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Control of the crop pest African cotton leafworm, <i>Spodoptera littoralis</i> (Boisduval), by chemical insecticides has led to serious resistance problems. <i>Ajuga</i> plants contain phytoecdysteroids (arthropod steroid hormone analogs regulating metamorphosis) and clerodanes (diterpenoids exhibiting antifeedant activity). We analyzed these compounds in leaf extracts of the Israeli <i>Ajuga iva</i> L. by liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) and thin-layer chromatography (TLC), and their efficiency at reducing <i>S.</i><i>littoralis</i> fitness. First and third instars of <i>S. littoralis</i> were fed castor bean leaves (<i>Ricinus communis</i>) smeared with an aqueous suspension of dried methanolic crude extract of <i>A. iva</i> phytoecdysteroids and clerodanes. Mortality, larval weight gain, relative growth rate and survival were compared to feeding on control leaves. We used ‘4’,6-diamidino-2-phenylindole (DAPI, a fluorescent stain) and phalloidin staining to localize <i>A. iva</i> crude leaf extract activity in the insect gut. <i>Ajuga iva</i> crude leaf extract (50, 100 and 250 µg/µL) significantly increased mortality of first-instar <i>S. littoralis</i> (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 per day compared to the relevant controls), ultimately resulting in few survivors. Crude leaf extract (250 µg/µL) reduced gut size, with relocation of nuclei and abnormal actin-filament organization. <i>Ajug iva</i> extract has potential for alternative, environmentally safe insect-pest control.