Cereals (DAC, 2016). Wheat is rich in carbohydrates

Cereals
are the most important food grains all over the world. Wheat is one of the
oldest and most important cereal crop, belonging to Triticaceae tribe of
Poaceae family (Briggle et al., 1963). Different ploidy levels of wheat
make it different from other cereal crops. Most cultivated varieties belong to Triticum
aestivum, which is a hexaploid wheat commonly called as bread wheat. Wheat
is a rabi season crop with a global production of 694 million metric tons (Ahmad et al.,
2017). It was the second most produced cereal in India with an estimated production
of 96.64 million MT during 2016-17 (DAC, 2016). Wheat is rich in carbohydrates
(55%) and it meets 20% of global calorie requirement (Breiman and Graur, 1995).
Apart from carbohydrates, wheat grains has a protein content of 10-18%, which
is far higher as compared with other cereals. Demand for wheat and its products
is also rising with population and change in food habits (World Bank, 1989).
Wheat holds a significant position in India’s economy (Chowdhury et al.,
1998).

Wheat
kernels remain for few weeks on plant after getting matured, but these have to
be stored for several months to years before processing (Hagstrum et al.,
1999). Storage loss of wheat may go up to 50% in some countries (Fornal et
al., 2007). In India, 12-16 million MT of post-harvest loss of food grains
has been estimated each year (Singh, 2010). A loss of 96 million MT of cereal
grains was reported due to storage pest attack, which was enough to feed 375
million people for a year (Dobrovsky, 1965). Most important reason for this
heavy storage loss is infestation of post-harvest insect pests and fungi (Maier
et al., 1997). Wheat is vulnerable to storage pest infestation resulting
in quantitative and qualitative losses, which vary with the pest species
(Sighamony et al., 1985). Abiotic factors such as temperature and
humidity play an important role in development of these pests in storage. The
losses owing to insects in storage should be prevented.

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Even
though chemical control of storage pests is successful, its indiscriminate use
has resulted in development of resistance, residues in food, environmental
pollution and high cost (Haq et al., 2005).

In order to achieve a
sustainable agricultural system with minimum pesticide usage, higher
productivity, profitability and higher seed production, resistant cultivars can
be used (Hall et al. 2003). Resistance is
the relative amount of transmissible characters possessed by a plant or its
materials (eg. seeds), which affect the extent of damage done by insects
(Mbata, 1997).

Protection
of grains with the use of resistant varieties is acceptable, as they are
environmentally safe, effective and cheap (Helbig, 1997). HPR can be incorporated
in crop plants, which impart the ability to prevent or delay or withstand pest
infestations (Kumar,1984) and thus improving the quality and yield of crop or
stored product. As far as farmers are concerned, one of the efficient and
economic way of controlling stored pest is by the use of resistant varieties. Among
the three approaches of HPR viz, tolerance, antibiosis and antixenosis,
antibiosis and antixenosis remain as sources of resistance against storage
insects.

In
order to develop a resistant variety, its sources of resistance need to be
identified, which can be done by revealing the causes of resistance (Somta et al., 2006). It
has been reported that wheat cultivars showed varying resistance levels against
storage insect pests (Cortez Rocha et al., 1993).The levels of
resistance were found to be influenced by nutritional and biochemical
components in cereal varieties along with the physical properties of grains. These
factors also influenced the key digestive enzymes of insects thereby affecting
their normal growth and development. Thus resistant varieties of wheat with
favourable physico-biochemical characteristics and digestive enzyme inhibitors can be used as an
effective management measure against insect pests in storage.

In
storage, seed quality, weight, viability and commercial value are reduced due
to insect pest infestation. Coleopterans constitute seventy percent of those
insect pests (Vinu uela, 1993). Insect
species found in stored wheat include Tribolium spp., Sitophilus
oryzae, Oryzaephilus surinamensis, Rhyzopertha dominica, Cryptolestes spp.,
Ahasverus advena and Typheae sp. (Hagstrum et al.,
1999). Most damaging ones are rice
weevil (Sitophilus oryzae) (Coleoptera: Curculionidae) and the lesser
grain borer (Rhyzopertha dominica) (Coleoptera: Bostrichidae) (Bekon and
Fleurat, 1992). In both cases, adults damage grains and larvae inhabit and feed
inside the grain. They feed on significant portion of kernel and contaminate
the stored grains with their faeces and skin castings. They also cause an
increase in heat and moisture content that cause secondary infestation by fungi
and bacteria.

Among the insect pests attacking wheat grains
in storage, Rice weevil- Sitophilus oryzae (L.) is the most devastating
one. S. oryzae originated in India and disseminated worldwide (Cotton,
1920). Favourable conditions for development are at a temperature range of
25-30°C and low RH (Batta, 2004). It is reported as the most ruinous and
extensive cereal pest in the world (Champ & Dyte, 1976), causing 18.30%
loss to grains in storage (Adams, 1976). S. oryzae is one among the
major restrictions for large scale wheat production. It can cause 48.4% of grain
loss in wheat (Tiwari et al., 1989) with its adult feeding on 19% and larvae on
16.2% of wheat grains.

Rice weevil also
feeds on corn, rice, oat, sorghum, buck wheat ear, barley and their products.
The life cycle of S. oryzae is comparatively short and hence population
builds up easily (Aitken, 1975). These are small, stout weevils with reddish
brown to black colour. Female lays only one egg in a grain and covers the hole
after egg laying with a gelatinous material. The larval period is for 18 to 34
days, during which the larva consumes grains at the rate of 14 mg grain / day
(Giolebiowska, 1968). Grains are reduced to hollow shells as a result of
feeding, especially on starchy content. Male and female of S. oryzae can
be distinguished by examining the rostrum, which is long and smooth in female
as compared to male. The infestation by adult weevils can result in rise in
temperature in grain heaps, further leading to mould growth (Suleman et al.,
2000).