Diterpenes insects

I. Kubo, T. Matsumoto and N. Ichikawa, Absolute configuration of crinitol, an acyclic diterpene insect growth inhibitor from the brown algae Sargassum tortile, Chem Lett., (1985) 249-252. 

Klocke, j. a., M. Hu, S. Chiu, and I. Kubo, Grayanoid diterpene insect antifeedants and insecticides from Rhododendron molle. Phytochemistry, 30, 1797-1800 (1991). 

Finally, the antifungal properties of a-pinene and limonene, two monoter-penes frequently found in the defensive secretion of Nasutitermes termites, was studied. In vitro assays showed that these molecules reduce spore germination of the fungus Metarhizium anisopliae through direct and indirect (vapor) contact [225]. Moreover, some diterpenes isolated from these insects have also an antibiotic activity on Bacillus subtilis, Staphylococcus aureus, and Enterococcus faecalis [226]. 

The unique structural character of these diterpenes and their exclusive distribution in the family Euphorblacae may allow their use as specific chemotaxonomlc markers within the family. Application of analytical HPLC methods developed in the Isolation of the jatrophane diterpenes to extracts of other E. esula accessions has revealed distinctively different unidentified jatrophane diterpenes (30) among the accessions. The rarity and exclusivity of these compounds within Euphorblacae warrants continued chemical examination and differentiation of E. esula accessions in relationship to successful Insect biological control. 

Bicyclic diterpenes-clerodanes. An important group of Insect antifeedants are the clerodane diterpenoids, which have been isolated from several different plant families (13)(Figure 1). Particularly well studied are the antifeedant activities of caryoptln and clerodin, and their derivatives, from Clerodendron and Caryopteris, Verbenaceae, against the tobacco cutworm Spodoptera litura L. (25-27). 

Cuticular diterpenes-duvanes and labdanes. Cutler have found that the cuticular diterpenes of green tobacco have both allelopathic and insect-deterrent effects (38). Present in the cuticle are duvane and/or labdane diterpenes (Figure 3) The levels of these specific cuticular components are believed to be responsible for the observed resistance of some types of tobacco to green peach aphids Myzus persicae (Sulzer), tobacco budworm Heliothis virescens (F.), and tobacco hornworm Manduca sexta (L.) (39). 

Table I. Diterpenes with Known Biological Activity against Insects... 

Source Family Diterpene(s) Insect affected Reference... 

Figure 6. Grayanoid Diterpenes with insect antifeedant activity.

As many of the same types of diterpenes play an active role in the resistance of other plants to insect attack (see above), it is interesting to investigate the role of diterpenes in Solidago and its interactions with insects. Solidago has the advantage that the behavior and the ecology of many of the associated insects are well known (80). 

A number of diterpenes are known to be active against herbivores ( ). The diterpenes abietic, dehydroabietic, 12-methoxy-abietic, sandaracopimaric, and isopimaric acid serve as feeding deterrents for the larch sawfly, Pristiphora erichsonii in single needles from new shoots of tamarack (Larix laricina) (133). The larvae of this insect do eat tufted needles on short shoots of the same trees. 

Diterpenes are found in nature, mainly in plants, but also in other natural sources, e.g. micro-organisms and insects. The following table presents just a few of these sources, and their major diterpenoidal components. 

The clerodanes are a notable example of feeding deterrents2 and further examples have been discovered recently. The natural insect antifeedant clerodin (100) forms the basis of the name of this type of diterpenes called the neo-clerodane diterpenes. Ateo-clerodanes are compounds with a clerodane skeleton and exactly the same absolute configuration as clerodin. Those that have the opposite absolute configuration are referred to as ent-neo- cl erod an es. 

Ajugarin I (clerodane diterpene) Ajuga remota (Lamiaceae) Insect antifeedant... 

Approximately half of the fifty species of Helianthus have shown some resistance to the major insect pests of cultivated sunflower in greenhouse and field trials (6-12). We began our studies by chemically investigating a small group of Helianthus species which were reported to be especially insect resistant. The major lipophilic secondary metabolites of the aerial parts of these plants proved to be terpenoid compound of two types sesquiterpene lactones and diterpenes. We have now studied 17 species of Helianthus chemically and have found high levels of sesquiterpene lactones and diterpenes in most of these (13-25). Both sesquiterpene lactones and diterpenes have been isolated from species of Helianthus by other workers (26-44). 

Terpenoids isolated in large quantities from resistant species were tested with several sunflower insect pests. Since sesquiterpene lactones were present in much higher concentrations than diterpenes in the most resistant species studied, our investigations emphasized sesquiter-pene lactones. Three species of insects were used in these studies Spodoptera eridania, Melanoplus sanquinipes and Homoeosoma electellum. 

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