Circadian change of dichlorvos lethality (LD 50) in the cockroach in LD 14:10 and continuous red light

To examine a possible circadian change in LD 50, male, nonvirgin female, and virgin female American cockroaches, Periplaneta americana, were standardized on a regimen of light for 14 hr, alternating with darkness for 10 hr (LD 14:10); they were tested for their sensitivity to an organophosphorus insecticide, dichlorvos, on this regimen and on a regimen providing for 1 added day's maintenance in continuous red light (RR). A marker rhythm in locomotor activity was established using running wheels for the three groups of cockroaches in LD 14:10. Series, consisting of the number of impulses per hour from each of ten roaches per group, were fitted with a 24-hr cosine curve; the zero amplitude assumption was rejected by single cosinor for all series (P less than 0.001) and by population-mean cosinor for each group. Individual circadian acrophases (phi) differed by 18 degrees, 13 degrees, and 6 degrees in the three groups. Group phis were -49 degrees, -48 degrees, and -47 degrees; (360 degrees identical to 24 hr; 0 degrees = light off), i.e., they were within 2 degrees (8 min) of each other with confidence intervals of 6 degrees, 5 degrees, and 11 degrees, respectively. In three separate groups (males and virgin and nonvirgin females) of 480 roaches each, susceptibility to dichlorvos was tested; six doses were applied, each to 10 roaches, 3 hr apart for 24 hr. The phis reflecting maximal resistance to the insecticide were at -200 degrees, -194 degrees, and -203 degrees, i.e., within 9 degrees of each other. Assuming sinusoidality, times of highest susceptibility (-20 degrees, -14 degrees, and -23 degrees) led the acrophase of the activity rhythm by about 2 hr.