Observations on Excessive Abundance of the Midge Chironomus plumosus at Lake Pepin

Abstract

Although midges (Chironomidae) are among the commonest of aquatic insects, instances of their occurrence in such numbers as to be a pest are unusual. Burrill ('13) reported that the giant midge (Chironomus plurnosus) had been abundant enough at Lake Winnebago, Wisconsin, in August i910 to become a serious pest. For the past several years swarms of midges of this same species have occurred at Lake Pepin-an expansion of the Mississippi River between Minnesota and Wisconsin-in such numbers as to be a serious annoyance to people living near the lake. These midges are nearly I/2 inch in length. An idea of their abundance can be gained from the fact that we counted 50 on one grape leaf, and caught 203 in a pail 8 1/2 inches in diameter placed against the wall of a tent. In the spring of I928 complaints in regard to these insects were made to several state departments and were referred to the Division of Entomology and Economic Zoology of the University of Minnesota. Because of these complaints, because the situation seemed an unusual and interesting one, and because the authors were already interested in the problem of factors affecting the abundance of fish food in lakes, a study was begun of the midge problem at Lake Pepin. It was not possible to begin the study until July i. In the meantime a number of facts relative to the problem were assembled. The head of the lake is about 55 miles below St. Paul. The lake is about 20 miles long and is on the average slightly less than 2 miles wide with an area of 38 I/2 square miles. The lake is for the most part hemmed in by hills. The shores are sand, gravel, or rock. Several sand and gravel points project into the lake from the Minnesota side. The bottom is of mud, and throughout most of the lake is at a fairly uniform depth of about 30 feet. Lake Pepin is formed by the natural damming of the Mississippi River by the delta of the Chippewa River. The Mississippi receives the sewage discharge of Minneapolis and St. Paul, and is seriously polluted for some distance below these cities. Its condition is greatly improved, however, before it enters the lake. The river, as it enters and as it leaves the lake, has a rather strong current. In the lake itself the drop is less than one quarter inch per mile, and no current is noticeable. The largest town on the lake is Lake City, Minnesota, with a