Dr. Murai Éva szerk.: Parasitologia Hungarica 23. (Budapest, 1990)

(SACCA 1964; SICK 1971; STEIN et al. 1977; BAUERMEISTER and SCHUMANN 1980; ADAMS 1984) including Hungary (MIHÁLYI 1975). The imagoes are a little smaller than the adults of M. domestica and are bronze-black with red palpi. The larvae occupy the same milieu in the manure as the house fly larvae and they can actively prey on other fly larvae. Despite the fact that Hydrotaea aenescens has already been used as a biocontrol agent against the house fly on pig farms (RIBBECK et al. 1987) and in poultry houses (RUSZLER 1989), we still lack sufficient detailed data on important aspects of the prédation effect of their larvae. In laboratory trials some scientists have observed that the larvae of Hydrotaea aenescens destroy the immature stages of Musca domestica; however, they used artificial medium instead of animal manure or their trials were not overall (HOGSETTE 1979; MÜLLER 1982; GEDEN et al. 1988; FARKAS and PAPP 1990). Our laboratory work presents an attempt to determine how the numbers and ages of the immature stages of both fly species influence the prédation rate using pig manure. MATERIALS AND METHODS The colony of Hydrotaea aenescens was derived from flies collected on a poultry farm outside Budapest. The Musca domestica strain has been maintained in our laboratory for a few years. The imagoes were fed on sugar and whole milk powder. Water was provided separately. Pig manure was used as larval medium. It was frozen at -20 °C for 24 hours before the trials in order to kill the living organisms. 100 g of manure was put into each of small plastic jars. Replicate groups of same (20-20) or different (30 to 150) numbers of predator and prey immatures (eggs and/or larvae) were placed together into the medium. The samples were put in a bigger jar with ca. 2 cm sawdust for fly pupation. All jars were covered with cloth and tightly sealed at the top with a rubber band to prevent larvae from escaping. The samples were kept at 25 ±2 °C and 60±10 % relative humidity. Three replicates were done for each predator-prey combination. In order to determine the mortality due to factors other than prédation control cups were used with only the predator omitted. After 14 days the developed imagoes were identified and counted. If control mortality was between 5 % and 20 %, the mortality data were corrected for control mortality using Abbott's formula. RESULTS AND DISCUSSION It is known that the larvae of Hydrotaea aenescens become carnivorous during a period of their development as well as the larvae of H. ignava (=0. leucostoma) and H. capensis (SÉGUY 1923; ANDERSON and POORBAUGH 1964; MULLER 1982; OLCKERS and HULLEY 1984). During this time the larvae attack, kill and eat larvae of other fly species including the house fly. KEILIN and TATE (1930) reported that Hydrotaea larvae are carnivorous only in later stages of their life, earlier they are saprophagous. This behaviour is related to the structure of their oral sclerites. The obtained data (Table 1) confirm that larvae of H. aenescens can destroy house fly maggots before they could reach the pupal stage but in pig manure the prédation rate also varies by the age and size of both predator and prey at the time when the experimental jar was set up. In the first part of the study, when the same immature stages were reared together, the mortality 104