The species composition and seasonal occurrence of spider mites and their natural enemies were compared in three Japanese pear orchards with different control pressures, from 1996 to 1999. The orchards were an agrochemical-free orchard, a pesticide-free orchard, and a conventionally controlled orchard. The differences in the dominant species of spider mites and phytoseiid mites among these three orchards were observed. Among the spider mites, Amphitetranychus viennensis was dominant throughout the four years in the agrochemical-free orchard whereas Panonychus citri was dominant in the pesticide-free orchard. In the conventionally controlled orchard, the dominant spider mite species was Tetranychus urticae, except for in the year 1998. The dominant phytoseiid mites were Amblyseius orientalis and Amblyseius sojaensis in the agrochemical-free orchard, Typhlodromus vulgaris in the pesticide-free orchard, and Amblyseius californicus in the conventionally controlled orchard. The predacious insects of spider mites which were abundant in the three orchards were Stethorus japonicus, Oligota spp., Scolothrips takahashii, and Feltiella sp. Unlike the spider mites and phytoseiid mites, the dominant species of predacious insect varied greatly from year to year within each orchard. This suggests that the species composition of the predatory insects may be less affected by agrochemicals than that of spider mites or phytoseiid mites. Differences among the peak densities of spider mites among the orchards seemed to be due to differences in the dominant spider mite species rather than to differences in control pressure with chemicals. The peak density in the orchards where P. citri was dominant was higher than in those where A. viennensis or T. urticae was dominant, nevertheless the predacious insects occurred in all orchards. This suggests that the predacious insects would be less effective in suppressing P. citri than in suppressing T. urticae and A. viennensis.

A population survey of phytoseiid mites and spider mites was conducted on peach leaves and wild plants in Japanese peach orchards having different pesticide practices. The phytoseiid mite species composition on peach leaves and wild plants, as estimated using quantitative sequencing, changed during the survey period. Moreover, it varied among study sites. The phytoseiid mite species compositions were similar between peach leaves and some wild plants, such as Veronica persica, Paederia foetida, Persicaria longiseta, and Oxalis corniculata with larger quantities of phytoseiid mites, especially after mid-summer. A PCR-based method to detect the ribosomal ITS sequences of Tetranychus kanzawai and Panonychus mori from phytoseiid mites was developed. Results showed that Euseius sojaensis (specialized pollen feeder/generalist predator) uses both spider mites as prey in the field.

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.

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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.