In the search for new plant‐derived products with antifeedant and insecticidal potential against economically important insects, 11 grindelanes (two as natural acids and nine as methylated derivatives) purified from the foliar tissue of Grindelia chiloensis (Cornel.) Cabrera (Asteraceae) were evaluated against the fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae). Larvae of this moth are considered to be the main pest of maize that causes severe losses on local agricultural production. Remarkable alterations in feeding behavior, larval weight, larval and pupal stage lengths, and adult fertility, as well as malformations and mortality in specimens during their life cycle were noticed in the feeding preference tests and no‐choice diet assays. Antifeedant indexes (AI%) calculated in food preference bioassays reached up to 79% for the lipophilic grindelane 3b, the most potent antifeedant agent. Grindelane 7b showed a better insecticidal activity (median lethal dose, LD50 = 26.49 µg g−1) and 8b was potentially neurotoxic due to acetylcholinesterase inhibition. Ultrastructure of the middle portion of the digestive tract of larvae fed with small amounts of natural grindelane 9a supplemented to the diet revealed cell damage, and thickening of the peritrophic membrane and a striking number of mitochondria, as evident signs of defense against toxic substances ingested by the larvae. Besides, this study demonstrated that these effective insecticides did not have inhibitory effects on beneficial plant and soil bacteria which would allow ecological pest management.
The new methylated grindelane diterpenoid, 7β-hydroxy-8(17)-dehydrogrindelic acid (1b), together with the known 7α-hydroxy-8(17)-dehydrogrindelic acid (2a), 6-oxogrindelic acid (3a), 4β-hydroxy-6-oxo-19-norgrindelic (4a), 19-hydroxygrindelic acid (5a), 18-hydroxygrindelic acid (6a), 4α-carboxygrindelic acid (7a), 17-hydroxygrindelic acid (8a), 6α-hydroxygrindelic acid (9a), 8,17-bisnor-8-oxagrindelic acid (10a), 7α,8α-epoxygrindelic acid (11a), and strictanonic acid (12a) as methyl esters were obtained from an Argentine collection of Grindelia chiloensis (Cornel.) Cabrera. Their structures and relative configurations were established on the basis of spectroscopic analysis. CHCl extract from the aerial parts and their pure compounds were evaluated for their antifungal and depigmenting effects. Methyl ester derivative of 10a (10b) exhibited a remarkable mycelial growth inhibition against Botritis cinerea with an IC of 13.5 μg ml . While the new grindelane 1b exerted a clear color reduction of the yellow-orange pigment developed by Fusarium oxysporum against UV-induced damage.
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