Relationship between insecticide resistance and performance in choice tests 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 were determined for seven strains. LT50s and LT90s with eight insecticides were determined for another three strains. Resistance ratios (RR) of > 10 to chlorpyrifos (LD50) were directly related to a significant decrease in activity in Ebeling choice-box tests. Using the LT method, the three field-collected strains were shown to possess zero to moderate 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 resistance (4.3-20) to cypermethrin, and high resistance (>17->35) to synergized 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 diazinon directly related to significantly lower efficacy. Low-level propoxur resistance (RR50 of 1.4-2.3) also reduced choice-box efficacy, but an RR50 of 0.9-1.3 for acephate did not reduce its performance. Cypermethrin deposits used in choice tests killed cockroaches exhibiting RR50s as high as 20 in the lethal time test, perhaps because resistance mechanisms could not detoxify the exceptional activity of the active ingredient formulated as a wettable powder. Because they were repellent, residual deposits of pyrethrins resulted in poor efficacy in choice tests with either susceptible or field-collected strains. Strains with low-level physiological resistance as measured by the time-mortality response method survived insecticide exposure in choice-box tests to certain organophosphate and carbamate insecticides, indicating that these compounds would not be effective in the field.