Abstract
Two diseases, caused by a microsporidan Pleistophora oncoperae Milner and Beaton and pox virus, were found to be common and widespread in populations of Oncopera alboguttata. In contrast, disease was rare in O. rufobrunnea populations and only P. oncoperae was found. Studies of field populations of O. alboguttata between 1971 and 1974 indicated that disease was responsible for the dramatic decrease in population density which occurred during 1973.