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

and P. saucia. Melanoplus sanguinipes suffered 50% larval mortality when
injected with 0.88 mol of the sesquiterpene lactone.
Azadirachtine, a tetramortriterpernoid from the neem tree (Azadirachta indica)
is the most effective and the most promising of all characterized antifeedants
(Jacobson 1983; Koul et al. 1990; Schmutterer 1990). Neem and its byproducts
have been used for centuries in India for protection of stored cereals and
various standing crops and as drugs for all kinds of aliments (Koul et al. 1990).
The greatest obstacle to its use is the limited shelf-life of neem seeds, which
contain the highest titre of oils. There are, nevertheless, several formulations
available in the subcontinent (Repelin, Weligro, Nimbasol, Biosol, and Neemark)
and one formulation has been approved by the Environmental Protection
Agency in the USA (Margosan-O).
Lignans
The toxicity of insecticidal compounds can be significantly increased by the
addition of synergists, i.e., chemicals that increase the lethality of insecticides
given in sublethal amounts (Brindley and Selim 1984). The identification of
sesamin in chrysanthemum flowers where it synergizes the activity of pyrethrum
(Donskotch and El Feraly 1969) triggered interest in the potential
interactions between the numerous secondary metabolites synthesized by
plants.
Lignans are one group of synergists that reduce the efficacy of detoxification
enzymes produced by insects that have been subjected to toxic compounds:
more than 200 lignans have been found in about 50 plant families and 146
species (MacRae and Towers 1984). They are most abundant in the Pinaceae,
Podophyllaceae, Rutaceae, and Lauraceae, with a few occurrences in the
Asteraceae. Lignans alone have limited activity on insects; their effectiveness
is felt in association with synthetic pesticides or with other plant compounds.
Sesamin, sesamolin, sesangolin, asarinin, myristicin, savinin, and hinokinin
are effective in enhancing the activity of a wide variety of insecticides, particularly
pyrethrins and carbamates. They have, therefore, been used to develop
numerous and economically important synergists.
Some lignans of the Asteraceae and Piperaceae (diasesartamin, sesamolin,
cubebin, epiyangambin, and dill apiole) have been examined as potential
inhibitors of the mixed-function oxydases of 0. nubilalis (Bernard et al. 1988).
At concentrations ranging from i0' to 10 M, diasesartamin, dill apiole,
sesamolin, and, to a lesser extent, cubebin were found to be inhibitors of
mixed-function oxydase activity. Epiyangambin had little effect.
Most lignans occur in plants with other toxins. It is, therefore, reasonable to
hypothesize that these compounds have a role in synergizing the biological
activity of these toxic compounds. More research is required to establish how
this chemical capability of plants can be successfully exploited in pestmanagement
strategies.
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