Structure- and species-dependent insecticidal effects of neo-clerodane diterpenes

Several natural neo-clerodane diterpenoids isolated from Linaria saxatilis and some semisynthetic derivatives were tested against several insect speci es with different feeding adaptations. The antifeedant tests showed that th e oliphagous Leptinotarsa decemlineata was the most sensitive insect, follo wed by the aphid Myzus persicae. The polyphagous Spodoptera littoralis was not deterred by these diterpenoids; however, following oral administration, some of these compounds did have postingestive antifeedant effects on this insect. In general terms, the antifeedant effects of these compounds were species-dependent and more selective than their toxic/postingestive effects ; The study of their structure-activity relationships showed that both the decalin moiety and the chain at C-9 determined their bioactivity. Furthermo re, the presence of a 4,18-epoxy/diol moiety was an important feature for b oth the antifeedant and the toxic/postingestive effects.