<jats:title>Abstract</jats:title><jats:p>The effects of crude extracts, fractions and isolated compounds from <jats:italic>Vitex polygama</jats:italic> Cham. and <jats:italic>Siphoneugena densiflora</jats:italic> Berg were evaluated on the development of <jats:italic>Spodoptera frugiperda</jats:italic> JE Smith, a destructive insect pest of corn and several other crops. The extracts and fractions were incorporated into an artificial diet at 1 mg g<jats:sup>−1</jats:sup> and offered to the insect during its larval stage. Length and viability of larval and pupal stages as well as pupal weight were assessed. Isolated compounds were tested through superficial contamination of the diet at 0.1 mg g<jats:sup>−1</jats:sup>. Weight and viability of ten‐day‐old larvae were determined. Methanolic and hydroalcoholic <jats:italic>S. densiflora</jats:italic> extracts caused 100% larval mortality, while leaf and fruit hydroalcoholic extracts from <jats:italic>V. polygama</jats:italic> were the most active. Among the isolated compounds, flavonoids presented the best insecticidal results, and tannins the best larval growth inhibition. Copyright © 2006 Society of Chemical Industry