RESISTANCE RISK ASSESSMENT OF 2 INSECT DEVELOPMENT INHIBITORS, DIFLUBENZURON AND CYROMAZINE, FOR CONTROL OF THE HOUSEFLY MUSCA-DOMESTICA L .2. EFFECT OF SELECTION PRESSURE IN LABORATORY AND FIELD POPULATIONS

Laboratory selection with diflubenzuron (DFB) and cyromazine (CYR) was carried out with strains of houseflies collected on Danish farms. The adult flies were multiresistant to organophosphorus. carbamate and pyrethroid insecticides. Susceptibility to the larvicidal effect of DFB or CYR was monitored on Danish farms where fly control with DFB or CYR had been carried out for one to nine years. On these farms the fly population continued during the winter, but at a low level. Four strains were selected with DFB in seven out of 13-18 laboratory generations. The selections varied from 56% to 94% mortality. The strains showed 1-5 times increase of LC50 and LC95 to DFB. The larvicidal effect of DFB was monitored on five farms where treatment of fly breeding sources in manure (1 g DFB m-2) was carried out regularly for one to nine fly seasons. On none of the farms was any general increase of tolerance to DFB found. Resistance ratios (R/S) compared to the WHO Standard Reference Strain ranged from 0.8 to 2.0 at LC50 and from 1.0 to 2.9 at LC95. Two strains were selected with CYR in five or six out of 13 laboratory generations. The selections varied from 62 to 97% mortality. The strains showed no increase of LC50 or LC95. The larvicidal effect of CYR was monitored on three farms where housefly breeding places in the animal houses were treated with CYR (0.5 g m-2) for two fly seasons. No reduction of susceptibility to CYR was found and the R/S was 0.7 to 0.9. The effect of DFB or CYR selection pressure is compared with other investigations and discussed. Moderate to high resistance to DFB or CYR can develop in housefly populations if the selection pressure is strong, especially when used as feed-through applications on poultry farms where all feed contains DFB or CYR. If the treatment of fly breeding sources is less complete, resistance problems may not develop.