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Molecular Entomology
19
of any other staple food in Africa (WARDA, 2008).
Four out of the eleven largest rice importing countries
in the world are within Sub Saharan Africa with Nigeria
as the world’s largest importer (WARDA, 2007).
The rice plant is vulnerable to attack by various kinds
of insects from the time the seed is planted until the
grain is harvested, during storage and distribution.
Rice is often attacked by various storage insects,
among these are
Sitophilus oryzae
Linn and
Sitophilus
zeamais
Motsch. The most important and most
destructive of these insect pests is
S. oryzae
which can
fly to the field and attack grain before it is harvested
in tropical and sub-tropical regions. Most of the
damage to grains is done by the larvae, which devour
most the whole endosperm leaving only the hull.
(Nilsa and Bosque–Perez, 1992), causing between
25% to 100% post-harvest losses in storage (Okonkwo,
1998). They build up in large numbers in stored grain
and causes unflavored taste and odour which made
rice unpalatable for consumer. (Metcalf, 1994; Charles
et al., 2003). In addition to causing direct loss,
S.
oryzae
also reduces seed viability and contaminates
the product with excrements, thus reducing the market
and nutritional value of the food (Lale, 1992). Owing
to their insidious feeding habits, they are often
undetected until damage has occurred.
Rice varieties exhibit varying degrees of susceptibility
to damage by insects. One of the ways of reducing
post-harvest losses of grains is breeding for varietal
resistance. The factors that confer resistance to the
grains against infestation by a variety of storage
insects are varied. Stout et al. (2001) documented
significant variation in the susceptibilities of modern
commercial varieties to infestation by weevil larvae.
Despite these studies, very little effort has been made
to incorporate weevil resistance into commercial
varieties or to integrate plant resistance into
management programs for this insect. Various works
have been done on the resistance or susceptibility of
different varieties of rice to attack by the rice weevil
(Enobakhare and Wey, 1996; Joda, 1998; Ashamo,
2005). However, there are a lot of varieties yet to be
tested against
S. oryzae
infestation, especially the local
varieties. Therefore in this study, the susceptibility of
some selected imported rice varieties and some local
rice varieties to
S. oryzae
infestation were investigated
under laboratory conditions.
1 Material and Methods
1.1 Experimental location and condition
The study was conducted in the Research Laboratory
of the Department of Crop, Soil and Pest Management
of the Federal University of Technology, Akure
(7°16'N, 5°12'E) Ondo State, Nigeria; under ambient
laboratory conditions of (28 ± 4)
℃
and (65 ± 10)%
relative humidity.
1.2 Source of rice substrates
The improved rice varieties used for this experiment
were collected from the International Institute of
Tropical Agriculture (IITA), Ibadan, Nigeria. While
the imported varieties were purchase from Erekesan
market, Akure, Nigeria and the local varieties were
sourced from the various growing localities associated
with the various varieties in Nigeria. The varieties and
some of their characteristics are provided in Table 1.
Table 1 Morphological characteristics of the selected rice varieties used in the experiment
Variety
Local name
Colour
Type
Shape
Form cultivated
Imported
Cisadene
White
Short grain
Fairly robust
Lowland
Caprice
Aroso
Shiny brown
Long grain
Slender
Lowland
Improved
ITA257/Faro 45
Yellow Mama Africa
Yellow white
Long grain
Robust
Upland
ITA 315/Faro 49
Sara Portland Rice
Dirty white
Short grain
Robust
Upland
ITA 222/ Faro 36 Local White Mama Africa
White
Long grain
Fairly robust
Lowland
Isan rice
Dirty white
Short grain
Robust
Lowland
Igbemo rice
Dull white
Long grain
Fairly robust
Upland
Ofada rice
Dirty white
Short grain
Robust
Upland
Erio rice
Dirty white
Short grain
Robust
Upland
Ayede Ekiti rice
Dull white
Short grain
Robust
Upland