Ocimum gratissimum Linn. (Lamiaceae) essential oil for the management of Exorista sorbillans Wiedemann (Diptera: Tachinidae) menace of silkworm in seriecosystem

Abstract

• Ocimum gratissimum is a potential candidate for management of E. sorbillans. • Non polar fraction of the leaf extract was found effective against uzi fly. • Essential oil was more effective than non-polar petroleum ether extract. • Thymol and carvacrol were the major constituents. • The efficacy of the oil is attributed to combination of individual oils. Although they act as biocontrol agents, Tachinid parasitoids attack economically beneficial insects, inviting the adoption of control measures against them. The uzi fly, Exorista sorbillans , infests silkworms incurring heavy loss. It is difficult to control such parasitoids as the larval stage is endoparasitic, and insecticides cannot reach them without subjecting the silkworms to potential risks. We selected the leaves of Ocimum gratissimum L. for testing their toxicity potential against E. sorboillans because of its low selective toxicity to Antheraea assamensis Helfer larvae. Assay for adulticidal efficacy of the leaf extracts showed the petroleum ether extract to be the most effective. But the essential oil (LC50 of 0.42%) was even more effective. GC–MS analysis of the oil revealed the presence of twenty-seven compounds with a high percentage of thymol (26.17%), indicating the plant oil as thymol rich chemotype. Thymol was the dominant constituent in the active fraction. The oil and constituents caused rapid death on topical application, indicating a higher penetration rate. We propose the exploration of the essential oil and its constituents as a sustainable solution for controlling E. sorbillans infestation of silkworms in the future.