolet-B-induced mortality of the two-spotted spider mite Tetranychus urticae

To determine whether the Bunsen-Roscoe reciprocity law (i.e., the extent of photochemical effects is determined by cumulative irradiance) is applicable to ultraviolet-B (UVB) damage in the twospotted spider mite Tetranychus urticae, egg hatchability and survival of individuals were assessed after irradiation with a UVB lamp using various combinations of intensity and time length. A positive linear correlation between probit mortality and cumulative UVB irradiance was detected in eggs, larvae, teleiochrysalis females, and adult females, regardless of UVB intensity (0.19-0.58 Wm(-2)). LD50 values were clearly higher in adult females, followed by teleiochrysalis females, larvae, and eggs. In eggs, reciprocity was obeyed not only at the UVB intensities listed above, but also at very low UVB intensity (0.014-0.023 Wm(-2)). Such reciprocity in the negative effects of UVB radiation was also observed for the developmental rate of juveniles and egg production of adult females. However, the LD50 value of eggs obtained using the UVB lamp (0.58 kJm(-2)) was lower than that elicited by solar UVB radiation in a previous outdoor experiment (about 50 kJm(-2)). These results suggest that a photoreactivation mechanism plays an important role in the survival of this mite under solar radiation.

UVB radiation and 17-hydroxygeranyllinalool diterpene glycosides provide durable resistance against mirid (Tupiocoris notatus) attack in field-grown Nicotiana attenuata plants

Depending on geographical location, plants are exposed to variable amounts of UVB radiation and herbivore attack. Because the role(s) of UVB in the priming and/or accumulation of plant defence metabolites against herbivores are not well understood, we used field-grown Nicotiana attenuata plants to explore the effects of UVB on herbivore performance. Consistent with previous reports, UVB-exposed plants accumulated higher levels of ultraviolet (UV)-absorbing compounds (rutin, chlorogenic acid, crypto-chlorogenic acid and dicaffeoylspermidine). Furthermore, UVB increased the accumulation of jasmonic acid, jasmonoyl-L-isoleucine and abscisic acid, all phytohormones which regulate plant defence against biotic and abiotic stress. In herbivore bioassays, N. attenuata plants experimentally protected from UVB were more infested by mirids in three consecutive field seasons. Among defence metabolites measured, 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) showed strongly altered accumulation patterns. While constitutive HGL-DTGs levels were higher under UVB, N. attenuata plants exposed to mirid bugs (Tupiocoris notatus) had still more HGL-DTGs under UVB, and mirids preferred to feed on HGL-DTGs-silenced plants when other UVB protecting factors were eliminated by UVB filters. We conclude that UVB exposure not only stimulates UV protective screens but also affects plant defence mechanisms, such as HGL-DTGs accumulation, and modulates ecological interactions of N. attenuata with its herbivores in nature.