Residual toxicity of bait formulations containing plant essential oils and commercial insecticides against the desert locust, Schestocerca gregaria (Forskäl)

Abstract

Essential oils extracted from seven different plants belonging to five families, and four commercial insecticides representing different chemical classes were tested against the 3rd nymphal instars of the desert locust, Schistocerca gregaria (Forskål) (Orthoptera: Acrididae) by bait application technique. The bait was prepared from wheat bran with molasses and fortified with different amounts of the studied toxicants, then introduced to the insect in cages. Methomyl, the most potent insecticide showed LC50 of 0.051μg/g. Fenvalerate, Fenitrothion, and Carbosulfan possessed potency accounted to 49.0%, 34.2%, and 9.0%, respectively, compared with Methomyl potency (100.0%). Cumin (Cuminum cyminum) oil was the highest toxic to the nymphs, while Parsley (Petroselinum sativum) was the lowest. Out of the eleven screened substances, 3 insecticides and 3 plant oils were subjected to detailed residual toxicity investigations at their corresponding LC90 values. The criteria of assessment included estimation of T50 and T20 values (time in days at which mortality declined to 50% and 20%, respectively), feeding activity, reproductive potential, and histopathological alterations in the rectal pads of the tested insect. For example, the T50 and T20 values for Methomyl equalled to 2.0 and 7.3 days, compared with 3.0 and 5.9 days, respectively, for Onion (Allium cepa) oil. Generally, Fenvalerate and Fenitrothion possessed T20 values higher than those obtained for Sweet Basil (Ocimum basilicum) and Cumin. Compared to control, Sweet Basil oil decreased severly consumption index (C.I.), efficiency of conversion of digested (E.C.D.) and ingested (E.C.I.) food. The growth rate was significanly declined by Fenitrothion, Fenvalerate, Sweet Basil, and Onion. Fenitrothion, Cumin, and Onion decreased approximate digestibility. The tested toxicants affected oviposition periods, number of egg pods and eggs laid per female and the total number of hatched eggs. The latter were found 305 eggs in control treatment, compared with 20, 74, 107, 120, 150, 117 eggs for Fenitrothion, Methomyl, Fenvalerate, Onion, Sweet Basil, and Cumin treatments, respectively. Except Cumin treatment, the other treatments caused anatomical alterations in the rectum of the adult S. gregaria represented by increasing length and width of rectal pads, as well as increase of epithelial cells. We may claim that the over all results of this study may be considered as novel findings in the course of searching for potent biopesticides against the locust, S. gregaria, and encourage further research under field conditions.