Insect deterrent

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 IV gives a few examples of terpenes shown to be Insect deterrents. As with the previous examples of Insect attractants In Tables II and III, the structural diversity of the deterrent compounds Is remarkable. There are no clear and logical structure-activity relationships among the compounds with these behavioral effects. Specialised and unique effects and behavioral adaptations are thus the rule In Interactions between species rather than the exception.

Not surprisingly, although the evolution of sequestered microbial toxins appears to be rather widespread in marine environments, sequestration of defensive alkaloids in the apparent absence of microorganisms may generally characterize the chemical defenses of terrestrial animals. Careful searches for possible microbial syntheses of defensive compounds (allomones) have not been generally implemented, but recent studies in a few laboratories raise the possibility that microbial endosymbionts may be of major importance in the biogenesis of selected insect deterrents. 

The various substituents (R) of glucosinolate (R-glucosinolate) compounds include alkyl, hydroxyalkyl, aryl (e.g. Phe-CH2, jft-HO-Phe, Phe-(CH2)2), indol-3-yl (Phe pyrrole), methylsulfonyl alkyl (GH3—S02—(GH2) ), methylsulfinylalkyl (GH3—SO—(CH2) ) and methylthioalkyl (CH3—S—(CH2) ) groups. These give rise to the corresponding isothiocyanates (R—N=C=S) that can have particular bioactivities such as insect attractant, insect deterrent, cytotoxic, lachrymatory, tastant and odorant activities. 

Echeverri, F., G. Cardona, F. Torres, C. Pelaez, W. Quinones, and E. Renteria, Ermanin An insect deterrent flavonoid from Passiflora foetida. Phytochemistry, 30, 153-155 (1991). 

Cardenolide structure is closely related to bufadienolides, but these 23C steroids possess a butenolide ring located at C17. As potent cardiotonics, they are widely distributed in plants mostly as glycosides. Digitoxigenin is a typical example of cardenolides. Besides, they are either toxins or insect deterrents. 

---