Insecticidal activity of the ether extract from the lichen Ramalina complanata and an isolated metabolite (divaricatic acid) against Sitophilus zeamais (Coleoptera, Curculionidae)

Abstract

The aim of the present study was to evaluate the insecticidal activity of ether extract from the lichen Ramalina complanata, and an isolated metabolite (divaricatic acid), against the maize weevil, Sitophilus zeamais. The evaluation of ingestion toxicity was performed using an artificial diet composed of the extract or compound (4 mg/g, 8 mg/g, or 12 mg/g) mixed with wheat flour. After 7 days of incubation, we determined the mortality rates and nutritional parameters. In the fumigation assay, the insects were incubated for 7 days in sealed chambers containing either the extract or divaricatic acid in 0.195–3.125 mg/L of air. The ingestion of the extract led to a maximum mortality rate of 33.33% (12 mg/g) and the extract had a deterrent effect ranging from weak (feeding deterrent index, FDI: 50%) at 4 mg/g to strong (FDI: 71%) at 12 mg/g. The mortality rate of the insects that ingested divaricatic acid reached 52.50% (12 mg/g) and the insects were also unable to convert the food into biomass. A moderate deterrent effect (FDI: 51%) of the divaricatic acid was found at a concentration of 12 mg/g. In the fumigation test, the mortality rates with the extract and divaricatic acid were 43.33-50.00% and 33.33-63.33%, respectively. An LC50 of 1.658 mg/L of air was determined for divaricatic acid. The results highlight the need for further studies on these kinds of compounds for the control of this insect, as well as for the investigation of the mechanisms by which lichen preparations work.