Completing the life cycle: detachment of an aquatic parasite (Arrenurus cuspidator, Hydrachnellae) from an aerial host (Coenagrion puella, Odonata)

Water mites are are very important parasites of aerial stages of aquatic insects. Their larvae parasitize semiaquatic hosts and must detach while the host is in a suitable habitat for reproduction of parasite and host. Therefore, water mites should respond to stimuli indicating this situation. Different stimuli were tested experimentally in the host – parasite system Coenagrion puella – Arrenurus cuspidator in outdoor cages; this method provides exact data on the initial intensity of mite larvae per host. It was found that mites detach during tandem oviposition by the host. The detachment rate does not correlate with the host's sex or with the intensity of mite larvae per host. Ectoparasitic water mites are apparently influenced by the host's condition because mites did not detach from dead hosts even in water. Proximity to water also seems to have an impact: mites exposed at a height of 10 mm above water detached, whereas mites exposed at 25 mm or higher did not. We suggest that detachment of mite larvae is triggered by a group of stimuli associated with the egg-deposition behaviour of the host.