Population dynamics of common filth flies (Diptera: Muscidae) on dairy farms: Importance of manure removal and mass release of parasitoids

In the dairy industry confined cattle produce large amount of manure in a short time, thus providing optimal conditions for the breeding of the common filth fly pests (Diptera: Muscidae), mainly houseflies Musca domestica Linnaeus and stable flies Stomoxys calcitrans (Linnaeus). Filth-fly control relies largely on manure management practices, mass trapping, use of insecticides, and augmentative release of chalcidoid pupal parasitoids. The effectiveness of the last measure was variable and mostly limited in previous studies performed over limited temporal and spatial scales. The objectives of the current study were to follow the population dynamics of these two major pests on dairy farms, and to test the effect of natural parasitism and mass-released parasitoids on fly populations over a larger scale. In two consecutive years, we monitored fly numbers on ten dairy farms in south-western Israel and released Muscidifurax raptor Girault & Sanders (Hymenoptera: Pteromalidae) and Spalangia cameroni Perkins (Hymenoptera: Spalangiidae) pa-rasitoids (ca. 1:1 ratio) on five of the farms in the second year. In both years and on all the farms, housefly numbers were ten-fold higher than those of stable flies. Although the populations sizes of both fly species differed significantly among farms, they showed similar seasonal dynamics with significant differences among sampling dates. Housefly populations started to peak in April in both years, with a decline in June and mid-July in the first and second years, respectively. Stable fly numbers decreased from April to July, probably reflecting an earlier population peak that preceded the monitoring period in this study. These temporal population dynamics of the flies may reflect changes in manure moisture levels. The clearing frequency of the manure pits on the study farms had an inconsistent effect on fly numbers. The release of parasitoid wasps did not affect adult housefly abundance significantly, possibly because the wasps were released too late in the season. The study provides useful information on temporal occurrence of two fly species on dairy farms, which is essential for the employment of site-specific preventive control measures to suppress fly populations.