Leaf Chemistry and Foliage Avoidance by the Thrips Frankliniella occidentalis and Heliothrips haemorrhoidalis in Glasshouse Collections

Observational studies on foliage avoidance by the polyphagous thrips species Frankliniella occidentalis (Pergande) and Heliothrips haemorrhoidalis (Bouch,) (Thysanoptera: Thripidae) identified six non-host species (Allagopappus dichotomus (Asteraceae), Gardenia posoquerioides (Rubiaceae), Plectranthus aff. barbatus, Plectranthus strigosus, Plectranthus zuluensis (Lamiaceae), and Sclerochiton harveyanus (Acanthaceae) among plants growing within a major glasshouse botanical collection. The effects of sequentially obtained acetone and aqueous methanol leaf extracts on mortality in first instar Frankliniella occidentalis were assessed. The acetone leaf extract of Sclerochiton harveyanus, which had the highest activity against the thrips, yielded four new iridoids, sclerochitonosides A-C, and sclerochitonoside B 4'-methyl ether. Mortality of F. occidentalis was increased on exposure to all four iridoids, and the most active iridoid was sclerochitonoside A (8-epiloganic acid 4'-hydroxyphenylethyl ester). Choice experiments demonstrated that this compound did not significantly deter H. haemorrhoidalis from treated leaf surfaces. The significance of iridoids in the defense mechanism of plants against thrips is discussed.