Dr. Murai Éva szerk.: Parasitologia Hungarica 19. (Budapest, 1986)

ment of resistance to DDT (SAWICKI, 1973; SHONO, 1985). Two of them are responsible for neutralizing the effect of DDT: the so-called Deh gene (DDT-dehydrochlorinase) acts on chromosome 2, whereas the md (microsomal detoxication) gene on chromosome 5. The recessive kdr (knock-down) gene is located on chromosome 3 and has no connection with DDT metabolism. The presence of the kdr gene reduces the sensitivity of the nervous system to DDT. Houseflies carrying this kdr gene, or its allele, the super kdr gene, are also resistant to pyrethroids, i.e. in this case a cross-resistance has developed. In most places of the world resistance to DDT is mediated by the dehydroclorinase enzyme, although in Denmark and Northern Europe the kdr gene also occurs rather frequently (KEIDING and SKOVM AND, 19 84), hindering housefly control. The resistance of the housefly to organochlorine insecticides is well known and has been studied by many researchers worldwide. Most of the data on the spread of resistance to DDT were reported in the'fifties (BABERS and PRATT, 1951; BROWN, 1958) and 'sixties. These data were reviewed in detail by BROWN and PAL (1971). Further reviews of the subject can be found in works by KEIDING (19 75, 1977, 1980). From these works one can conclude that in most places the DDT resistance of houseflies is still very high and no-where has it disappeared completely even though it has been out of use for years or decades. This fact is confirmed by recent surveys conducted by researchers from England (CHAPMAN and LLOYD, 19 81; SAWICKI et al., 1981) and Czechoslovakia (RUPES et al., 1983). In Hungary, no other insect has been studied for DDT resistance as intensively as the housefly. BÁNKI and LONKAI (1959) ran laboratory experiments to study the development of DDT resistance, and found that field populations possessed 2 to 3 times higher resistance. SZTANKAY-GULYÁS and ZOLTAI (1962, 1963) found DDT-resistant houseflies in the region of Baja and Dömsöd. During a survey, SZTANKAY-GULYÁS and ERŐSS (1970, 1970a) examined 17 strains, 76. 5 % of which proved to be resistant to DDT. The method recommended by WHO was used and the LDQQ values were published. No sensitive strain was used for comparison, and thus resistance indices could not be calculated. The same authors mentioned that inHungary DDT resistance tests had been carried out by M. ARADI and I. KECSKEMÉTI. Recently SZABÓ (1984) performed such tests on two housefly populations and found a more than 1000fold DDT resistance level. After DDT had become ineffective, HCH was introduced into hou se fly control; however, houseflies soon developed resistance to this agent as well (OPPENORTH, 19 56; BRIDGES and COX, 1959). Resistance to HCH did not prove as persistent as that to DDT, and by now it has dropped to a very low level in most places (KEIDING, 1980). At the same time, during a survey conducted in the United Kingdom, CHAPMAN and LLOYD (1981) found numerous housefly populations with high or very high resistance levels. TANAKA (1981) also observed high resistance in a Japanese housefly population. So far no numerical data have been available concerning the HCH resistance of houseflies in Hungary. SZTANKAY-GULYÁS and ERŐSS (1970) reported reduced sensitivity to HCH in some housefly populations. SZARÔ (1984a) examined two populations and found resistance indices below 10. MATERIALS AND METHODS Housefly strains Field housefly populations were collected from pig farms located in 18 different areas of Hungary, and from one chicken house. The populations were numbered from 1 to 18, and are further on referred to using these numbers even if certain populations were not tested for resistance to certain agents. The populations were collected in the following localities of Hungary (Fig. 1); 1: Küngös, 2: Pápa, 3: Bábolna I, 4: Bábolna II, 5: Agárd, 6: Zichyújfalu, 7: Rábapordány, 8: Szil, 9: Mezőlak, 10: Simaság, ll:Újkér, 12: Lövő, 13: Solt, 14: Nagymajor, 15: Csongrád, 16: Városföld, 17: Bácsbokod, 18: Bácsborsod. Mainly pig farms using battery management system were sampled, where themanure accumulates under the batteries and is not removed for a prolonged period. In such places flies occur in large numbers owing to the presence of conditions essential for their life. The larvae and pupae present in the ma-