Fertility life table parameters of the cabbage aphid, Brevicoryne brassicae ‎affected by sublethal concentration of BotanAphid aphidicide on two ‎cabbage varieties

Cabbage aphid, Brevicoryne brassicae L. is one of the important pests of brassicaceous plants ‎worldwide, causing considerable damage to them. It not only damages host plants by directly ‎sucking the tissue sap and excreting honeydew, but also transmits some plant pathogenic viruses ‎and further weakens the host crops. In one hand, due to the adverse effects of irregular chemical ‎pesticide use on vegetables such as cabbage, and indecisive effects of biocontrol agents in pest ‎control, exploring alternative methods for cabbage aphid management such as botanical pesticide ‎application sounds appropriate and necessary. Therefore, in the present study, acute toxicity of the ‎botanical insecticide, BotanAphid was studied against cabbage aphid on white and red cabbages by ‎determining their corresponding LC50 values. Moreover, their sublethal (corresponding LC25 ‎values) effect was assessed on the fertility life table parameters of the pest in both white and red ‎cabbages.‎

After preliminary bioassays based on recommended concentration (2 % for cotton aphid) in both ‎plant cultivars, five concentrations were estimated by logarithmic intervals in white (0.45, 0.81, ‎‎1.48, 2.69 and 4.9 percent) and red (0.5, 0.89, 1.58, 2.82 and 5.01 percent) cabbages. Control ‎aphids were treated with distilled water containing 0.05% citowett as surfactant. Thirty newly ‎emerged adult aphids were randomly selected for each treatment. Ten insects were placed on each ‎leaf disc and treated uniformly by a hand sprayer. After drying, insects were placed within Petri ‎dish with ventilation. The number of dead insects were counted after 24 h. Bioassay data were ‎analyzed by probit analysis using SAS 24.0. To study the sublethal effects of BotanAphid on ‎fertility life table parameters of B. brassicae, the calculated LC25 values (0.675 and 0.765 percents ‎respectively for white and red cabbage cultivars) of BotanAphid were used. For each treatment and ‎control, 50 adult aphids were treated similar to bioassays. All treated insects were removed after 24 ‎h and the number of 50 newly emerged first instar nymphs from treated adults were randomly ‎selected and kept individually. The number of dead insects and produced nymphs were recorded ‎daily and removed from experimental units. Life table data were analyzed using Twosex-MsChart ‎program and the means and standard errors of parameters were calculated by bootstrap method ‎using 100000 replicates.‎

According to the bioassays, the mean lethal concentrations (LC50 values) of this pesticide on ‎cabbage aphid were 1.552 and 1.704 percents respectively in white and red cabbages. In life table ‎study with LC25 values of aphicide, the lowest fertility and reproduction period of B. brassicae ‎were remarkably decreased on BotanAphid treatment in both white and red cabbages compared to ‎corresponding control (insecticide-free) units. Furthermore, pre-adult duration was prolonged and ‎survival of treated aphids was decreased considerably when compared to control. In addition, ‎insecticide treatment caused intrinsic rate of increase (r), as the most important population growth ‎parameter, of the cabbage aphids to be lowered significantly compared to corresponding control ‎insects. Also, BotanAphid sublethal concentration resulted in considerable decrease in other life ‎table parameters when compared to appropriate control insects in both cabbage cultivars. ‎Reproduction values and life expectancies were the highest in control aphids and the lowest in ‎LC25-treated insects.‎

Our results revealed that B. brassicae suffered considerable mortality from BotanAphid, as a ‎botanical insecticide in nearly recommended concentrations (2 %). Comparison of LC50 confidence ‎intervals (95%) showed that susceptibility of the pest was not statistically different in treated aphids ‎on white and red cultivars. This showed that the plant cultivar did not impose significant impact on ‎aphid mortality. Given that the chemical content of sap and nutritional quality of two cultivars are ‎varying in different cultivars, these did not affect adult aphids’ mortality encountering BotanAphid. ‎Using LC25 value in each cultivar for analyzing life table parameters demonstrated the its negative ‎influence on all parameters compared to control insects in both cultivars. Besides these results, due ‎to botanical nature of BotanAphid and its low risk for non-target organisms especially human, this ‎pesticide application is recommendable on fresh-consuming vegetables such as different cabbage ‎cultivars.‎