ABSTRACT
The maize weevil is one of the most important storage pests
of maize in Ghana and Africa as a whole. It causes damage from as low as 20% to
as high as 100% in untreated varieties. Developing resistant varieties has been
identified as an important and environmentally friendly aspect of the
integrated pest management system. However, little is known about the genetic
control of resistance to the maize weevil in Ghana. The main objective of this
research was to understand the genetic control and heritability of resistance
to the maize weevil. The specific objectives were to identify promising
genotypes with resistance to the maize weevil. Five parents were crossed in a
complete Diallel mating design to obtain 20 hybrids. The 25 genotypes were
planted again with two local checks to obtain their seeds. The seeds obtained
from these genotypes were subsequently used in the laboratory evaluation for
the identification of resistance of the genotypes to the three regional
collections of the maize weevils. The laboratory assessment identified parent
TZEEQI 111 as the best parental line for resistance to the maize weevil. It
exhibited highly significant and negative GCA effects for weevil progeny
emergence, percentage weight loss, percentage grain damage and susceptibility
index. It also exhibited a positive and significant GCA effect for Median
development period. Hybrids TZEEQI 111 × TZEEQI 139, TZEEQI 111 × TZEEQI 12,
TZEEQI 111 × TZEEQI 61 and TZEEQI 12 × TZEEQI 66 exhibited significant SCA
effects. Heritability estimates revealed high narrow sense heritability for F1
weevil progeny emergence, percentage grain damage and susceptibility index.
These results suggest the presence of additive and non-additive gene action in
the control of resistance to the maize weevil. Parental lines TZEEQI 111, TZEEQI
139 and TZEEQI 66 performed very well and as such should be considered when
forming base population to initiate breeding programs for resistance to maize
weevils.
CHAPTER ONE
1.0 INTRODUCTION
Maize (Zea mays L.) is cultivated
extensively worldwide (FAOSTAT, 2014) and the highest ranked cereal in terms of
grain yield per hectare in the world (M’mboyi et al., 2010). Worldwide
production of maize in 2014 amounted to 2,039,153,437 tonnes (FAOSTAT, 2014).
The United States of America is the highest producer of maize in the world
(FAOSTAT, 2014). The top ten maize producing countries in Africa are South
Africa, Nigeria, Ethiopia, Tanzania, Malawi, Kenya, Zambia, Uganda, Ghana and
Mozambique (FAOSTAT, 2014). In sub-Saharan Africa, maize is the most important
and the most widely cultivated staple food, occupying an area which is in
excess of 33 million hectares annually (Macauley, 2015). An estimated yield of
less than 1.8 t/ha is realized on farmers’ fields in Africa as compared to the
average worldwide yield of 5 t/ha (Macauley, 2015). Maize yield in Ghana on
farmers’ fields is estimated at 1.7 t/ha (MoFA, 2011).
Maize has several uses for
different people all over the world including human food, livestock feed, and its
use in several industrial products (Zunjare et al., 2015). About 66% of the
maize produced worldwide is used in the livestock industry for the feeding of
livestock, 25% for human consumption and 9% for industrial purposes (Verheye,
2010). In developing countries, however, over 50% of the maize produced is
consumed as food by humans. In sub-Saharan Africa, maize production is so
important that low maize production is frequently linked with famine and
scarcity of food (Oppong, 2013).
Maize comprises approximately 10%
protein, 72% starch, and 4% fat, contributing 365 Kcal/100 g of energy (Ranum et
al., 2014). Maize also provides most of the vitamin B’s but lacks vitamin B12
and vitamin C. It is also a good source of fibre. Maize is however lacking in
two important amino acids, specifically tryptophan and lysine (Ngaboyisonga and
Njoroge, 2014).
In Ghana, maize is the most
important cereal (FAOSTAT, 2014). However, maize is produced predominantly by
smallholder farmers in Ghana under rainfed conditions (Ragasa et al., 2013).
It is estimated that a greater part
of the maize produced in West Africa yearly, is damaged in storage before
reaching the consumer (Hell et al., 2000). It is also estimated that about $4
billion worth of maize grains is lost after harvest in sub-Saharan Africa each
year (FAOSTAT, 2014). The greatest damage is caused by insects (Ukeh et al.,
2012). Insects damage 15 to 50% of the total maize produced each year in
developing countries (Suleiman et al., 2015).
The maize weevil (Sitophilus
zeamais Motschulsky (Coleoptera: Curculionidae)) together with the Larger grain
borer (Prostephanus truncatus Horn (Coleoptera: Bostrychidae)) are the most
important storage pests of maize (Derera et al., 2014). Sitophilus zeamais
infestation commences on-field and continues into storage (Demissie et al.,
2008). The maize weevil is reported to cause damage to untreated storage maize
from trace levels to as high as 80% grain damage when conditions are favourable
(Tefera et al., 2010).
The problem of protein deficiency
in maize was solved with the development of maize fortified with Lysine and
Tryptophan. Quality Protein Maize was introduced into Ghana with the development of Obatanpa
(Asiedu et al., 2001). Quality Protein Maize developed earlier had soft
endosperm, chalky and dull kernel appearance and were susceptible to storage
grain pests (Ignjatovic-Micic et al., 2011). This situation was alleviated with
the successful development of quality protein maize that was genetically
improved and possessed hard endosperm (Vivek et al., 2008).
The maize weevil is controlled
predominantly by the use of synthetic insecticides. However, increased public
awareness and concern for environmental safety and health considerations are
gradually making the use of these chemicals unpopular (Kanyamasoro et al.,
2012). Hence the need to find alternative methods of controlling these insects.
The use of resistant varieties provides a safer, more practical and economic
method of controlling the maize weevil than any other control technique (Abebe et
al., 2009). A number of factors, whether present alone or in combination with
other factors confer resistance to maize. Some of these factors include kernel
hardiness, good husk cover, kernel size and texture, starchy amylose content,
phenolic content etc. (Gudrups et al., 2001).
However, little is known about the
genetic control of resistance to the maize weevil and the mode of inheritance
and how easily the resistance can be transferred to the next generation. The
lack of knowledge on the genetic control of resistance to the maize weevil is
hampering further improvement of Quality Protein Maize. The main objective of this
study therefore was to understand the genetic control and heritability of
resistance to the maize weevil.
I. estimate the general combining
ability (GCA) and specific combining ability (SCA) of the parental lines and
their hybrids respectively, for yield and resistance to maize weevil
II. estimate mid-parent and better
parent heterosis for resistance to maize weevil III. estimate broad sense and
narrow sense heritability of resistance to maize weevil
IV. identify
promising genotypes with resistance to maize weevil.
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