Sterilization makes a difference: demographic analysis of spirodiclofen effects on Tetranychus urticae (Acari: Tetranychidae)

Spirodiclofen is characterized by a relatively slow acaricidal action against adult females of the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), with reduction of their fecundity (eggs laid/female) and fertility (eggs hatched/female). Exposure of pre-ovipositing T. urticae females to the acaricide may affect population growth, considering that a high reproduction of young females is crucial in the population biology of this colonizing species. Effects of spirodiclofen on life-history traits and population parameters of T. urticae were evaluated in demographic bioassay using the age-stage two-sex life table, constructed in fecundity-based and fertility-based variants. The acaricide was applied against pre-ovipositing females in a series of nine concentrations, starting from the recommended field rate (96 mg/l). The treatments with concentrations ranging from 12 - 96 mg/l significantly reduced fecundity and longevity, while 27 - 40% of females didn’t lay eggs. Exposure was lethal to 2 - 21% of the females, of whom a large majority didn’t lay eggs. A considerable part of surviving females also failed to lay eggs within the first four post-treatment days (when around 50% of all eggs in the control were laid) i.e., they were sterilized by the acaricide. At the same time, the percentage of dead females rose to 17 - 55%, mainly due to the mortality of sterilized females. The three highest concentrations (24 – 96 mg/l) significantly reduced the net reproductive rate (R0), intrinsic rate of increase (r) and finite rate of increase (λ), by 49-72%, 20-34%, and 4-6%, respectively, compared to the control. This reduction was mainly the result of sterilization and high mortality of treated females, in combination with reduced fecundity and longevity of reproductive ones. Application of the fertility-based life table showed significant decrease of r and λ (by 23-40%, and 4-7%, respectively, compared to the control), in the treatments with the three highest concentrations. The short-lived transovarial toxic effect observed in the fertility-based life table was not sufficient to cause a significant reduction in population parameters, compared to those acquired by the fecundity-based life table.