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

lations as well. In the various experiments the values of the regression coefficient (b) differ only slightly and the confidence interval is narrow. As it will appear from the accompanying papers, such differences occurred only for some of the 9 active substances tested. In addition, it should be noted that the two highest doses used by us gave a slight crystallization at the injection needle at the time of treatment. However, this could not account for the differences observed. The cause of the phenomenon remains unknown, but it is presumably related to the findings that the DDT-dehydrochlorinase and in vitro microsomal oxidase activities of the WHO/SRS strain are rather Iowas compared to other strains (YTJ and TERRIERE 1973; TERRIERE and YU, 1973), and its microsomal oxidases differ in several parameters from those of the test strains (TERRIERE, SCHONBROD and YU, 1975). Data provided by the WHO (1965), referring to generations F2g_4g, were published in the 2 to 3 years following the establishment of the strain. It is assumed that since that time the strain has undergone changes resulting in altered DDT sensitivity. In our tests generations F

305.325 were used. In the case of HCH the heterogeneity of the strain is of lower degree, as indicated by the smaller deviation of the LD^Q values and the regression coefficient ranging between 2 and 3 (Table 2). Topical HCH toxicity data concerning the WHO/SRS housefly strain were not available in the literature. DDT and HCH resistance of field populations Seventeen housefly field populations were monitored for the development of resistance to DDT (Table 3). According to the nomenclature used by KEIDING (1980), all of the populations possessed very high resistance as shown by the resistance index calculated from the LD50 values. There were marked differences between the various populations in sensitivity to DDT. This is clear from Fig. 2 which gives the probit-regression line for the populations. The highest, i.e. 1820-fold, resistance was obtained for housefly population no. 5. Six populations had a resistance index higher than 1000, 5 populations had RIs between 500 and 1000, another 5 between 250 and 500, while for one population a resistance index of 180 was found. With knowledge of the results obtained for the other active compounds tested by us, we can state that housefly populations most resistant to DDT also had higher resistance to the other active substances. Examining the coefficient of the probit-regression line we can see that the values range between 1.5 and 3.4, indicating a varying homogeneity of the populations as regards DDT sensitivity. This is indicated by the different slopes of the probit-regression lines. As regards the relationship between the level of DDT resistance and its geographical distribution, there is no direct connection between the two (Fig. 1, Table 3). There exist marked differences in resistance level even between testing sites close to one another. Therefore, in a given place the level of resistance depends on the extent of selection applied there and the genetic potential of the housefly population. That is easy to understand since at the different animal farms rather isolated and closed housefly populations develop. Fourteen populations were tested for resistance to HCH. The degree of HCH resistance was found to be rather low. Only for one population did we obtain a resistance index higher than 10, for 3 populations the RI was between 5 and 10, whereas for the remaining ones it was below 5. The coefficient of the probit-regression line was between 1 and 1. 5 for most populations, and values higher or lower than that were rare. The natural populations were more heterogeneous in HCH sensitivity than the reference strain used for comparison. This is shown in Fig. 3 which gives the probit-regression lines of the populations. The dose-mortality lines of the natural populations are much less steep than those found for the sensitive strain used for comparison. DISCUSSION The data presented allow us to conclude that in Hungary housefly populations still possess a very high resistance to DDT although the agent has been out of use for almost twenty years. These results are consistent with those reported in the special literature (KEIDING, 1967; RUPES, 1983). During their surveys conducted in Hungary, SZTANKAY-GULYÁS and ERŐSS