Abstract
Simple SummaryPest insects cause tremendous damage and losses to global agriculture, and their control relies mainly on chemical insecticides, which are greatly harmful for human health and the environment. Many Ajuga plant species have secondary metabolites such as phytoecdysteroids (analogues of insect steroid hormones—ecdysteroid) that control insect development and reproduction. In this study, the effect of ingestion of Ajuga iva phytoecdysteroid plant extract on the growth and development of the African cotton leafworm Spodoptera littoralis was carried out. Our results clearly showed the susceptibility of S. littoralis to phytoecdysteroid ingestion. Crude leaf extracts and fractionated phytoecdysteroids significantly increased mortality of first-instar S. littoralis by up to 87%. Third-instar larval weight gain decreased significantly by 52%, and crude leaf extract (250 ppm) reduced gut size, with relocation of nuclei and abnormal actin-filament organization compared to the controls. In conclusion, the use of phytoecdysteroids against pest insects is not an alternative for chemical insecticides, but they could have an important role in integrated pest management strategies for controlling S. littoralis and possibly other lepidopterans.Control of the crop pest African cotton leafworm, Spodoptera littoralis (Boisduval), by chemical insecticides has led to serious resistance problems. Ajuga 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 Ajuga iva L. by liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) and thin-layer chromatography (TLC), and their efficiency at reducing S. littoralis fitness. First and third instars of S. littoralis were fed castor bean leaves (Ricinus communis) smeared with an aqueous suspension of dried methanolic crude extract of A. iva 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 A. iva crude leaf extract activity in the insect gut. Ajuga iva crude leaf extract (50, 100 and 250 µg/µL) significantly increased mortality of first-instar S. littoralis (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. Ajug iva extract has potential for alternative, environmentally safe insect-pest control.
Highlights
The African cotton leafworm Spodoptera littoralis (Boisduval) is considered one of the most serious pests of cotton, maize, rice, alfalfa, potato, tomato, ornamentals, and orchard trees [1]
We examined the potential use of phytoecdysteroids and clerodanes extracted from Ajuga (Lamiaceae) plants to control S. littoralis
The first and third instars of S. littoralis used for the bioassays were from Murad Ghanim’s laboratory, Department of Entomology, Agricultural Research Organization (African cotton leafworm colony) reared on castor bean leaves under laboratory conditions (25 ± 1 ◦ C, 40% relative humidity with a 12–12 h light–dark cycle)
Summary
The African cotton leafworm Spodoptera littoralis (Boisduval) is considered one of the most serious pests of cotton, maize, rice, alfalfa, potato, tomato, ornamentals, and orchard trees [1]. It feeds year-round on the leaves of numerous old- and new-world plant species [2]. Most of them possess a cholest-7-en-6-one carbon skeleton (C27) and are synthesized from phytosterols in the cytosol through the mevalonic acid pathway [4] They can mimic insect 20-hydroxyecdysteroid, bind insect ecdysone receptors, and elicit the same responses [7]. Ecdysteroids are not toxic to mammals, because their structure is quite different from mammalian steroids, and they are not expected to bind to vertebrate steroid receptors [8]
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