Chemical control of polyphagous pests on their auxiliary hosts can minimize insecticide resistance: A case study of Spodoptera exigua Hübner (Lepidoptera: Noctuidae) in cotton agroecosystem

Abstract

Use of insecticides for agricultural pest management have significant environmental impacts. Using integrated approaches to minimize the frequency of insecticide application by incorporating cultural control tactics can keep environmental hazards to the minimum. The alternate hosts of polyphagous pests have significant role in population development and success of chemical control. Spodoptera exigua Hübner is a polyphagous insect pest of frequently cultivated crops (castor, cauliflower, okra and spinach) in cotton agroecosystem of Pakistan. Since the timeline of cultivation of these crops coincide and the field are often adjoining, S. exigua therefore keeps on migrating from one crop to the other both in winter and summer. Such shifting has a significant effect on the susceptibility of the larvae against the commonly used insecticides. To reveal these effects, toxicity of chlorpyrifos and cypermethrin were tested against the larvae fed on their natal hosts, then shifted to their auxiliary hosts and finally upon their reversal to the natal host. Results have demonstrated that resistance of S. exigua decreased significantly when the populations are cultured on a single host in the laboratory. Field population collected from castor crop demonstrated the highest decrease in resistance ratios against chlorpyrifos and cypermethrin (i.e., 210- and 403-folds, respectively) compared to its field population. Moreover, alternation of generations among cotton and alternate crops, in general, decreased the resistance against chlorpyrifos and cypermethrin. But when these populations return to cotton again, their resistance has been found to be increased significantly. A similar but more obvious change in resistance ratio is observed when the pest population from cotton hosts is fed on spinach (a host available in the later developmental stages of cotton) and vice versa upon their return to cotton. Hence it is suggested that for effective management of S. exigua a continual routine of pest control on available alternate hosts, especially castor and spinach should be administered at critical stages of cotton crop development, i.e. early seedling stage and the late fruiting stage.