RESISTANCE RISK ASSESSMENT OF 2 INSECT DEVELOPMENT INHIBITORS, DIFLUBENZURON AND CYROMAZINE, FOR CONTROL OF THE HOUSEFLY MUSCA-DOMESTICA .1. LARVICIDAL TESTS WITH INSECTICIDE-RESISTANT LABORATORY AND DANISH FIELD POPULATIONS

Larvicide tests with diflubenzuron (DFB) and cyromazine (CYR) were carried out against 15 laboratory housefly strains and 89 field strains collected from 87 Danish and 2 Swedish farms, 1975-89. The strains represented a wide range of adult insecticide resistance and R-mechanisms. The larvicide tests were done by treating larval medium with serial or discriminating dosages of the larvicide, seeding it with eggs and calculating the mortality during development to adults. The WHO susceptible strain (S) was used as a reference. Dose-response tests with DFB gave resistance ratios (R/S) from 1.1 to 4.1 at LC50 and 0.3 to 3.4 at LC95 and, with CYR, R/S from 0.6 to 1.8 at LC50 and 0.6 to 2.9 at LC95. It was concluded that the relatively small variation in susceptibility between strains was not generally correlated with resistance in adult flies to organophosphorus, pyrethroid or other conventional insecticides (neurotoxins). Tests with discriminating dosages of DFB (59 farms) and CYR (63 farms) showed no indication of resistance to either product. The results of investigations by other workers on the relation between resistance to DFB or CYR and resistance to conventional insecticides are discussed.