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

Tox

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city test For the toxicity tests 3- to 5-day-old female imagos were used. The average body weight of the WHO/SRS imagos ranged between 15 and 20 mg. In most cases imagos of the wild (field) populations exceeded 20 mg in body weight. In the case of the field populations, a preliminary experiment was carried out to determine the dose rate to be applied. A dilution series consisting of 5-7 doses was used in which the doses formed a geometric progression. The initial concentration was always given as w/v per cent of the active substance and acetone, and the solution was always prepared immediately before treatment. For populations 1-5 10 treatments per dose and 10 flies per treatment, whereas for the remaining populations 3 treatments per dose and 20 flies per treatment were used. For the WHO/SRS strain the test was performed at 10 different times per active substance, and 5-7 doses were applied. Ten treatments per dose were performed, with 10 flies per treatment. To calculate the resistance indices, the average of 10 measurements was taken. An untreated and an acetone-treated control was applied for each active substance and in each test. The sex of the houseflies narcotized with CO2 was determined, and the females were treated: 1 jul solution was dropped onto the dorsal part of the thorax with an LV-65 type (ARNOLD, 1965) automatic microapplicator. High doses of DDT were administered in several fractions: on one occasion only 2 .nl was given. After the first drops had dried, in case of need the treatment was repeated. After each treatment the treated flies were placed into plastics dishes of 250 cm^ volume and the dishes were covered with tulle. During exposure, the flies were kept in a climatized chamber at a temperature of 24JT°C, at 65+5% relative humidity and a light period of 16 h (200-2 50 lux). Evaluation was performed 24 h later, by counting paralysed specimens that had obviously died. Statistical evaluation of data Data were processed in a VDT-52122-type computer, using a programme that performs control mortality correction according to ABBOTT (1925) and probit analysis as described by FINNEY (1971). As a rule, control mortality did not exceed 5%; nevertheless, the tests were repeated if mortality exceeded 10 %. In probit analysis minimum 4, but usually 5-6, data were used in the calculations for each active substance. LD50 and LDQQ values are expressed in jug/female units, but for WHO/SRS flies also in jug/g body weight to ensure comparability. The regression coefficient and its fiducial limits at a 95 % probability level are given to characterize the probit regression line. The resistance index is the ratio of the LD50 value obtained for the field (wild) population and that found for the sensitive strain. RESULTS Data on the sensitive strain The sensitive reference strain serving as a basis by comparison is of great importance in resistance tests. Consequently, it is not indifferent what strain is used and what toxicological parameters it has. Unfortunately, as stated by KEIDING (1980), in this respect there exist very large differences in the literature. There is a considerable difference between the DDT resistance values published for the WHO/SRS fly strain (WHO, 19 65) and those determined by us. The LDgQ values found by us (Table 1) are almost 10 times, whereas the LDQQ values 20 times, higher than those previously reported for that strain. Differences between measurements performed at different times are also large, indicating heterogeneity of the strain. Heterogeneity is indicated by the very low ( < 1) value of the regression coefficient as well. The question arises what the source of the above-mentioned differences is: is it the unreliability of the tests or the fact that the DDT resistance of the strain has changed since it was determined 20 years ago. In our opinion, the tests are sufficiently reliable. The flies are bred and the toxocity tests performed always in the same manner, as described under M aterials and methods, by the same staff of technicians, under reproducible circumstances, using large numbers of flies. The reliability of the tests is supported by the results of statistical calcu-