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

index calculated from the LD^^ values shows that only few populations possess low or moderate resistance. Only on one population was the resistance index higher than 10, in two populations it was between 5 and 10, whereas in the remaining populations below 5. The resistance index calculated from the LD95 values did not markedly differ from the above figures. The populations were homogeneous for DDVP resistance and did not contain specimens carrying genes responsible for high resistance. This is illustrated by the probit-regression lines in Fig. 2. Also.it can be seen that in population no, 16 a more intensive selection had started. Fenitrothion resistance data of the populations tested are shown in Table 6. The resistance indices calculated from EDgg values show that most of the populations were characterized by moderate or low resistance. The resistance index was higher than 20, between 10 and 20, between 5 and 10, and below 5 in 1, 3, 9 and 3 housefly populations, respectively. The resistance indices derived from the LD95 values were similar. Therefore, the populations were homogeneous as regards fenitrothion resistance, i.e. they were devoid of specimens carrying genes responsible for high resistance. The coefficient of the probit-regression line varied between wide ranges; however, for most populations it was between 1. 5 and 2, 5, somewhat exceeding the value obtained for the sensitive strain. This is clear from Fig. 3 which shows the probit-regression lines. DISCU SSION Of the organophosphorus insecticides tested, trichlorfon was the one to which the highest resistance had developed. As has been pointed out above, the same holds for countries where trichlorfon was used widely and over a prolonged period. Based upon the results, it is not advisable to use trichlorfon as a spraying agent against Hungarian housefly populations. Separate experiments are needed to assess its usability as a bait. The populations contained many houseflies characterized by very high levels of resistance. This means that it would be pointless to raise the dose since a higher selection pressure could only lead to a rapid selection of the highly resistant specimens. The situation is more favourable as concerns DDVP to which only slight resistance has developed. The fairly high values of the slopes of the probit-regression line (mostly between 2. 5 and 3.0) indicate that the populations are homogeneous as regards DDVP sensitivity, i.e. they do not contain housefly specimens carrying genes responsible for high resistance. Provided that an appropriate rotation of drugs is applied, DDVP can remain an effective spraying agent for controlling houseflies for a long time to come. The situation of fenitrothion resistance is not so clear-cut. Despite the fact that this agent had not practically been used in Hungary, about 25 % of the housefly populations tested exhibited a moderate resistance to it, and only in 3 populations was the resistance index lower than 5. The resistance observed by us was probably due to cross-resistance, for which there are precedents in the literature (RUPES and PINTEROVA, 1975; SHONO and MASUHISA, 1983). Therefore, the use of fenitrothion in housefly control does not seem expedient, since there is a high risk of the rapid development of resistance.