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

Permethrin resistance data are summarized in Table 7. Of the 17 populations tested for permethrin resistance, 2, 3 and 6 had more than 10-fold, between 5- and 10-fold and between 1- and 5-fold resistance, respectively, whereas 6 populations proved to be more sensitive than the control (WHO/SRS) strain. The highest values were obtained for populations no. 5 and 10. These populations were characterized by moderate resistance, which is indicated by the less steep slopes as compared to those of the sensitive strain (Fig. 3). Deltamethrin resistance data are shown in Table 8. Of the 13 populations tested for deltamethrin resistance, 1, 2, 4 and 6 had a resistance index above 20, between 10 and 20, between 5 and 10, and between 1 and 5, respectively. Of the four compounds tested, the highest resistance developed to deltamethrin, disregardin the two populations exhibiting strikingly high resistance to tetramethrin. This is also supported by the fact that, as opposed to the other compounds, none of the populations was more sensitive to deltamethrin than the control strain (Fig. 4). Also to deltamethrin, the highest resistance value (25.7 -fold) was obtained for population no. 5. A moderate deltamethrin resistance was found in populations no. 6 and 10 which behaved similarly against other compounds as well. Contemplating the values of the regression coefficient, we can make an interesting observation, namely that the regression coefficients of the field populations exceed the average found for the WHO/SRS strain in all the cases but twox We can state that the majority of the populations tested were not, or were only moderately, resistant to the four pyrethroids. At the same time, resistance values as low as 1-5 also deserve attention, since these indicate a real decline in susceptibility. This is supported by the fact that with all agents but deltamethrin there were populations whose sensitivity was even lower than that of the WHO/SRS strain used for comparison. Actually only these populations can be considered sensitive, and only these express the real level of sensitivity. Using the sensitive housefly strain "Cooper", in England SAWICKI et al. (1981) observed the opposite, i.e. that even the most sensitive field populations were at least 2 to 3 times as tolerant as the control strain. Following the categorization recommended by KEIDING (1980), two populations had high, one had moderate, while the remaining ones had low resistance to the photostable tetramethrin. In the two populations the development of resistance must have been due to the earlier use of tetramethrin aerosols. Of the three photostable pyrethroids, the highest resistance had developed to deltamethrin. All housefly populations but one exhibited a manifold decrease in sensitivity. This may be accounted for by the fact that most pyrethroids used in Hungary for housefly control so far contained deltamethrin as active substance. At the same time, this finding must be regarded as a warning sign since the most rapid development of resistance was observed precisely to deltamethrin. According to the special literature, in various parts of the world permethrin has been the photostable pyrethroid to which resistance develops most frequently. In Hungary, this is not the case, since permethrin has rarely been used so far. The lowest resistance was found to Cypermethrin. The results reported above unequivocally prove that in some populations sensitivity has decreased to almost all of the active compounds tested. If we contemplate the geographic distribution of these housefly populations (see Fig. 1 in the I.part of the accompanying papers ), three foci can be distinguished. Each focus involves two neighbouring populations. One focus is constituted by populations no. 3 and 4: unfortunately, these have not been tested for Cypermethrin and deltamethrin resistance. Another focus consists of populations no. 5 and 6 which exhibit a markedly reduced sensitivity to all the compounds tested. Pyrethroids have been used in both populations occasionally, but in population no. 6 their use has been more intensive. This fact might account for the reduced sensitivity. Populations no. 10 and 11, in which no pyrethroids have been applied, are even more interesting: population no. 10 clearly has developed resistance, but population no. 11 also exhibits reduced sensitivity, although to a less extent. In these populations the reduction of sensitivity might be attributed to a crossresistance with DDT. However, further experiments are warranted to prove the latter hypothesis. DISCUSSION