Biological activity of Ajuga iva extracts against the African cotton leafworm Spodoptera littoralis

The African cotton leafworm Spodoptera littoralis, a major crop pest worldwide, is controlled by chemical insecticides, leading to serious resistance problems. Ajuga plants contain phytoecdysteroids (analogs of arthropod steroid hormones that regulate metamorphoses) and clerodanes (diterpenoids exhibiting antifeedant activity). We analyzed phytoecdysteroids and clerodanes in leaf extracts of the Israeli Ajuga iva by LC.TOF.MS and TLC, and their efficiency at reducing S. littoralis fitness. Castor bean leaves were smeared with an aqueous suspension of dried methanolic crude extract of phytoecdysteroid and clerodanes from A. iva leaves (50, 100 and 250 microgram/microliter). First and third instars of S. littoralis larvae were fed with 1 treated leaf for 3 and 4 days, respectively. Mortality, larval weight gain, relative growth rate and survival were compared to feeding on control leaves. To evaluate and localize A. iva crude leaf extract activity in the insect gut, we used DAPI and phalloidin staining. Crude extract of A. iva leaves (50, 100 and 250 microgram/microliter) significantly increased mortality of first instar S. littoralis larvae (36%, 70% and 87%, respectively) compared to controls (6%). Third instar larval weight gain decreased significantly (by 52%, 44% and 30%, respectively), as did relative growth rate (minus 0.05 g/g day, compared to the relevant controls). S. littoralis larvae were further affected at later stages, with few survivors. Insect-gut staining showed that 250 microgram/microliter crude leaf extract reduces gut size, with relocation of nuclei and abnormal actin-filament organization. Our results demonstrate the potential of A. iva extract for alternative, environmentally safe insect-pest control.