Oligophagous

The antiherbivory effects of phytoalexins have been studied in our laboratory using the Mexican bean beetle and the soybean looper (44). The Mexican bean beetle, Epilachna varivestis. is an oligophagous species that feeds preferentially on legume hosts. 

Insect herbivores can be divided into two large groups whose strategies with respect to the plant s defense chemistry differ substantially (75). The polyphagous species can exploit a wide range of host plants, whereas the mono-/oligophagous insects are often specialized on one or a small number of (often systematically related) hosts. 

In contrast, mono- and oligophagous species often select their host plants with respect to the composition of the nutrients and secondary metabolites present. For these specialists the originally noxious defense compounds are often attractive feeding and oviposition stimulants. These insects either tolerate the natural products or, more often, actively sequester and exploit them for their own defense against predators or for other purposes 1,4,10-12,14-17,28,31,33,494-496). These observations seem to contradict the first statement, that secondary metabolites are primarily defense compounds, and a number of renowned authors have fallen into this logical pit, such as Mothes 35) and Robinson 505). However, these specialized insects are exceptions to the general rule. For these specialists, the defense chemistry of the host plant is usually not toxic, but they are susceptible to the toxicity of natural toxins from non-host plants 32). As compared to the enormous number of potential herbivores, the number of adapted monophagous species is usually very small for a particular plant species. 

In summary, the Delphinium diterpene alkaloids cardiopetamine (30) and 15-acetylcardiopetamine (27) are potent insect antifeedants active on two insect species with different feeding adaptations (a polyphagous Lepidopteran and an oligophagous Chrysomelid beetle), suggesting a potential broad range of antifeedant action for this class of compounds [20]. 

On the other hand the oligophagous Mexican bean beetle clearly was deterred from feeding on the phytoalexin-rich tissue. Since this insect is not easily reared on artificial diets, the direct toxicity of glyceollin to the Mexican bean beetle could not be determined in these experiments. This is a common experimental problem when determining the effect of a given compound in vivo. 

Other antifeedants such as shiromodiol diacetate (50) have been isolated from Parabenzoin trilobum (syn. Lind-era, Lauraceae). These compounds are active against feeding by polyphagous and oligophagous insects at the 0.05-0.1% level in artificial diets (Mabry and Gill, 1979 Munakata, 1977). 

Soybean saponins have been shown to be detrimental to the larvae of Callosobruchus chinensis, an oligophagous insect that lives on several legumes but that cannot live on soy beans, peanuts, and certain other legumes. 

Thorsteinson, A. J. (1953) The chemotactic basis of host plant selection in an oligophagous insect (Plutella maculipennis (Curt.)). Can, J, ZooL, 31, 52-72. 

Cottee, P.K., E.A. Bemays J.A. Mordue. 1988. Comparisons of deterrency and toxicity of selected secondary plant compounds to an oligophagous and a polyphagous acridid. Entomol. Exp. Appl. 46 241-247. 

In a broad spectrum feeding study, which included some steroidal alkaloids as well as many other secondary plant products, an oligophagous locust, Locusta migratoria, was offered more than 100 different plants as food material. Most dicots, which contained higher quantities of alkaloids, were rejected and monocots were by far the preferred food source. Extracts of the rejected dicots inhibited feeding of artificial diets while extracts from the monocots did not affect feeding of the artificial diets (2). Of more than 100 chemicals which were added to artificial diets to test effectiveness as a feeding deterrent, alkaloids and monoterpenoids were consistently the most effective at the lowest concentrations. The alkaloids which were tested in this study were as shown in Table 2.2. 

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