The Biology of Chironomus plumosus (Diptera: Chironomidae) in Lake Winnebago, Wisconsin1

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Abstract
Eggs of Chironomus plumosus (L.) are laid on the lake surface in masses averaging 1676 eggs. These masses absorb water, swell, and sink to the lake bottom, where the eggs hatch in 3–14 days. Larval stages range from 1.4-mm, colorless, first instars to 30-mm, dark red, fourth instars. The newly hatched first instars are free-swimming and positively phototrophic. All instars, except perhaps the first, construct U-shaped tubes in the bottom mud and, at temperatures above 5°C, feed indiscriminately by “filter feeding.” There is evidence that larvae may occasionally become limnetic, but normally they remain in their tubes and eventually pupate there. Female larvae must weigh 60 mg or more to pupate, but male larvae will pupate when they weigh 48–56 mg. Emergence may occur at any hour, day or night, but is most prevalent just before sunset. Newly emerged flies fly weakly with the wind, eventually accumulating on the lee shore. Swarms of males form about 1 hr before sunrise and disperse about 1 hr after sunset in response to light. The swarms, typically containing several thousand flies, orient over objects that contrast with the background. Females fly into the swarms for mating, which occurs most commonly in the early morning. Mated females, after a preoviposition period of 1½–5 days, fly out over the lake to oviposit, extruding their egg masses onto their hind legs while in flight; oviposition occurs commonly at all hours of day or night. In the laboratory, adults lived 3–11 days, longest at cool temperatures and 100% RII. The larvae are eaten by many fish and by 1 species of leech, are parasitized by a microsporidian, and are occasionally killed by unknown diseases. There are normally 2 generations a year in Lake Winnebago. Overwintering fourth-instar larvae pupate and emerge during mid-May; their progeny emerge from mid-July to early September. Rate of development and time of emergence are dependent on mud temperatures. If unknown stimuli necessary for pupation are absent, a generation may be delayed or entirely missed.

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