Host plant acceptance by the phytophagous mite Tetranychus kanzawai Kishida is affected by the availability of a refuge on the leaf surface

Spider mites avoid caterpillar traces to prevent intraguild predation

The phytophagous spider mites Tetranychus kanzawai and Tetranychus urticae can be as small as < 0.5 mm; thus, they are often incidentally consumed along with food plant leaves by voracious lepidopteran larvae (hereafter, 'caterpillars'). Therefore, the ability to avoid such intraguild predation should confer a selective advantage to mites. We experimentally demonstrated that adult females of both mite species avoided settling on food plant leaves with traces of all tested caterpillar species (Bombyx mori, Papilio xuthus, Spodoptera litura and Theretra oldenlandiae). We examined additional interactions using B. mori and T. kanzawai and found that B. mori trace avoidance by T. kanzawai lasted for more than 48 h. Tetranychus kanzawai also avoided B. mori traces on plant stems, along which mites access leaves. Moreover, T. kanzawai avoided acetone extracts of B. mori traces applied to filter paper, indicating that chemical substances of caterpillar traces are responsible for the avoidance. This study is the first demonstration of a repellent effect of herbivore trace chemicals on heterospecific herbivores. Although spider mites have developed resistance against many synthetic pesticides, these results predict that natural compounds simulating caterpillar traces may repel spider mites from agricultural crops.

Avoidance of ant chemical traces by spider mites and its interpretation

Spider mites become easy prey for ants when they leave their protective webs; therefore, the ability to avoid traces of ongoing ant activity should confer a selective advantage to mites. We examined avoidance of ant traces by the spider mites Tetranychus kanzawai and Tetranychus urticae. Both mite species avoided host plant leaves with active traces of Pristomyrmex punctatus or Formica japonica ants. Pristomyrmex punctatus trace avoidance by T. kanzawai lasted for more than 1 h, but not more than 3 h. Tetranychus kanzawai also avoided P. punctatus traces on plant stems, along which the mites access leaves. Moreover, T. kanzawai avoided hexane extracts of P. punctatus or F. japonica applied to a filter paper pathway. This study represents the first demonstration of a repellent effect of ant chemical traces on spider mites. Considering the substantial abundance and global distribution of ants in nature, such repellent effects may help to answer the long-standing question of why only a small fraction of available plant resources is used by herbivores. Although spider mites have developed resistance against many synthetic pesticides, natural compounds that simulate ant chemical traces may repel spider mites from agricultural crops.

Ant-mediated indirect negative effects of aphids on spider mites living on the same plant

Some aphid species are known to have mutualistic relationships with tending ants; that is, the aphids supply the ants with honeydew and are protected by the ants. Although spider mites and honeydew-producing aphids often live on the same host plant, it has not previously been determined whether the ants tending these aphids affect spider mite survival. Using replicated microcosms, each containing an artificial ant nest, we compared experimentally the survival of two-spotted spider mites on kidney bean plants with and without cowpea aphids. Our results showed significantly fewer spider mites on plants with aphids, indicating that spider mites were preyed upon by ants tending aphids. On the other hand, there was no detectable plant-mediated indirect effect of aphids on mite performance in the microcosms. Therefore, we conclude that aphids indirectly reduced the survival of spider mites living on the same host plant via their tending ants. Nonetheless, spider mites did not avoid settling on plant leaves infested with aphids.

Atmospheric Humidity Influences Oviposition Rate of Tetranychus urticae (Acari: Tetranychidae) Through Morphological Responses of Host Cucumis sativus Leaves

We investigated the effects of morphology of host cucumber, Cucumis sativus L., leaves acclimatized to different atmospheric humidity levels on oviposition by adult females of the twospotted spider mite, Tetranychus urticae Koch. Cucumber seedlings were grown at a vapor pressure deficit (VPD) of 0.4, 1.9, or 3.0 kPa at 28°C (90%, 50%, or 20% relative humidity, respectively) in growth chambers until the second true leaves had expanded. Adult females of T. urticae were released on the adaxial surfaces of leaf squares cut from first and second true leaves in each treatment group, and held in the same humidity condition. Eggs were counted 2 d after release. The lower acclimatization humidity (higher VPD) increased trichome (leaf hair) density of the host leaves and oviposition rate, but the relationship between the trichome and oviposition differed between leaf positions. The leaf mass per area (LMA) was greater in first true leaves than in second true leaves, but was not influenced by VPD. A linear regression model with oviposition rate as the dependent variable and trichome density and LMA as independent variables showed that both variables influenced the oviposition rate approximately equally. We conclude that oviposition was accelerated under low humidity (high VPD) conditions indirectly probably through an increase in the trichome density of host leaves.

Suspension of Egg Hatching Caused by High Humidity and Submergence in Spider Mites

We tested the effects of high humidity and submergence on egg hatching of spider mites. In both the high humidity and submergence treatments, many Tetranychus and Panonychus eggs did not hatch until after the hatching peak of the lower humidity or unsubmerged controls. However, after humidity decreased or water was drained, many eggs hatched within 1-3 h. This was observed regardless of when high humidity or submergence treatments were implemented: either immediately after oviposition or immediately before hatching was due. Normal eyespot formation was observed in most eggs in the high humidity and submergence treatments, which indicates that spider mite embryos develop even when eggs are underwater. Therefore, delays in hatching are not caused by delayed embryonic development. A delay in hatching was always observed in Panonychus citri (McGregor) but was more variable in Tetranychus urticae Koch and Tetranychus kanzawai Kishida. The high humidity and submergence treatments affected but did not suppress larval development in these species. In contrast, many Oligonychus eggs died following the high humidity treatments. In Tetranychus and Panonychus spider mites, suspension of egg hatching may mitigate the adverse effects of rainfall.

Do plant mites commonly prefer the underside of leaves?

The adaxial (upper) and abaxial (lower) surfaces of a plant leaf provide heterogeneous habitats for small arthropods with different environmental conditions, such as light, humidity, and surface morphology. As for plant mites, some agricultural pest species and their natural enemies have been observed to favor the abaxial leaf surface, which is considered an adaptation to avoid rain or solar ultraviolet radiation. However, whether such a preference for the leaf underside is a common behavioral trait in mites on wild vegetation remains unknown. The authors conducted a 2-year survey on the foliar mite assemblage found on Viburnum erosum var. punctatum, a deciduous shrub on which several mite taxa occur throughout the seasons, and 14 sympatric tree or shrub species in secondary broadleaf-forest sites in Kyoto, west-central Japan. We compared adaxial-abaxial surface distributions of mites among mite taxa, seasons, and morphology of host leaves (presence/absence of hairs and domatia). On V. erosum var. punctatum, seven of 11 distinguished mite taxa were significantly distributed in favor of abaxial leaf surfaces and the trend was seasonally stable, except for Eriophyoidea. Mite assemblages on 15 plant species were significantly biased towards the abaxial leaf surfaces, regardless of surface morphology. Our data suggest that many mite taxa commonly prefer to stay on abaxial leaf surfaces in wild vegetation. Oribatida displayed a relatively neutral distribution, and in Tenuipalpidae, the ratio of eggs collected from the adaxial versus the abaxial side was significantly higher than the ratio of the motile individuals, implying that some mite taxa exploit adaxial leaf surfaces as habitat.