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

Abstract

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 {micro}g/{micro}l). 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 {micro}g/{micro}l) 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 (-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 {micro}g/{micro}l 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. Key MessageInsects cause severe damage to numerous crops and their control relies on pesticides. Green control is becoming increasingly popular due to concerns about the negative impacts of pesticides on the environment. Phytoecdysteroids are found in Ajuga plants and affect a wide range of insects at very low concentrations. Here we demonstrate that crude extract from Ajuga iva alters the development of Spodoptera littoralis. Phytoecdysteroids may therefore be beneficial in IPM programs.