Parasitism of water mites (Acari: Hydrachnidiae) on Appasus japonicus in a paddy field in Sagamihara City, Kanagawa Prefecture, Japan

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.