Releases of Spalangia nigroaenea and Muscidifurax zaraptor (Hymenoptera: Pteromalidae) increase rates of parasitism and total mortality of stable fly and house fly (Diptera: Muscidae) pupae in Illinois cattle feedlots

Weekly releases of Spalangia nigroaenea Curtis and Muscidifurax zaraptor Kogan & Legner from May through August of 1991-1993 at small, owner-operated cattle feedlots in Illinois provided weekly emergence of 100-300 parasitoids of each species per feedlot animal. In assessments based on ny and parasitoid emergence from >47,000 stable ny and house ny puparia collected during the 3-yr period, total stable ny mortality was greater in lots where releases were made (60.7%) than in paired, untreated control lots (51.7%) (P = 0.04; paired t-test); parasitism of stable fly pupae by S. nigroaenea averaged 11.6% where releases were made and 6.4% in paired control lots (P = 0.0016). In lots where releases were made, total mortality of house fly pupae was greater (68.7 versus 56.1%; P = 0.0001); unexplained mortality was greater (55.5 versus 46.1%; P = 0.0018); and parasitism Ly Muscidifurax spp. was greater (2.4 versus 1.4%; P = 0.07) than in paired control lots. Parasitism, unexplained mortality, and total mortality of both ny species varied significantly from 1991 to 1992 in lots that received the same treatment each year, presumably due primarily to weather. Over the 3-yr period, releasing these species, particularly S. nigroaenea, significantly reduced production of stable ny and house ny adults in cattle feedlots. The potential value of such reductions is likely to vary as a result of feedlot conditions and weather.