Insecticide activity of Greek oregano essential oil and entomopathogenic fungus Metarhizium pemphigi against Diabrotica virgifera virgifera LeConte

The western corn rootworm (WCR), Diabrotica virgifera virgifera , is an invasive alien species and an important pest of maize in Europe. The insecticide potential of Greek oregano, Origanum vulgare subsp. hirtum, essential oil (EO) and the entomopathogenic fungus Metarhizium pemphigi were evaluated against WCR adults in laboratory conditions as alternative control agents. Pure undissolved Greek oregano EO applied at a volume of 3, 5 and 10 µl caused 97–100% mean corrected mortality of the test individuals with a rapid lethal effect. Treatment of WCR with four concentrations of EO (0.01, 0.1, 1 and 10 µl/ml) showed a strong concentration–time effect. Mortality rate increased with increasing concentration and exposure time. The median lethal concentration LC 50 was 0.03 µl/ml. The time needed for Greek oregano EO to cause LT 50 of D. v. virgifera ranged from 17.95 days for the lowest concentration (0.01 µl/ml) to 90 min for the highest concentration (10 µl/ml). Metarhizium pemphigi , which was tested in five conidia concentrations (2 × 10 3 –2 × 10 7 conidia/ml), showed relatively lower effectiveness against D. v. virgifera adults. At the end of the bioassay (ten days post-treatment), the mean corrected mortality of the test specimens varied from 14 to 73%. LC 50 was 3.3 × 10 6 conidia/ml. The results showed that Greek oregano EO had a bioinsecticidal effect against WCR adults at low concentrations and they are a basis for further investigations on the EO as an environmentally friendly alternative to chemical insecticides.