Studies on the stability of population equilibrium of the diamondback moth (Plutella xylostella (Lep.: Plutellidae)) using perturbation method

Nowadays, a basic understanding of population biology is necessary to establish ecological strategies of pest management. In the present study, the population behavior of the diamondback moth after a perturbation was examined. For this purpose, a long-term experiment with three treatments (control, a perturbation using density reduction and a perturbation using density increase) was performed. The experiment was started with 10 pairs of adult moths in ventilated cages under the standard constant environment. The experiment was continued for 10 generations. After five generations, the perturbation was applied and each treatment was replicated 10 times. Adults and larvae were fed with honey solution (20%) and Chinese cabbage, respectively. The population trend and stability were monitored by weekly census counts of live adults, and the data used as a measure of abundance for the population dynamics. The results showed that the populations of the diamondback moth persisted at equilibrial levels when there was no perturbation. On the contrary, when the populations were perturbed using density reduction, the population equilibrium was shifted to a lower level; in this situation perturbations using sustainable strategies, such as the release of natural enemies or the application of microbial insecticides, may set the equilibrium beneath economic injury threshold. While, a perturbation using density increase can cause instability of population equilibrium toward a decreasing trend that may drive the population to extinction. The causes and effects of the findings in relation with the moth life history, resource competition and its ecological consequences on the population behavior were discussed