The parasitic nature of water mites on aquatic heteropterans was investigated at a paddy field in Sagamihara City in August 2016. During the survey, three heteropteran species, Appasus japonicus, Aquarius paludum, and Ranatra chinensis, were collected. Among these species, water mites were found only on A. japonicus. The water mites were all in a post-larval resting stage and identified as the Hydrachna sp. The mite prevalence or intensity has no relation to the developmental stage or sex of A. japonicus. However, the host individuals infested with Hydrachna sp. were significantly larger than non-infested ones, and the mite intensity on each A. japonicus was apt to correlate with the host body size. Consequently, the Hydrachna sp. is opportunistically parasitic on larger A. japonicus individuals, regardless of the developmental stage or sex of the host. Though the Hydrachna sp. did not exhibit a preference for particular body parts of the juvenile A. japonicus, the mites preferred the forewing to the mesothorax, metathorax, foreleg, midleg, and hindleg of the adult one. The mites probably select large, inactive body parts rather than small, active parts for attachment on the adult host.

Tetranychus phaselus (Acari: Tetranychidae) lives chiefly on the mulberry weed Fatoua villosa (Moraceae), and reaches high densities in the autumn. To test the prediction that T. phaselus is adapted to F. villosa, we investigated the suitability of six host plants available in this species’ habitat. We supplementarily investigated the association between the density of T. phaselus and the density of an important predatory mite, Neoseiulus womersleyi (Acari: Phytoseiidae) on F. villosa leaves to infer the predation pressure. To know the characteristics of host plant use in T. phaselus, these characteristics were compared with the population of the common polyphagous species T. kanzawai on ramie (Boehmeria nivea, Urticaceae) leave. With respect to host-plant suitability, T. phaselus had one of the highest egg production and the survival rate on F. villosa among all six host plants examined, which partly supports the prediction that T. phaselus is adapted to F. villosa. On the other hand, T. phaselus egg production and development time on F. villosa was similar to that of T. kanzawai reared on F. villosa. Nevertheless, T. phaselus was found only on F. villosa in the field, and the density per leaf area of T. phaselus on F. villosa was higher than that of T. kanzawai on ramie leaves. Considering that the density of N. womersleyi was not significantly associated with the density of T. phaselus, the high density of T. phaselus could be partly attributed to the scarcity of N. womersleyi on F. villosa leaves.