Phytochemical Studies of some Indigenous Plants as Grain Protect Ants Against Sitophilus Zeamais (Maize Weevil)
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Date
2013-10-17
Authors
Ng'eno, K Livingstone
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Abstract
Maize farming, though successful in some parts of Africa is greatly hampered by the post harvest
handling procedures. Resource farmers in developing countries like Kenya use different plant
materials to protect stored grains against pest infestation by mixing grains with protectants made
up of plant products. Dry ground plant material, were tested for their activity against the maize
weevil; Sitophilus zeamais; (Coleoptera: Curculionidae). All showed no reasonable protectant
ability. Essential oils extracted were analyzed by gas chromatography-mass spectrometry (Ge-
MS) and evaluated for repellent and insecticidal activities against Sitophilus zeamais using the
area preference and contact methods. Most oil components identified by analyzing their mass
spectra to the NBS library of mass spectra using computer routing were oxygenated
monoterpenoids or phenolic compounds and the oils were active in repelling at doses between
0.005 and 0.125 ppm. The mean mortality values for Lippia kituiensis and Chrysanthemum
cinerariaefolium (pyrethrum) was not significantly different after 24 hrs of exposure with an
average activity of 79 %. Chenopodium chenopoides, Ajuga remota and Ricinus communis were
not significantly different after 24 and 48 hrs of exposure with a mean of 18 and 20% activity,
respectively. The mortality rate of S. zeamais by contact increased with the exposure time and
the concentrations of the essential oils. The most active oil extracts of plants were those of
Eucalyptus globulus, Rosemarinus officinallis. Lantana camara, Lippia kituiensis, Azadirachta
melia and Azadirachta indica with LOso values as 0.609, 0.199, 0.171, 0.220, 0.399 and 0.333,
respectively. These oil extracts were reasonably comparable to the already known adulticidal and
repellent pesticides such as pyrethrum with its LDso being 0.245. The constituents which may be
more responsible for the maize grain protectant ability includes 1,8-cineole, limonene, u-pinene,
~-pinene, a-terpineol, terpinen-4-ol, and globulol. Blends of plants whose activity is known were
made and tested against the maize weevil.' A blend of Eucalyptus globulus and Rosemarinus
officinallis at 1.0% concentration gave a PD and WPI of 3.5 and 7.2, respectively. These values
are far much lower than their individual PO and WPI values of 15.3 and 12.3, 25.2 and 21.3,
respectively. This therefore gives the impression that blend effects in the bioactivity of plant
products may well be the norm to follow. Mortality tests by inert dusts were also carried out.
Woodash, Kaolin and Bentonite showed high activities of upto 70 % at 5 % concentration after
/68 hours. Gypsum Whiting dusts was not comparatively active since they only exhibited
activities below 40 % even at 10 % concentrations after 168 hours. This implies that a conclusive
research carried out using standard conditions could reveal all the responsible components for
such high activities in all the tests carried out.
Description
Chemistry Department