Application of insecticides to control the German cockroach, Blattella germania (L.)

Abstract

Control strategies for the German cockroach, Blattella germanica (L.), are usually based on the application of liquid insecticides in or near infested harborages. Cockroach mortality occurs when they are exposed to insecticide residue by walking on insecticide-treated surfaces. Cockroach walking depends on the three pairs of legs. Only tarsal pads and arolium on each leg are involved in traveling and picking up insecticide residue. The total contact area at each step was 0.1879 mm² and 0.1771 mm² for laboratory susceptible (VPI) and field-collected (RHA) cockroach strains, respectively. There were no significant differences (P > 0.05) in the walking distance in two steps among three legs, or between the two strains. However, the walking movement rate greatly influences the amount of insecticide accumulated on tarsal pads in a distinct time period. RHA-strain cockroach walked a significantly (P < 0.05) greater distance than VPI-strain on untreated and 10 ug/cm² of cypermethrin treated glass plates. RHA-strain cockroaches picked up significantly (P < 0.05) more cypermethrin than VPI-strain in 5 min, but there was no difference when exposed for 30 movement units. Knockdown time was significantly correlated (P < 0.05) to estimated dose transfer in 0.025 μg/cm² and 0.049 μg/cm² treatments for the VPI-strain, and in 0.245 μg/cm² and 0.392 μg/cm² for the RHA strain. Cockroach knockdown time and mortality were determined by cockroaches walking on different numbers of cypermethrin droplets on treated glass plates. Insecticide dose could be reduced from 10 μg/cm² to 0.025 μg/cm², but resulted in increasing the KT₅₀ from 5.4 min to 15.9 min in VPI-strain cockroaches. To achieve 50% cockroach knockdown, a VPI-strain cockroach must walk on at least 33 droplets (130 μm in diameter) of 0.1% cypermethrin; an RHA-strain cockroach must walk on at least 3174 droplets. High insecticide dosage increased the amount of antennal grooming in VPI-strain cockroaches and stimulated leg grooming in RHA-strain. Walking movement activities decreased in response to increased antennal and leg grooming.