Antifeedant and toxic activity of some plant extracts against larvae of cotton leafworm Spodoptera littoralis (Lepidoptera: Noctuidae).

The biological activity of crude petroleum ether extracts of Oshar (Calotropis procera); Harmal (Rhazya stricta) and Hargal (Solenostemma argel) were assessed using the 4th larval instar of cotton leafworm Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae). All extracts exhibited a significant antifeedant activity at the LC50 levels. Harmal extract deterred feeding potential of insect larvae by 52.96% but decreased to 26.76 and 18.00% for Hargal and Oshar, respectively. In nutritional assays, all plant extracts affected Growth Rate (GR mg) where Harmal caused the highest rate of efficiency and followed by Oshar against 4th larval instar fed for two days on castor leaves treated with botanical extracts (LC50) and three days on un-treated leaves after that. Hargal didn't show a significant effect on growth rate compared with un-treated larvae. The differences between Consumption Index (CI mg) of larvae treated with Hramal and Hargal after five days of feeding in comparison with un-treated larvae were significant. Efficiency of Conversion of Ingested food (ECI%) to biomass reached 31.81 +/- 1.49, 26.06 +/- 1.89 and 48.67 +/- 1.54 after five days of treatment by Oshar, Harmal and Hargal, respectively. These values were 49.61 +/- 1.72, 39.12 +/- 0.54 and 53.20 +/- 0.96 for digested food utilization (ECD%) in larvae treated with the aforementioned extracts after five days, respectively. There was a remarkable inhibitory activity of plant extracts on the digestive carbohydrate enzymes, amylase and invertase in vitro. It was noticed that Harmal had a remarkable inhibitory action causing an average of 42.58% inhibition rate on amylase and 16.27% on invertase followed by Hargel and Oshar with inhibition rates of 33.27 and 19.58% against amylase, while these values averaged 11.19 and 5.97% of inhibition in case of invertase, respectively.