Toxicity of essential plant oils, in comparison with conventional insecticides, against the desert locust, Schistocerca gregaria (Forskål)

• Essential plant oils showed high potency against the locust, S. gregaria.
• Some oils possessed toxicity as high as that of chemical insecticides.
• Combining plant oils with insecticides induced synergistic/antagonistic effects.
• The insecticidal activity of tested plant oils magnifies their industrial values.

Essential oils extracted from ten different plants belonging to five families were tested against the 3rd nymphal instars of the desert locust, Schistocerca gregaria (Forskål) (Orthoptera: Acrididae) by topical application technique. Also, the toxicity of four chemical insecticides, as well as the joint action resulted from mixing botanicals and insecticides were evaluated. Either at LD50 or LD90 values, the Allium cepa-oil proved to be the most toxic where the above mentioned values equaled to 1.11 and 1.42 ppm, respectively followed by the Petroselinum sativum (1.34 and 1.61 ppm, respectively). The LD50 and LD90 values for the oils of Pelargonium radula, Cuminum cyminum, Ocimum basilicum, Origanum vulgare and Matricaria chamomilla ranged between 1.54–1.59 ppm and between 1.84–1.91 ppm, respectively. GC/MS analyses revealed the presence of known biologically active compounds such as p-Cymene, Linalool, Thymol, Caryophyllene and Carvacrol in some of the analyzed oils. Fenitrothion was the most toxic (LD50 = 0.33 ppm), while fenvalerate was the least (LD50 = 1.48 ppm). The other 2 insecticides possessed intermediate and nearly equitoxic values (LD50 = 1.2 ppm). Surprisingly, the A. cepa oil was more toxic than methomyl, and the other plant oils possessed toxicity accounted to 74–80% and 83–98% of methomyl toxicity when comparisons were made at the LD50 and LD90 levels, respectively. Combining plant oils with insecticides resulted in different types of interaction (e.g., synergism, additive and antagonism). Such results may be considered as novel findings in the course of searching for potent botanical insecticides against the locust, S. gregaria, and magnify the industrial value of such plant oils in the era of biopesticides.