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

MATERIALS AND METHODS Characterisation of housefly strains, description of toxicity test and statistical evaluation of data are the same as in the 1 part (p. 81-91). The only difference that in here presented the LDg

5 value instead of LDy

0 and the resistance index calculated for it too. High doses of trichlorfon were administered in several fractions: on one occasion only 2 pi was given. After the first drops had dried, in case of need the treatment was repeated. Insecticides Trichlorfon (dimethyl-2, 2, 2-trichloro-1-hydroxyethylphosphonate), dichlorvos (DDVP; 2,2dichlorovinyl dimethylphosphate) and fenitrothion (0,0-dimethyl 0-3-methyl-4-nitrophenyl phosphorotioate) were obtained from Riedel-de Haen, Inc. The agents were of at least 99 % purity. They were dissolved in acetone of 95% purity immediately before use. RESULTS Data on the sensitive strain Data concerning the trichlorfon resistance of the WHO/SRS housefly, the strain serving for comparison, are shown in Table 1. There is only a moderate deviation between LD50 values determined in the various experimental series. On the average of the ten experiments, the LDgQ is 0.9225^g/female. A still higher homology is found when LD^Q values are related to 1 g body weight. The coefficient of the probit-regression line varies between 1.4 and 2. 8, Baseline DDVP resistance data of the susceptible strain are given in Table 2, The LDgQ value obtained as the average of the ten experiments is 0. 044jig/female, avalue consistent with data reported in the literature. The LD

50 and LD

g5 values indicate that there was a very low deviation between the different experiments. The regression coefficient varies between 1, 8 and 3. 3. Baseline fenitrothion resistance data of the sensitive strain are shown in Table 3. The average LD50 value computed on the basis of the ten experiments is 0. 3069 jug/female. This value exceeds those reported in the literature. The coefficient of the probit-regression line varies between rather narrow bounds and is rather low. This is clearly seen from Fig. 3 which illustrates the probit-regression lines. Data on the populations tested Trichlorfon resistance values of the housefly populations tested are shown in Table 4. All populations proved to be resistant and, according to the categories proposed by KEIDING (1980), their resistance was of very high degree. The resistance index calculated from LD50 values was 17.8, between 50 and 100, between 100 and 200, and above 200 for 1, 2, 10 and 4 housefly populations, respectively. The resistance indices calculated from LDyg values are higher than those derived from the LDg

0 values: these were lower than 100, between 100 and 300, between 300 and 500, and above 500 for 3, 7, 4 and 3 populations, respectively. The difference between the two resistance indices allows us to conclude that in natural (field) housefly populations there are specimens with a very high level of resistance. The highest trichlorfon resistance index (79 5) was obtained for population no. 10. The coefficient of the probit-regression line varies within wide ranges (from 0.9 to 2.4); however, for most populations it is between 1 and 1, 5. This value is lower than that found for the sensitive strain, a fact indicating that most populations are very heterogeneous as regards trichlorfon resistance, i.e. they contain housefly specimens that carry genes responsible for high or low levels of resistance. This is clearly shown by the different slopes of the probit-regression lines given in Fig. 1. DDVP sensitivity data of the populations tested are summarized in Table 5. The resistance