Impact of - IDL-BNC @ IDRC - International Development Research ...

made possible by close collaboration among plant scientists, entomologists,
chemists, and toxicologists (Miller and Miller 1986; Morgan and Mandava
1990).
Developments in plantinsect studies
The selective pressures brought about by phytophagous insects and other
plant-eating organisms have resulted in the development, in plants, of a
multitude of compounds that are either toxic or modifiers of physiology or
behaviour. Current research has led to the identification and characterization
of some particularly interesting molecules (Morgan and Mandava 1990). It has
also permitted a more meaningful characterization of biochemically based
resistance in plants.
Natural photosensitizers
Light is often forgotten or misunderstood as a factor in the study of insects
(Philogene 1982) and, until recently, little attention has been paid to its role in
plantinsect relations. Our research team looked specifically at the activation
of secondary plant substances by light and their subsequent photosensitizing
effects on insects (Arnason et al. 1983).
Secondary plant compounds with phototoxic action have been identified in
several plant families, but particularly in the Asteraceae. The secondary
metabolites of plants that are capable of photoscnsitizing insects include
furanocoumarins, furanoquinoline alkaloids, beta-carboline alkaloids, polyacetylenes
and their thiophene derivatives, and extended quinones. These
compounds may have direct lethal effects, retard larval development, or be
ovicidal.
Polyacetylenes and their thiophene derivatives are toxic to a broad range of
organisms, especially some insect species. One compound in this group
particularly attracted our attention: aipha-terthienyl (a-T), which occurs naturally
in marigold (Tagetes). We investigated its phototoxic effect on mosquito
larvae and plant-feeding Lepidoptera (Philogène et al. 1985; Champagne et al.
1986; Iyengar et al. 1987). The results have been so encouraging that we are
contemplating the commercialization of ct-T.
Phototoxic acetylenes and thiophenes presumably provide enhanced protection
to the plant by virtue of their involvement in high-energy photochemical
processes and the catalytic nature of singlet-oxygen generation, which they
mediate (Amason et al. 1987a). Without photosensitizing radiation, these
compounds still possess many of the insect-deterrent effects observed with
other nonphotosensitizing secondary plant metabolites: feeding deterrence,
growth reduction, and reduced nutrient utilization. Because a-T is not a
281