Crude foliar extracts of sixty species from six subfamilies of Australian Lamiaceae were screened by whole organism contact toxicity on the polyphagous mite Tetranychus urticae Koch (Acari: Tetranychidae). In addition, cytotoxicity assessments against insect cell lines from Spodoptera frugiperda J.E.Smith (Lepidoptera : Noctuidae) and Drosophila melanogaster Meigen (Diptera: Drosophilidae) were also made. The findings indicated that the Spodoptera cell line was more susceptible to extracts than the Drosophila cell line and no direct correlation was observed between the two screening methods. However, several interesting relationships were identified. Extracts from sub-families Ajugoideae, Scutellarioideae, Chloanthoideae, Viticoideae and Nepetoideae showed acaricidal activity, whilst only those from Ajugoideae and Nepetoideae displayed potent cytotoxic effects. A range of activities was observed for the 25 species of Plectranthus, 14 of which showed moderate to high contact toxicity against T. urticae. Overall, least promising bioactivity was observed for extracts from the plant subfamily Prostantheroideae, which showed little contact toxicity or cytotoxicity for the 18 extracts studied.
Greatest cytotoxicity was observed from a methanol extract of leaf material of Glossocarya calcicola Domin. Bioassay guided fractionation led to the discovery of three novel clerodane diterpenes, which were not present in the stems or at all in the closely related species G. hemiderma. Compound 1 was characterised as (rel)-10âH-trans-12î-(2-methylbut-2(E)-enoyl)-1â-(isobutanoyl)-6á,13î-dihydroxyclerodan-4(20),8(18)-dien-7,15-dione-15,16-oxide, to which the trivial name calcicolin-A was assigned. The other two compounds had the same skeletal structure and C-12 substituent but in compound 2 (calcicolin-B), the C-1 esterifying group became 2-methylbut-2(E)-enoic acid and in 3 (calcicolin-C) it became 2-methylbutanoic acid. Although insect antifeeding activity was not observed for G. calcicola, cytotoxicity against insect and human carcinoma cell lines was detected.
Greatest acaricidal activity was observed for Plectranthus diversus S.T.Blake when topically applied (LC50 = 0.25% (w/v)). The major compound present in the extract of P. diversus was identified as the known compound 7á,18-dihydroxy-isopimara-8(14),15-diene. These findings were of chemotaxonomic significance as this is the first known occurrence of a pimarane diterpene in Plectranthus, a genus characterised by abietane diterpenes. A number of abietanes were isolated and identified in this study, though none of which were novel. Promising cytotoxicity was observed from P. fasciculatus P.I.Forst., which was attributed to coleon U, a known abietane diterpene with cytotoxic activity.
Other species of Plectranthus showed potential acaricidal activity as a crude extract in particular P. graveolens R.Br. (LC50 = 0.76% (w/v)). Bioassay guided fractionation did reveal some interesting behavioural effects on mites but in general the fractions were less toxic than the whole extract. The hexane-soluble components of the methanol extract were extremely effective against mites and while the mode of action was not fully understood, it is possible that a smothering effect could be the cause. A potted plant trial was also carried out for this species to determine the effects of crude extract. At time = 112 h post treatment, a concentration response in mite mortality was observed. The control and 0.5% (w/v) treatment were not significantly different to one another but had significantly larger numbers of mites than the 1.0 and 1.5% (w/v) treatment. These results showed that the crude methanol extract of Plectranthus graveolens showed acaricidal efficacy against Tetranychus urticae in a leaf disc and a potted plant trial at concentrations of 1% (w/v) or above with no phytotoxic effects.
The repellency and antifeeding effects of some plant extracts were evaluated against Plutella xylostella L. (Lepidoptera: Plutellidae) in a choice and no-choice bioassay. The most outstanding results were observed for Ajuga australis L. against both second and third instar larvae. The active constituents were present as minor compounds, thus detailed spectroscopic analysis was not adequately performed. However, the compounds were believed to be known clerodane diterpenes that have been previously isolated from this and other species of Ajuga.