- University of New Brunswick
Laboratory-reared first-instar juvenile Gerris comatus Drake & Hottes, Gerris alacris Hussey, and Gerris buenoi Kirkaldy were exposed to larval Limnochares aquatica (L.). Development and mortality of parasitized juvenile water striders were monitored and compared with those of unparasitized insects. Duration of infestation was from 6 to 13 days during which surviving hosts moulted several times, with engorging larvae transferring to the teneral host during the moulting process. Most mites dropped off the host during the host's second instar, and all mites were off the host by the host's fourth instar. Parasitism by larval mites significantly increased mortality, duration of instars, and variance in age at first moult for parasitized water striders. Mortality and duration of first instars were directly correlated with number of mites per host. Parasite-induced mortality was not evident until 2 days after attachment of mites, and almost all mortality occurred within the host's first instar. Gerris comatus and G. alacris apparently did not differ in response to parasitism; however, G. buenoi showed greater mortality than the other gerrid species when carrying comparable numbers of mites. The effects of parasitism on juvenile water striders within natural populations are predicted to be severe, and would reduce recruitment while also increasing variation in age structure.
This research was conducted at the Queen's University Biological Station, located near Chaffeys Locks (ca. 80 km northeast of Kingston), Ontario. Adult water striders were collected from Deadlock Bay of Lake Opinicon in late May and maintained in the laboratory. Gerrids were fed various midges (Diptera; Chironomidae) and caddisflies (Trichoptera) that were collected in the evening at an ultraviolet light located by the lake. Insects for food were kept frozen until needed. Water striders in the laboratory readily laid eggs on small squares of brown paper towel floated on the surface of rearing containers. These eggs were then isolated in individual 200 mL plastic cups (82 mm high, 74 mm diameter at top, 55 mm diameter at base) half-filled with water. Newly hatched first nymphal water striders were transferred to fresh cups daily.