Sublethal effects of spirodiclofen on life history and life-table parameters of two-spotted spider mite (Tetranychus urticae)

Abstract

Laboratory bioassays were conducted to evaluate the effects of spirodiclofen on life history and life-table parameters of two-spotted spider mite (Tetranychus urticae Koch) females treated at pre-ovipositional period with a series of acaricide concentrations starting with the concentration discriminative for eggs and immatures i.e. the lowest concentration that causes 100% mortality of those stages. After a 24 h exposure, the proportion of females that survived treatments was 0.71 (6 mg/l), 0.51 (12 mg/l), 0.41 (24 mg/l), 0.30 (48 mg/l) and 0.25 (96 mg/l). At the end of the trial, the survival rate of females treated with the lowest concentration was significantly lower than the survival rate of untreated females but it remained above that of females treated with higher concentrations. Total fecundity/fertility significantly decreased as concentrations of spirodiclofen increased. Viable eggs were laid by females treated with 6, 12 and 24 mg/l, and total fertility was reduced by 42, 84 and 97%, respectively. Compared with control, the gross fecundity/fertility of the treated females was significantly reduced throughout the trial, except in females treated with 6 mg/l. All concentrations caused a significant reduction in the net fecundity/fertility throughout the trial. The females treated with 12 mg/l had significantly reduced net reproductive rate (R0 = 6.45), compared to females treated with 6 mg/l (R0 = 23.35) and to untreated females (R0 = 28.92); there was no significant difference between the last two treatments. The intrinsic rate of increase (r(m)) and finite rate of increase (lambda) were significantly reduced in treated females (r(m)) = 0.141, lambda = 1.156 and r(m) = 0.214, lambda = 1.232; 12 and 6 mg/l, respectively), compared to control (r(m) = 0.251, lambda = 1.276). The reduction was significantly greater in females treated with the highest concentration. As a result of the lowered r(m), the doubling time in treated females was significantly extended. Sublethal effects of spirodiclofen and its impact on T. urticae management are discussed.