Insect antifeedant activity of flavones and chromones against Spodoptera litura.

The antifeedant polymethylated flavones 5-hydroxy-3,6,7,8,4'-heptamethoxyflavone, 5-hydroxy-3,6,7,8-tetramethoxyflavone, and 5,6-dihydroxy-3,7-dimethoxyflavone have been isolated from the cudweed, Gnaphalium affine D. Don (Compositae). These flavonoids and authentic analogues showed insect antifeedant activity against the common cutworm (Spodoptera litura F.). In a previous paper, it was suggested that there was no substituent on the B-ring of the flavonoid for the beneficial antifeedant activity against the common cutworm. These flavonoids having a phenyl group as the B-ring and the chromone as elimination of the B-ring from the flavonoids were used to test the hypothesis of the previously described B-ring effect. The known fact is that Sculletaria baicarensis (Rutaceae) produced the 2-phenyl flavone. Test compounds and their methylated derivatives were prepared from this material for the structure-activity relationship (SAR) study of insect antifeedant activity. In spite of the 2-phenyl flavonoids, some tested compounds did not show any insect antifeedant activity against the common cutworm, although these inactive flavonoids were deficient in the 6-substituent group on the A-ring of the flavonoid. This 6-position-substituted derivative almost showed strong insect antifeedant activity against common cutworm. Moreover, the tested flavonoids having a hydroxyl group as a substituent on any of the positions tended to increase the activity. These results suggested the importance of the 6-position substitution on the flavonoid; however, hydrophilic substituents decreased the activity. Baicalein (5,6,7-trihydroxyflavone) derivatives did not show any activity despite having the 6-substituent derivative. Although the activity of some chromones increased the activity of the flavone, the bulky B-ring was a disadvantage for the antifeedant activity. It was suggested that the charge on C(3) and C(5) of the flavonoid was important for the biological activity. Additionally, an adequate hydrogen bonding property, which is different from lipophilicity, was an advantage for the activity on the basis of a QSAR analysis.

Antifeedant activity of an anthraquinone aldehyde in Galium aparine L. against Spodoptera litura F.

The insect antifeedant anthraquinone aldehyde nordamnacanthal (1,3-dihydroxy-anthraquinone-2-al) was identified in Galium aparine L., and isolated from the root powder of akane (Rubia akane), a member of the Rubiaceae. Structure-activity relationship (SAR) studies using a series of anthraquinone analogues suggested that the aldehyde group on the anthraquinone was more important than the quinone moiety for antifeedant activity against the common cutworm (Spodoptera litura). High levels of nordamnacanthal were found in the seed leaf stage and in callus tissue induced from seedlings of G. aparine, but its concentration decreased with plant development. Since these compounds are natural pigments for dying textiles, we also evaluated the antifeedant activity against the carpet beetle (Attagenus japonicus ), a textile pest was also evaluated. While nordamnacanthal had strong antifeedant activity against the common cutworm, it did not show any antifeedant activity against the carpet beetle. The most effective antifeedant against the carpet beetle was the major constituent in the extract of R. trictorum, lucidin-3-O-primeveroside, a food pigment.