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

neurotoxin but a cytotoxin, cross-resistance to organophosphates does not
seem to occur.
The isoquinoline alkaloid, berberine, which is present in over 60 plants and 9
botanical families, was found to possess antifeeding activity on Euxoa messoria
(Devitt et al. 1980) and insecticidal action against larvae of Aedes atropalpus
(Philogene et al. 1984). A slow singlet-oxygen generator capable of producing
cytogenetic damage, berberine had an LC50 (lethal concentration to 50% of
organisms tested) of 8.8 ppm under near-ultraviolet light for 24 h. Larval,
pupal, and adult survival were all significantly affected after a brief initial
exposure of the larval stage, suggesting long-term carry-over effects. Lamentably,
because berberine can intercalate with the DNA molecule, it is an
unlikely candidate as a commercially useful insecticide.
Antifeed ants
Many compounds have been known for years to act as feeding deterrents or
antifeedants (Philogène 1974). Plants contain quite an array of such molecules,
which could be exploited either as components of plant-resistance mechanisms
or as exogenous compounds.
Most known antifeedants are complex molecules that are difficult to synthesize.
They are active at very low concentrations and, consequently, are not
found in large quantities in plants. Even with the best extraction procedures,
it is impossible to obtain quantities that could be used on a large scale and the
process is costly. In spite of this, the search goes on and new molecules are
reported frequently. Over 350 compounds have been identified since 1976
(Wharten and Morgan 1990); only a small number of these molecules are
effective antifeedants.
Our research team has investigated one group - the sesquiterpene lactones.
These secondary plant substances from the Asteraceae have a number of
biological effects on phytophagous insects, suggesting that they have evolved
as deterrents to insect herbivory. These compounds also deter oviposition,
inhibit growth, lengthen development, and are toxic.
The effects of tenulin, a sesquiterpene lactone from bitterweed (Helenium
atnarum) on the European corn borer (Ostrinia nubilalis), the variegated cutworm
(Peridroma saucia), and a polyphagous grasshopper (Melanoplus
sanguinipes) have been examined (Arnason et at. 198Th). The influence of the
compound on the biological activity of these insects was observed at levels
that were much lower than its concentration in leaves of H. amarum (15-
25 mol/g fresh weight). When applied to leaf disks of corn, tenulin reduced
feeding by 0. nubilalis at a concentration of 0.3 mol /g fresh weight and above.
In spite of the potent antifeedant effects, larvae could be induced to feed on
artificial diets containing tenulin at low concentrations. At 5 mol/g in artificial
diets, tenulin reduced growth and delayed larval development of 0. nubilalis
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