RELATIONSHIP BETWEEN INSECTICIDE RESISTANCE AND PERFORMANCE IN CHOICETESTS OF FIELD-COLLECTED GERMAN COCKROACHES (DICTYOPTERA, BLATTELLIDAE)

Topical application (LD) and time-mortality response (LT) methods were used to determine resistance levels of field-collected strains of the German cockroach, Blattella germanica (L.). LD50s for chlorpyrifos we re determined for seven strains. LT50s and LT90s with eight insecticid es were determined for another three strains. Resistance ratios (RR) o f >10 to chlorpyrifos (LD50) were directly related to a significant de crease in activity in Ebeling choice-box tests. Using the LT method, t he three field-collected strains were shown to possess zero to moderat e resistance at the RR50 level (0.9-5.3) to organophosphates, moderate to high resistance to (1.4->21.0) to carbamates, moderate to high res istance (4.3-20) to cypermethrin, and high resistance (>17->35) to syn ergized pyrethrins. When the three field-collected strains were tested in choice boxes, RR50s of 1.2-2.2 for chlorpyrifos and 2.5-5.3 for di azinon directly related of significantly lower efficacy. Low-level pro poxur resistance (RR50 of 1.4-2.3) also reduced choice-box efficacy, b ut an RR50 of 0.9-1.3 for acephate did not reduce its performance. Cyp ermethrin deposits used in choice tests killed cockroaches exhibiting RR50s as high as 20 in the lethal time test, perhaps because resistanc e mechanisms could not detoxify the exceptional activity of the active ingredient formulated as a wettable powder. Because they were repelle nt, residual deposits of pyrethrins resulted in poor efficacy in choic e tests with either susceptible or field-collected strains. Strains wi th low-level physiological resistance as measured by the time-mortalit y response method survived insecticide exposure in choice-box tests to certain organophosphate and carbamate insecticides, indicating that t hese compounds would not be effective in the field.