Aggregation of Lady Beetles on the Shores of Lakes (Coleoptera: Coccinellidae)

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Aggregations of lady beetles, predominately Hippodamia convergens and H . tredecimpunctata, were commonly observed on the shores of lakes in the Upper Midwest during the autumn and spring. The beetles remain on the shore for only a short time, usually dispersing within 2-3 weeks. Lady bettles from autumn shore aggregations and overwintering aggregations are characterized by the presence of large amounts of fat, reproductive inactivitiy, empty digestive tracts, a skew in the sex ratio favoring females and the behavioral tendency to form aggregations. Hippodamia convergens, the most commonly observed coccinellid in shore aggregations, undergoes long migratory flights to and from overwintering sites in California. I t is suggested that similar migratory activity occurs in the Upper Midwest and that during these flights the beetles may be blown into the water and washed ashore, thus forming the aggregations. However, in contrast with the overwintering aggregations in California, the shore aggregations reported in this study appear to be temporary shoreline collections and not the overwintering site. INTRODUCTION The aggregation of lady beetles on the shores of lakes in the Upper Midwest is a phenomenon commonly known to local residents, although it has received little attention in the scientific literature. The beetles in these shore asgregations may be so numerous that 5000-10,000 living individuals may be collected in 1 hr. As early as 1850, LeConte reported collections of lady beetles on the shore of Lake Superior. Additional observations of coccinellids on lake shores were made by Wheeler ( 1887), Snow (1902), Needham (1900, 1904, 1917) and Park (1930) on Lake Michigan, Schwarz (1890) on Lake Superior, and Smith (1966) on Lake Ontario. In 1975, Simpson and Welborn reported a mixed aggregation of lady beetles and alfalfa weevils [Hypera postica (Gyllenhal)] along a reseivoir in Colorado. Savoiskaya (1965) referred to masses of coccinellids on the shores of lakes, including Lake Ala-kul and Lake Balkhash in Kazakhstan, USSR. Accumulations of coccinellids have also been reported on ocean shores in England (Marriner, 1939; Riggall, 1953)) the United States (Hagen, 1962) and Egypt (Oliver, 1943). Oliver describes a drift line of dead Coccinella 11-punctata L. at least 13 miles long with 70,000 beetles per ft run. These reports are restricted to species lists and general notes based upon limited observations at single shore sites. In order to determine the significance shore aggregations have in the biology of lady beetles, additional information is required. The purpose of this study is twofold: (1 ) to provide a detailed description of these shore aggregations, and ( 2 ) to determine the relationship of these aggregations to the life history of coccinellids. METHODSAND MATERIALS Periodically from April through November in the years 1975-1978, the shores of lakes were examined for the presence of lady beetle aggregations. A shore site was considered to have an aggregation if it was possible to collect 50 or more beetles in a 15-20 min search, while sites without aggregations generally yielded fewer than five individuals during the search period. However, on most occasions when an aggregation was present several hundred and often more than 1000 beetles could Present address: Department of Biology, University of Houston, Central Campus, Houston, Texas 77004.