Biology of a large mayfly, Hexagenia bilineata (Say), of the upper Mississippi river

Fremling, Calvin
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Hexagenia bilineata (Say) and Hexagenia limbata ( Serville) create nuisance problems by their sheer numbers in many cities along the Mississippi River. They also constitute a navigation hazard and may cause allergies.

H. bilineata is generally more abundant than H. limbata on the Upper Mississippi River, but H. limbata becomes increasingly abundant northward. The absence of intermediate-sized nymphs in late summer indicates that H. bilineata completes a generation in 1 year in the Keokuk area. H. bilineata nymphs live in burrows in the river bottom and are most abundant in impounded areas where there is little current and where the river bottom is silty. Keokuk is less bothered by Hexagenia spp. than are other river cities because it lies only partially along a silted, impounded area. Other cities which lie entirely along silted areas, such as Fort Madison, Iowa, receive greater quantities of adult Hexagenia spp. Detailed observations were made of H. bilineata mating flights and the emergence of the subimago from its nymphal exuviae. An analysis of 150 observations of H. bilineata emergences on the Upper Mississippi River indicated that waves of emergence occur at intervals of about 6-11 days, with the maximum emergence occurring in mid-July. A single wave of emergence usually occurred almost simultaneously throughout the river segment. During the maximum wave, 30 observations were made during a 3-night period over a 440-mile expanse of river.

H. bilineata nymphs, subimagoes and imagoes were heavily parasitized by metacercariae which were thought to be those of Megalogonia ictaluri and Crepidostomum cooperi.

Hexagenia nymphs, which occur predominantly in shallow, slow-water areas, may be vulnerable to wettable powder or granular insecticides. The nymphs are a prime food of fish, however, and such control measures may adversely affect fish populations. Modifications in lighting may alleviate the mayfly problem to some extent.