Insect-host plant relationships

Jermy, T., Insect host-plant relationships Coevolution or sequential evolution, Symp. Biol. Hung., 16, 109-113 (1976). 

Feeding. The feeding behavior of phytophagous Insects has been studied much more widely than other aspects of the Insect/plant relationship. The reason for this probably lies In the relative ease with which bloassays can be performed and the results Interpreted. Many Insects can be reared on artificial diets, and the effects of added plant constituents can readily be determined. Some early studies by Dethler (29) demonstrated a correlation between larval food choice and the presence of specific chemicals In the umbelliferous host plants of Paplllo polyxenes. However, many of the compounds typically found In the Umbelliferae are also present in other... 

A rather precise relation exists between dosage and insect response. The dosage, in the case of oil sprays, is generally expressed in terms of concentration of toxicant in the spray mixture, even though there may be little relationship between the actual oil deposit and the concentration of oil in the spray mixture (3). Comparisons made between oil emulsions of equal concentrations but prepared with different emulsifiers have resulted in considerable confusion. Dosage in terms of oil concentration in the spray is directly proportional to the actual dosage deposited on the host plant only when the emulsifier as well as the concentration is kept constant. 

Byers J. A., Wood D. L., Browne L. E., Fish R. H., Piatek B. and Hendry L. B. (1979) Relationship between a host plant compound, myrcene, and pheromone production in the bark beetle, Ips paraconfusus. J. Insect Physiol. 25, All-Ail. 

Bioratlonal methods of control based on altered insect-plant relationships are being effectively used, if we extend this term to describe application of host plant resistance. Such control methods have been developed empirically but the rational basis for further development will be the outcome of new genetic studies. 

Potential research topics. Because of their steroidal structure, the brassinosteroids offer many potential avenues for research both in phytochemistry and zoochemistry. If, for example, the presence of the brassinosteroids is relatively ubiquitous in plants, is it then possible that they play a role in host parasite relationships It is of note that many plant parasitic and intestinal nematodes must obtain their cholesterol precursors from their respective hosts (18), a paradigm of the former being the plant gall inducing nematode Ditylenchus dipsaci and of the latter the somewhat large intestinal nematode Ascaris lumbricoides. Certain phytopathogenic fungi also have a sterol requirement for completion of their life cycle (19), as do certain insects (20). 

Jermy, T. (1976) Insect/hostplant relationship - Co-evolution or sequential evolution In The Host-plant In Relation to Insect Behavior and Reproduction (Jermy, T., ed.) pp. 109-13. Plenum Press, New York. 

The precise role of plant secondary metabolites and their interactions with insect herbivores have been focal points for research by chemists, botanists and entomologists for many years. Alkaloids and their glycosides are frequently involved as feeding deterrents. Chapter 2 treats the relationships between the chemistry of alkaloids in host plants and the effects that these compounds may have on insect herbivores. Interestingly, an alkaloid produced by a plant may manifest different effects on different insects. 

The great diversity of parasitoids and the complexity of parasitoid-host relationships is probably influenced by attempts of potential host insects to escape their predators, parasites and parasitoids. There is considerable speculation on the role that parasites (parasitoids) play in herbivore evolution. There are many examples where a host on different plants is attacked by different parasitoid species (see Vinson, 1981). As discussed by Zwolfer and Kraus (1957), and Vinson (1981), plants play an important role in the host selection process, probably by providing cues to the location of a potential host community. Theoretically, a host could escape a particular parasitoid by attacking a plant lacking those stimuli used by the parasitoid to locate the potential host community. This idea is supported by the observation that there is less tendency for parasitoids to select phylogenetically related hosts than to favor a range of hosts on a particular plant (Askew and Shaw, 1978 Cross and Chesnut, 1971). 

For convenience we have included the chemical structures of the alkaloids discussed in Table 2.3. Table 2.4 is a representative sample of specific host alkaloids that have been linked to a specific biological activity with a particular insect. Although Table 2.4 is not comprehensive for all alkaloids and all plant/insect interactions, it does serve as a useful guide for further investigations of the evaluation of structure/function relationships in alkaloids. 

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