The effect of crude methanolic extracts of Adhatoda vasica leaves on the feeding and performance of Spodoptera littoralis larvae was investigated in the laboratory. Feeding on fresh leaves resulted in 100% mortality of larvae after 26 days of unsubstantial growth. The extract exhibited strong antifeedant and toxic activity against the larvae when applied either on leaf discs or incorporated into artificial diet. Under choice conditions the antifeedant index calculated over 72 h for neonate larvae increased significantly (from 71.5 ± 3.2 to 92.1 ± 4.2) as the concentration of extract in the treated diet increased from 200 to 1000 ppm. Consumption by the sixth instar larvae of leaf discs dipped in 0.01, 0.1 and 0.2% extract solutions was significantly lower than consumption of control discs in both choice and no‐choice tests. The latter two concentrations deterred feeding by 63.4 and 90.4%, respectively, under choice conditions, while only the 0.2% extract solution deterred feeding by 56.8% in the no‐choice test. Toxicity of the extracts was manifested by a high mortality, reduced growth rates, and low weight gain by larvae fed on diets containing 200–2000 ppm of the extract. No larvae survived to pupation under the latter concentration. The time to pupation increased from 15.8 ± 0.4 to 37.9 ± 4.1 days as the extract concentration in diet increased from 0 to 1000 ppm. When fed to the fifth instar larvae, the crude extract significantly reduced consumption, growth, utilization of ingested and digested food, and approximate digestibility. The consumption‐dependent growth efficiency of animals fed on extract‐free diet was significantly higher than the growth efficiency of animals fed on extract‐containing diets, suggesting both antifeedant and toxic activities of the extract.
Summary1. Plants produce herbivore-induced plant volatiles (HIPVs) in response to damage by herbivores. Although HIPVs are known to enhance plant resistance by affecting herbivore host plant preferences and by attracting natural enemies, little is known about the role of HIPVs on the resistance of neighbouring plants and the mechanism behind this associational resistance. 2. This study examined the effect of HIPVs from herbivore-damaged host plants (alfalfa (Medicago sativa), clover (Trifolium alexandrinum) and cotton (Gossypium hirsutum)) on oviposition by Egyptian cotton leafworm (Spodoptera littoralis) (Lepidoptera: Noctuidae) on neighbouring, undamaged host plants.3. There was a significant reduction in oviposition by S. littoralis on undamaged plants adjacent to herbivore-damaged cotton plants under both field and laboratory conditions. The results showed that the associational resistance by HIPVs depends on direct effects on oviposition behaviour in S. littoralis. There were also indications that other mechanisms may be involved. 4. Associational resistance via HIPVs was not observed for all plant species tested. Emission of HIPVs from damaged cotton increased the resistance of undamaged cotton and alfalfa plants to oviposition by S. littoralis, but HIPVs from damaged alfalfa and clover neighbours did not provide resistance to undamaged cotton plants. 5. Synthesis. Our results suggest that the presence of HIPV-emitting plant neighbours can reduce herbivory on undamaged plants and enhance plant resistance by affecting oviposition behaviour in insect herbivores.
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