Article Title

Winter Drift, Sex Ratios, and Size Distribution of Giant Water Bugs, Lethocerus Americanus, (Heteroptera: Belostomatidae) in a Lake Superior Tributary

Document Type

Peer-Review Article


More than 2,100 adult, drifting giant water bugs (Lethocerus americanus) were collected from the upstream face of the uppermost raceway screens at a run-of-river trout hatchery on a Wisconsin tributary to Lake Superior during December 1991 May 1997. This drift was greatest from mid-October through March at water lower than 4° C, and was rare during summer. Individuals that were collected from the screens, marked, and released above the hatchery were rarely recovered, hence the location of the source population and drift distances are speculative. The sex ratio overall was close to 1:1, with males dominating numerically in 1994 and 1997, and females in 1993 and 1996. Females averaged 56.5 mm in total length (range 51-63 mm), and males 50.7 mm (range 47-56 mm). There was significant interannual variation in the mean lengths of males, but not of females. No differences in mean lengths or sex ratios between early- and late-winter collections were evident for any year. We suggest that the drift of these insects is best explained as a dispersal mechanism following the final molt and that winter is selected for dispersal to mini-mize predation risks.

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