Photochemistry and Photobiology, 2013, 89: 424–431
Tolerance to Solar Ultraviolet-B Radiation in the Citrus Red Mite, An Upper Surface User of Host Plant Leaves Midori Fukaya1, Ryuji Uesugi1, Hirokazu Ohashi2, Yuta Sakai1, Masaaki Sudo1, Atsushi Kasai1, Hidenari Kishimoto3 and Masahiro Osakabe*1 1
Laboratory of Ecological Information, Graduate School of Agriculture, Kyoto University, Kyoto, Japan Fruit Tree Experiment Station, Wakayama Research Center of Agriculture, Wakayama, Japan 3 Kuchinotsu Citrus Research Station, National Institute of Fruit Tree Science, Nagasaki, Japan 2
Received 30 March 2012, accepted 12 September 2012, DOI: 10.1111/php.12001
ABSTRACT
reside on the upper leaf surfaces of their host plants (6–9). However, the effects of UVB radiation on the fitness of Panonychus species have not yet been assessed. Given that the avoidance of solar UVB radiation is the major factor determining the within leaf distribution of other plant-dwelling mites, we hypothesized that Panonychus species have acquired mechanisms of protection against the impacts of ambient UVB radiation. The citrus red mite, Panonychus citri (McGregor), is distributed in midlatitude areas and occurs mainly on citrus trees. Although the UVB protection functions of leaf hairs, trichomes and pubescence have been described previously (10–12), citrus leaves are mostly hairless on both adaxial and abaxial surfaces and lack trichomes. Therefore, if female mites lay eggs on the sunny parts of leaves, those eggs are exposed directly to solar radiation, including UVB. Because of the potential impact of solar UVB radiation, if eggs are not guarded against UVB damage, females might avoid ovipositing on the upper surfaces of sunny leaves. In T. urticae, eggs are more sensitive to UVB radiation than are adult females (13). In the present study, we investigated whether the upper leaf surfaces are favorable to P. citri and to what extent these mites are tolerant to UVB radiation. We conducted laboratory experiments with a UVB lamp, semioutdoor experiments that included manipulating the levels of solar UVB radiation and field studies in a citrus grove. In the laboratory experiments, we compared the dose response in the hatchability of P. citri eggs with that of T. urticae eggs after exposure to radiation from the UVB lamp. In the semioutdoor experiments, we determined the effects of solar radiation on the hatchability of P. citri eggs, and on the survival of hatched juveniles, under near-ambient and UV-attenuated conditions. In the field study, we examined the distribution of P. citri on upper and lower leaf surfaces, and on sunny and shaded leaves, and determined the hatchability of eggs on collected citrus leaves. To evaluate the nutritional value of the upper and lower leaf surfaces, we collected sunny and shaded leaves from the citrus grove and compared egg production and juvenile development of P. citri on the various leaf surface types.
Plant-dwelling mites are potentially exposed to solar ultraviolet-B (UVB) radiation that causes deleterious and often lethal effects, leading most mites to inhabit the lower (underside) leaf surfaces. However, in species of spider mite belonging to the Genus Panonychus, a substantial portion of individuals occur on upper leaf surfaces. We investigated whether the upper leaf surfaces of citrus trees are favorable for P. citri, and to what extent they are tolerant to UVB radiation. If eggs are not adequately protected from UVB damage, females may avoid ovipositing on the upper surfaces of sunny leaves. To test this, we conducted laboratory experiments using a UVB lamp, and semioutdoor manipulative experiments. As a result, P. citri eggs are tolerant to UVB. Field studies revealed that the ratio of eggs and adult females on upper leaf surfaces were larger for shaded than for sunny leaves. However, 64–89% of eggs hatched successfully even on sunny upper leaf surfaces. Nutritional evaluation revealed that whether on sunny or shaded leaves, in fecundity and juvenile development P. citri reaped the fitness benefits of upper leaf surfaces. Consequently, P. citri is tolerant to UVB damage, and inhabiting the upper surfaces of shaded leaves is advantageous to this mite.
INTRODUCTION Plant-dwelling mites experience harsh environments on the upper leaf surfaces of their host plants including high temperatures (or radiant heat), desiccation and other weather phenomena. Moreover, mites are potentially exposed to the impacts of solar ultraviolet-B (UVB, wavelength 280–315 nm) radiation that causes deleterious and often lethal effects (1–3). These environmental factors may cause mites to preferentially inhabit lower (underside) leaf surfaces (4) where they are sheltered from such threats. Of those threats, solar UVB radiation is a major factor that influences the behavior of the twospotted spider mite, Tetranychus urticae Koch, causing it to remain on the underside of leaf surfaces in the field (5). In contrast to the majority of plant-dwelling mites, a substantial portion of individuals of Panonychus spider mite species
MATERIALS AND METHODS Mites. Panonychus citri used for the laboratory and semioutdoor experiments was originally collected from citrus groves (“Tsunokaori”; 14) at the National Institute of Fruit Tree Science (32°36′N, 130°11′E) on 1 June 2007, and reared on citrus (sour orange: Citrus aurantium L.), Japanese pear (Pyrus pyrifolia Nakai) or kidney bean (Phaseolus vulgaris L.)
*Corresponding author email:
[email protected] (Masahiro Osakabe) © 2012 Wiley Periodicals, Inc. Photochemistry and Photobiology © 2012 The American Society of Photobiology 0031-8655/13
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Photochemistry and Photobiology, 2013, 89 leaves placed on water-soaked cotton in Petri dishes (9 cm diameter) in a laboratory at 25°C under 16 h L: 8 h D light cycles. The T. urticae population (the yellow-green type) had been established from several different localities in Japan and cultured on potted kidney bean plants in the laboratory for at least 6 years. The UVB dose response in P. citri and T. urticae eggs. We prepared four Petri dishes (9 cm in diameter). Three citrus (or kidney bean) leaf disks (2 9 2 cm) were placed upside-up on water-soaked cotton in each Petri dish. Ten adult P. citri females developed on citrus (or kidney bean) leaves were introduced to each leaf disk in the four Petri dishes. For T. urticae, we introduced five adult females to each of the kidney bean leaf disk. The Petri dishes were kept in a laboratory at 25°C, 16 h L: 8 h D light cycles. In the laboratory, fluorescent lights were turned on at 07:00 A.M. and off at 23:00 P.M. The next morning, we removed the females and counted the eggs laid on the leaf disks. Consequently, we obtained 127–176 and 98–170 P. citri eggs per Petri dish on kidney bean and citrus leaves, respectively, and 90–146 T. urticae eggs per Petri dish. Then three of these Petri dishes were placed on three shelves at different distances from an overhead UVB lamp (YGRFX21701GH; Panasonic Co., Osaka, Japan) at 25°C. The frame of the shelves was covered with UV-opaque film (0.1 mm thick polyvinyl chloride film, Cutaceclean Kirinain; MKV Platech, Tokyo, Japan) that filtered out UV as described in the next section. The remaining Petri dish was placed on an adjoining shelf outside of the UV-opaque film. Eggs on leaf disks were irradiated with UVB for 1 h. The intensity of UVB (280–320 nm) received by the Petri dishes was 0, 0.19 (UVA + UVB: 0.27), 0.31 (0.45) and 0.58 (1.0) W m 2, resulting in cumulative doses of 0, 0.684, 1.116 and 2.088 kJ m 2, respectively. The output of the UV lamp peaked at wavelength of 310 nm, and the laboratory was illuminated with fluorescent lamps. The spectrum of relative intensity was measured using a spectrometer (UFV-VIS F; Spectra Co-op Co., Tokyo, Japan; Fig. 1a). After UVB irradiation the Petri dishes were kept in the laboratory at 25°C, 16 h L:8 h D light cycles. We observed egg hatching daily from day 6 to 13 for P. citri and from day 4 to 9 for T. urticae (day 1 was the day that females were removed from leaf disks). No eggs hatched before days 6 and 4, or after days 10 and 8, in P. citri and T. urticae, respectively.
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Figure 1. Wavelength spectrums on shelves where UVB irradiances of 0.19 (UVA + UVB: 0.27) W m 2 (chain line), 0.31 (0.45) W m 2 (broken line), 0.58 (1.0) W m 2 (solid line) and 0 W m 2 (outside of the frame of shelves; dotted line) were used for the radiation dose–response experiments using a UV lamp and fluorescent lights (a) and transmission spectrum of light in UV-opaque polyvinyl chloride film (solid line), and UV-transparent polyethylene film (broken line) (b).
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We replicated this experiment three times for each mite type (i.e. P. citri on citrus or kidney bean leaves) and species independently. Prior to the statistical analyses, we combined the numbers of eggs hatched and dead on the three leaf disks in the same Petri dish. Differences in the UVB radiation dose response between mite types and species were evaluated using a generalized linear model (GLM; logit-link, binomial error) that included mite types, UVB dose and interaction factors. We constructed the models with modules “glm” using the package MASS in R (version 2.14.0; 15). Semioutdoor experiments under near-ambient and UV-attenuated conditions. We used Japanese pear leaves instead of citrus leaves for convenience in this experiment. The hatchability of P. citri eggs on Japanese pear leaves was intermediate between the eggs on citrus and kidney bean leaves, when they were irradiated with the UVB lamp. The hatchability on Japanese pear leaves was 85.8, 70.0 and 23.0% after UVB irradiance at 0.19, 0.31 and 0.58 W m 2 for 1 h (0.684, 1.116 and 2.088 kJ m 2), respectively, in a preliminary experiment. Eight P. citri adult females reared on Japanese pear leaves were introduced to each of four Japanese pear leaf disks (2 9 2 cm) placed on watersoaked cotton in a Petri dish in June, and eight Japanese pear leaf disks in two Petri dishes (four leaf disks per dish) in July and September. The mites were allowed to lay eggs in a laboratory at 25°C, 16 h L: 8 h D light cycles. The next day morning, adult females were removed. Then, the leaf disks containing P. citri eggs within 36 h of oviposition were exposed either to solar UV-attenuated conditions (UV ) or near-ambient UV conditions (UV+) from June 13 to 19 (50 eggs were used both for UV+ and UV [two leaf disks each]), July 5–10 (100 eggs both for UV+ and UV [four leaf disks each]), and September 24–30 [69 and 72 eggs for UV+ and UV (four leaf disks each)], 2008. The exposure was done for 10:00 A.M.– 18:00 P.M. except when rainy. We had rain on 19 June, 8 July (10:00 A.M.–12:00 P.M.) and 28 September (13:30 P.M.–18:00 P.M.), 29 and 30 (12:00 P.M.–18:00 P.M.). We recorded temperature and relative humidity (RH) under UV+ and UV conditions with data loggers (Hygrochron; KN Laboratories, Osaka, Japan) at 30-min intervals. Average temperature and RH for exposing the samples to solar radiation in the experimental periods were 31.5 ± 4.8°C and 42.1 ± 10.6% (mean ± SD) in UV+ and 32.7 ± 5.6°C and 45.1 ± 8.2% in UV in June, 33.9 ± 3.6°C and 53.3 ± 1.5% in UV+ and 33.9 ± 4.0°C and 50.8 ± 5.7% in UV in July and 24.7 ± 2.1°C and 57.9 ± 1.8% in UV+ and 24.4 ± 1.9°C and 57.4 ± 2.3% in UV in September. The leaf disks were placed in the laboratory (25°C, 16 h L:8 h D light cycles) during nighttime (18:00 P.M.–10:00 P.M.) and when rainy. We counted the number of hatched and unhatched eggs and the number of individuals survived and died on the day following the final day of the exposure. Simultaneously, we recorded the developmental stages of survived individuals. To achieve UV and UV+ conditions, UV-opaque film and UV-transparent film (0.03 mm thick polyethylene film; Dainichi, Osaka, Japan) were used, respectively, in the same manner as in Ohtsuka and Osakabe (13). The UV-opaque film transmitted 80% of visible light (wavelengths >400 nm) while filtering out UV (|z|) |z|) = 7.22 9 10 7; P. citri on kidney bean: coefficient = 0.5979 ± 0.2166, Pr(>|z|) = 0.00578). Although the interaction between P. citri eggs on kidney bean and UVB dose was not significant, the interaction between T. urticae eggs and UVB dose was significant (T. urticae 9 UVB dose: coefficient = 1.5602 ± 0.2180, Pr(>| z|) = 8.33 9 10 13). Therefore, the response of egg hatchability to UVB exposure was different between T. urticae eggs and P. citri eggs on citrus and kidney bean. Semioutdoor experiments on the effects of solar radiation on egg hatchability
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Cumulative dose of UVB radiation (kJ m-2) Figure 2. Dose response to UVB irradiation by UV lamp in eggs of Panonychus citri laid on citrus (solid triangles) and kidney bean (solid circles) leaves, and Tetranychus urticae on kidney bean (open circles). Vertical lines above and below each plot indicate 95% confidential interval.
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Field study of the upper/lower leaf distribution and egg hatchability in citrus groves Upper/lower distribution of adult females and juveniles. Most adult females stayed on the upper surfaces of shaded leaves, whereas fewer remained on the upper surfaces of sunny leaves (Fig. 4a). In contrast, juveniles largely remained on the lower leaf surfaces regardless of sunny or shaded conditions, but preferred the upper surfaces on shaded leaves in April 2009 (Fig. 4b). Using data on sunny and shaded leaves combined over all trees in the same date, we evaluated the effects of date, sunlight (sunny and shaded), air temperature and RH on the upper/lower distribution of adult females and juveniles. As a
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The hatchability of P. citri eggs under UV was greater than 90%, and decreased more in the UV+ than in the UV condition in June and July (Fig. 3a). Hatchability under UV+ was marginally lower than that under UV (Wald test: d.f. = 1, v2 = 3.7239, Pr(>v2) = 0.05364) but did not differ among months (d.f. = 2, v2 = 2.7285, Pr(>v2) = 0.25557). The interaction between month and UV treatment was also significant (d.f. = 2, v2 = 6.4864, Pr(>v2) = 0.03904). The hatchability under UV+ was 76% in June and 97.1% in September. Hatchability under UV+ that was slightly higher than that under UV (91.7%) in September might be due to cloudy and rainy weather; only 2 days, 24 and 27 September, were sunny. The survival of juveniles was higher under UV (87–98%) than UV+ over the season (Wald test: d.f. = 1, v2 = 56.864, Pr(>v2) = 4.671 9 10 14) and survival was also influenced by month (d.f. = 2, v2 = 38.288, Pr(>v2) = 4.852 9 10 9; Fig. 3b). The interaction between month and UV treatment was also significant (d.f. = 2, v2 = 33.433, Pr(>v2) = 5.496 9 10 8). Only 15.5% of hatched individuals survived under UV+ in July, but 94% survived under UV+ in September (Fig. 3b). By the end of the July experiments, all survivors under UV+ conditions were larvae. This was also true for the June experiments, although a small number of individuals developed into quiescent larvae. In September, more than half of the survivors developed into quiescent larvae, and a small portion made it to the protonymph stage under both UV+ and UV conditions. However, even in September, development occurred more quickly under UV than under UV+ conditions.
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Figure 3. Seasonal fluctuations in hatchability of Panonychus citri eggs (a) and survival of hatched larvae (b) under nearly ambient UV radiation (UV+) and solar UV-attenuated (UV ) conditions. Dark gray, light gray and open regions of the bars in (b) indicate the ratios of larvae, quiescent larvae and protonymphs, respectively. A vertical line on the top of bars represents 95% confidential interval of true ratio.
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Figure 4. Seasonal fluctuations in the distribution of Panonychus citri adult females (n = 42–244 for a bar) (a), larvae and nymphs (n = 30– 141) (b) and eggs (n = 166–968) (c) between upper and lower leaf surfaces in sunny (open bars) and shaded leaves (gray bars). The figures in and above the bars show the total numbers of individuals or eggs on the upper and lower leaf surfaces. A vertical line on the top of each bar represents a 95% confidential interval of true ratio. Air temperature and relative humidity (RH) were calculated from five records measured above (sunny; open circles) and below (shaded; solid circles) the canopy of citrus trees in 30 min intervals from 11:30 to 13:30. Vertical lines above and below each plot indicate the standard deviation.
result, only sunlight had the negative significant effects for the distribution to upper leaf surfaces in both adult females (GLM; coefficient = 1.65297 ± 0.42103 [SE], Pr(>|z|) = 8.64 9 10 5) and juveniles (coefficient = 3.39161 ± 0.80340, Pr(>|z|) = 2.43 9 10 5). However, date, air temperature and RH showed no significant effects on the distribution (Table S2). Upper/lower distribution and hatchability of eggs. The number of eggs laid on the upper leaf surfaces was equivalent to or greater than those laid on the lower surfaces of the shaded leaves, whereas the ratio of upper-to-lower eggs was lower on sunny leaves throughout the observation period (Fig. 4c). Using data on sunny and shaded leaves combined over all trees in the same date, we evaluated the effects of date, sunlight (sunny and shaded), air temperature and RH on the upper/lower distribution of eggs. As a result, only sunlight had the negative significant effects for the distribution to upper leaf surfaces (GLM; coefficient = 0.80133 ± 0.12189 [SE], Pr(>| z|) = 4.9 9 10 11). In contrast, air temperature showed the posi-
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Table 1. Hatchability of Panonychus citri eggs laid on upper and lower leaf surfaces of sunny and shaded leaves collected from citrus trees in the field. Sunny
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14 27 23 12
237 51 164 255
64.1 80.4 89.0 76.7
170 99 237 260
75.3 88.9 92.0 66.2
280 96 187 169
87.9 88.5 95.2 77.5
76 73 140 166
93.4 89.0 93.6 75.3
tive significant effects for the distribution to upper leaf surfaces (coefficient = 0.80133 ± 0.12189 [SE], Pr(>|z|) = 4.9 9 10 11). This result likely reflected the fact that more eggs were on the upper leaf surfaces regardless on the sunny or shaded leaves in July and on the sunny leaves in June in comparison with October and April (Fig. 4c, Table S2). RH showed no significant effects on the distribution of eggs (Table S2). Hatchability tended to be lower for eggs on sunny rather than shaded leaves and lower on upper versus lower leaf surfaces except June (Table 1). Model selection supported a GLMM including date, sunlight, surface and interactions between date and sunlight, and between date and surface (AIC = 293.3; Table S3). Significant negative coefficients were detected in sunlight (sunny: coefficient = 1.398066 ± 0.2187, Pr(>|z|) = 1.62 9 10 10) and on the leaf upper surface (upper: coefficient = 0.520210 ± 0.2068, Pr(>|z|) = 0.0119), suggesting reduced hatchability on the upper surfaces of sunny leaves. Significant positive coefficients of interaction between date and sunlight in October 2008, April 2009 and June 2009 (October 9 sunny: coefficient = 1.274688 ± 0.4532, Pr(>|z|) = 0.00491; April 9 sunny: coefficient = 0.889088 ± 0.3799, Pr(>|z|) = 0.0193; June 9 sunny: coefficient = 1.138405 ± 0.2805, Pr(>|z|) = 4.93 9 10 5) suggested that the reduction in hatchability on sunny leaves was reduced in these months compared with July. On the other hand, unlike other observation dates, egg hatchability on lower leaf surfaces was lower than that on the upper leaf surfaces of sunny leaves in June 2009 (June 9 upper: coefficient = 0.915753 ± 0.2652, Pr(>|z|) = 5.54 9 10 4). However, this temporal difference could not be explained as a function of solar UVB radiation, and thus another factor must have influenced egg hatchability in June 2009. Evaluation of nutritional conditions on the upper and lower surfaces of citrus leaves Juvenile development. The percentage of successfully developed individuals was 100 and 80% on the upper and lower surfaces of sunny leaves, and 88 and 72% on the upper and lower surfaces of Table 2. Duration of mite development on upper and lower surfaces of sunny and shaded leaves. Female Leaf surface
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Egg production. The number of eggs laid on the upper leaf surfaces was more than twice that on the lower leaf surfaces during peak production (Fig. 5; n = 17 and 19 for each surface of sunny and shaded leaves, respectively). Model selection supported a GLMM including leaf surfaces and interaction between leaf surface and days after emergence (AIC = 672.1; Table S4), suggesting that the major factor affecting egg production was differing nutritional benefits arising from differences in the quality and quantity of nutrients or in the physical properties of the leaves (upper surface: coefficient = 1.149398 ± 0.11335, Pr(>|z|) F) = 0.09194) and males (d.f. = 3, F = 1.0659, Pr(>F) = 0.3778). Juvenile development was shorter on upper than lower leaf surfaces (Table 2; two-way ANOVA; females: F[1,46] = 24.180, Pr(>F) = 1.16 9 10 5, males: F[1,31] = 10.120, Pr(>F) = 0.00332). The development of males was also faster on sunny leaves than on shaded leaves (F[1,31] = 5.322, Pr(>F) =0.02790), whereas the difference was marginal in females (F[1,46] = 3.417, Pr(>F) = 0.071). The interaction between sunlight and surfaces was also significant in males (F[1,31] = 5.454, Pr(>F) = 0.02617), i.e. the difference in the nutritional effects on developmental rate between leaf surfaces was greater on the shaded leaves. However, no interaction was detected in females (F[1,46] = 1.066, Pr(>F) = 0.307).
No. of eggs per female
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0.176 0.26 0.17 0.31
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Days after the last molt Figure 5. Egg production of unmated Panonychus citri females on the upper and lower surfaces of sunny and shaded leaves collected from the field. Open circle: upper surface of sunny leaf; solid circle: lower surface of sunny leaf; open triangle: upper surface of shaded leaf and solid triangle: lower surface of shaded leaf.
Photochemistry and Photobiology, 2013, 89 at the time of peak egg production (upper surface 9 day: coefficient = 0.077595 ± 0.01074, Pr(>|z|) = 4.89 9 10 13).
DISCUSSION Plants accumulate phenolic compounds (e.g. flavonoids) to reduce the penetration of solar UVB radiation through the epidermis and to protect sensitive targets in the mesophyll cells (16), thus providing a habitat safe from solar UVB radiation for spider mites on the lower leaf surface (13). For instance, during a 2year field study in northern Japan, Osakabe et al. (9) found that most T. urticae settled on the lower leaf surfaces of apple trees throughout the experimental period, while less than 1.4% of adult females occurred on the upper leaf surfaces. Such biased distribution may be a consequence of the adaptation of T. urticae adult females to avoid solar ultraviolet radiation by sheltering on the lower leaf surfaces (5). According to Foott (6), 95.3 and 100% of T. urticae eggs were found on the lower leaf surfaces of peach and apple mature trees in an orchard. Sakai and Osakabe (5) reported that fecundity of T. urticae increased 16–21% on adaxial than abaxial leaf surfaces of kidney bean. However, Sakai et al. (17) found that the fecundity advantage on the adaxial leaf surfaces (9–21% increased) was largely canceled out by the effects of gravity direction; females laid 9–21% more eggs when they were facing downward than facing upward. Sakai and Osakabe (5) also reported that T. urticae laid 5–11% more eggs on leaves grown up in shade than sunny place. In contrast, Jones and Parrella (7) reported that, in P. citri, 44.1% of all active stages occurred on the upper leaf surfaces of lemons in California from spring to summer. They also pointed out that the upper surface was populated by 59.6% of the adult females, whereas 77.2% of the immature stages were found on the lower leaf surfaces (7). Such spatial distribution is consistent with our results on the distribution of adult females and juveniles. However, the ultimate and proximate factors that influence the peculiar distribution of Panonychus mites remain largely unknown. Sakai et al. (3) showed that the deleterious effects of solar UVB radiation on the eggs of T. urticae became weaker in autumn when the intensity of solar UVB radiation decreased rapidly. We found a larger portion of adult females of P. citri on the upper surfaces of sunny leaves in late October (mid autumn) than we did on other dates. Moreover, the portion of adult females on the upper leaf surfaces was larger on shaded leaves than sunny leaves, suggesting that they preferred upper leaf surfaces, but tended to avoid stronger solar UVB radiation. This means that P. citri adult females balance the intrinsically, mutually exclusive matters, preference for the upper leaf surfaces and avoidance of UVB radiation including other harsh environmental conditions such as high temperature and desiccation. Egg hatchability also seemed to be affected by incoming solar UVB radiation, due to the reduction in the hatchability on the upper surfaces of sunny leaves in summer. However, the hatchability of P. citri eggs on sunny leaves during spring and summer (59.5–89.0%) has been found to be much higher than that of T. urticae eggs (10–55%) reported by Sakai et al. (3). Our dose– response experiment also revealed the UVB resistance of P. citri eggs compared with those of T. urticae. We also found that, with regard to both egg production and juvenile development, upper leaf surfaces were nutritionally advantageous to P. citri reproduc-
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tion. Nevertheless, the proportions of eggs on upper leaf surfaces were also larger on shaded leaves than sunny leaves, suggesting that females tended to avoid laying eggs on the sunny part of leaves. The semioutdoor experiments showed that, even many eggs hatched over the three experiments, hatched juveniles did not successfully develop under solar UVB radiation (UV+) in midsummer. In contrast to adult females, many individuals in immature stages of development remained on the lower leaf surfaces in the field, a finding shared by Jones and Parrella (7). Although juvenile development was shorter on the upper than on the lower leaf surfaces, the vulnerability of juveniles to UVB radiation causes a distribution bias toward lower leaf surfaces. Nutritional evaluation revealed that in terms of juvenile development and egg production, whether on sunny or shaded leaves, P. citri can reap the fitness benefit of being on upper leaf surfaces. Therefore, inhabiting the upper surfaces of shaded leaves, mainly being present on the north facing side, is most advantageous for this mite. The vulnerability of juveniles may be important for both the intraspecific and interspecific interactions with competitors and predators. Juvenile Panonychus mites can become entangled in the complicated webs of Tetranychus mites and frequently die (8). Nevertheless, larvae of the European red mite, Panonychus ulmi (Koch), that hatched on upper leaf surfaces immediately move to lower leaf surfaces (6). This may be a major mechanism of coexistence by which T. urticae constrains the population development of P. ulmi in the field (9); the solar UVB radiation could enhance this effect. On the other hand, phytoseiid mites, primary predators of spider mites, are also vulnerable to UVB radiation (13,18). They avoid both UV and visible light, preferring to remain in shaded areas (18,19). In addition, phytoseiid mites prefer to stay near and/or inside three-dimensional microstructures such as veins, leaf domatia and tomenta (19,21), which are abundant on lower leaf surfaces. Therefore, staying on lower leaf surfaces may increase the predation risk for Panonychus juveniles that have no protection mechanisms, such as the complicated webs of Tetranychus mites (22–24). Vulnerability to UVB radiation is potentially the ultimate factor causing Panonychus juveniles to stay on the lower leaf surfaces. However, such a response may not be advantageous for exploiting resource availability and reducing competition or risks of predation. Consequently, for Panonychus juveniles, the UVB threat likely supersedes those advantages and risks. Conversely, for adult females and eggs of Panonychus species, acquisition of enemy- and competitor-free space may be the prime reason for remaining on upper leaf surfaces. There is growing evidence that solar UVB radiation impacts terrestrial plant-dwelling arthropods (25). The intensity of insect herbivory often increases if solar UVB radiation is attenuated (26,27). Although it is well documented that the effects of UV-B on insect herbivory are indirect and mediated by changes in host-plant chemistry (17,28–30), UVB may also directly affect insect behavior (31,32) and survivorship (33,34). However, the ecological/evolutional functions of solar UVB radiation in the plant-dwelling arthropod communities have been largely unknown. Further studies on the effect of solar UVB radiation on interspecific interactions within the communities are required to further elucidate these mechanisms.
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A proximate factor that allows Panonychus mites to exploit upper leaf surfaces may be the acquisition of tolerance to UVB radiation. However, the mechanism of such tolerance remains unknown. Suzuki et al. (2) reported that diapausing adult females of T. urticae became tolerant to UVB radiation compared with summer (nondiapausing) females. The diapausing females cumulated keto-carotenoids including astaxanthin (described as a saponified product, astacene, in the literature; 35). In human dermal fibroblasts, astaxanthin reduces reactive oxygen species and abrogates apoptotic responses to UV radiation more efficiently than either b-carotene or lutein (36). Metcalf and Newell (37) found significant pigments expected to be various esters of astaxanthin or protein complexes in P. citri females by paper chromatography, suggesting constitutive and substantial synthesis of astaxanthin. The relationship between carotenoid composition and UVB damage in leaf mite species may be worthy of future research attention. Acknowledgements—We thank Prof. Y. Saito, Hokkaido University, for his valuable advice and participants in Panasonic Co. for supplying the UVB lamp used for the dose–response analyses. This study was supported by the 21st Century COE Program of Innovative Food and Environmental Studies Pioneered by Entomomimetic Sciences at Kyoto University and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (22380036).
SUPPORTING INFORMATION Additional Supporting Information may be found in the online version of this article: Table S1. Generalized linear model analysis of the effects of UVB cumulative dose on egg hatchability of Panonychus citri laid on citrus and kidney bean leaves and that of Tetranychus urticae on kidney bean (hatchability mite type + UVB dose + mite type 9 UVB dose, logit-link, binomial, true = hatch). Table S2. Generalized linear model analyses of the effects of date, sunlight, air temperature and relative humidity on the distribution between upper and lower leaf surfaces of adult females, juveniles and eggs on citrus trees in the field (distribution date + sunlight + air temperature + RH). Distribution data on sunny and shaded leaves were combined over all trees in the same date. Table S3. GLMM analysis of factors affecting hatching of eggs collected from citrus trees in the field (hatchability date + sunlight + surface + date 9 sunlight + date 9 surface, logit-link, binomial, cluster = tree, true = hatch), AIC = 293.3. Table S4. GLMM analysis of factors affecting egg production on field-collected leaves (egg surface + day 9 surface, logit-link, Poisson, cluster = leaf, AIC = 672.1; AIC in the full model = 677.1). Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article.
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