Abstract:

The maize weevil (Sitophilus zeamais Motschulsky) is one of the most destructive storage
insect pest of maize (Zea mays L.) in tropical Africa and worldwide, especially when susceptible
varieties are grown. Therefore, grain resistance against the maize weevil should be part of a
major component of an integrated maize weevil management strategy. The specific objectives
of this study were to: i) determine farmers’ perceptions about weevil resistance in maize
cultivars; ii) determine the genotypic variation for maize weevil resistance in eastern and
southern Africa maize germplasm lines; iii) study the gene action conditioning weevil resistance
in the inbred line populations from eastern and southern Africa maize germplasm and to
measure their combining ability for yield and weevil resistance; iv) determine the effectiveness
of two cycles of modified S, recurrent selection in improving a tropical maize population
“Longe5” for weevil resistance and agronomic superiority and v) evaluate the effectiveness of
the "weevil warehouse techniques" compared to the “laboratory bioassay technique" as
methods of maize screening against the maize weevil.
A participatory rural appraisal (PRA) was conducted in three districts between December 2010
and January 2011, to gather information on the maize weevil pest status in Uganda and
farmers’ perceptions about improved maize varieties and the major attributes desired in new
maize varieties. Over 95% of farmers knew the maize weevil and its pest status, and were
reportedly controlling the maize weevil using wood ashes, red pepper and Cupressus
sempervirens. The estimated postharvest weight losses attributed to weevil damage was over
20% within a storage period of four months. The most highly ranked attributes desired in the
new maize varieties included high grain yield, tolerance to drought and low nitrogen stresses,
resistance to field pests and diseases, good storability and resistance to storage pests.
In the search for new sources of weevil resistance, a total of 180 inbred lines from three
different geographical areas were screened for weevil resistance using the laboratory bioassay
technique. Eight inbred lines (MV21, MV23, MV75, MV102, MV142, MV154, MV157, and
MV170) were consistently grouped in the resistant class, and therefore selected as potential
donors for weevil resistance in the maize improvement programs. Large significant genetic
variations for weevil resistance, and high levels of heritability (89 - 96%) were observed. The
results revealed that there was no significant association between maize weevil resistance and grain yield; suggesting that breeding for maize weevil resistance can be achieved without
compromising grain yield.
Eight weevil resistant and two susceptible inbred line parents were crossed in a 10 x 10 full
diallel mating design and the resulting 45 experimental hybrids and their reciprocal crosses
evaluated for grain yield and secondary traits under four environments, and also to determine
the gene action regulating their expression. The F1 hybrid seed, F2 full-sib and F2 half-sib grain
generated from the 45 experimental hybrids and their reciprocals under two environments in
Namulonge, were evaluated for weevil resistance using F, weevil progeny emergence, median
development period (MDP), Dobie’s index of susceptibility (DIS), and parental weevil mortality
as susceptibility parameters. The general combing ability (GCA), specific combining ability
(SCA), and reciprocal effects were all significant for grain yield, with SCA accounting for over
80% of the hybrid sum of squares. Inbred line parent MV44 exhibited positive significant GCA
for grain yield and thus can be utilized in the development of synthetics and hybrids. Hybrids
MV21 x MV13, MV154 x MV44, and MV154 x MV102 and all hybrids between parent MV142
and the rest of the parental lines exhibited positive and significant SCA effects. For the weevil
resistance parameters, the general combining ability (GCA), specific combining ability (SCA)
and reciprocal effects were all significant for F, weevil progeny emergence, MDP, and DIS in the
three seed categories. The results revealed that weevil resistance was governed by additive
gene action, non-additive, and maternal effects. Parents MV170 and MV142 were consistently
exhibiting weevil resistance in the three seed categories and thus recommended for future
breeding strategies. Furthermore, most of the hybrids generated from parental line M142 were
noted to exhibit outstanding performance in terms of grain yield and weevil resistance.
Another study was conducted to determine the effectiveness of two cycles of modified S,
recurrent selection towards the improvement of weevil resistance in a maize population Longe5.
Over 540 selfed ears were selected from the source population (Co) and screened for weevil
resistance in the laboratory at Namulonge. Based on weevil resistance characteristics, 162
genotypes were selected from Co and recombined in an isolated field to generate cycle C,. The
same procedure was used for generating cycle C2 from cycle C,, but instead 190 weevil
resistant Ci genotypes were selected and recombined to form C2. Seed from cycles C, and C2,
together with that from the source population (Co), was used to plant an evaluation trial in three
locations, to compare the performance of the three cycles in terms of grain yield and reaction to
the major foliar diseases, and also to produce seed for subsequent screening against weevil
infestation. A total of 54 seed samples were screened for weevil resistance in a laboratory at
Namulonge, in an experiment laid out in a randomized complete block design. A reduction in
grain weight loss of 65% was registered in the C2 seed, whereas in Ci seed it was 15%. A
similar trend was observed for Fi weevil progeny emergence and grain damage. Grain yield
results indicated a yield gain of 19% realized from cycle C2 while a yield gain of 7% was realized
from cycle Cv Furthermore, reductions in disease severity of 27%, 10% and 13% were exhibited
for Turcicum leaf blight (TLB), grey leaf spot (GLS) and rust disease, respectively in cycle C2.
The results indicated that Longe5 can be improved for maize weevil resistance, grain yield, and
resistance to foliar diseases through selection. Further recurrent selection cycles would be
recommended.
The last study was aimed at evaluating the potential of shelled grain and suspended ear options
of the weevil warehouse technique in discriminating maize genotypes into different susceptibility
classes, based on genotype response to weevil attack. It involved comparing the effectiveness
of the two options under the weevil warehouse technique with the laboratory bioassay technique
using grain damage and grain weight loss as the maize grain susceptibility parameters.
Fourteen maize genotypes were screened using the weevil warehouse and the laboratory
bioassay techniques at Namulonge. On grouping the 14 genotypes into different response
classes, high levels of consistency were observed in the three screening techniques. Therefore,
the two weevil warehouse screening options being faster and effective in discriminating maize
cultivars towards weevil attack, they were found to be better than the laboratory bioassay
technique. The minimum evaluation period required to discriminate genotypes by the two weevil
warehouse options was two months from the onset of the experiment.
The maize weevil was noted to be an important storage pest constraining maize production in
Uganda. The major weevil control measures included proper postharvest handling procedures
and use of indigenous technical knowledge. The results also revealed that host plant resistance
could significantly reduce grain damage. It was further revealed that grain resistance against the
maize weevil could be enhanced through hybridization and recurrent selection; thus the
germplasm identified in the study can provide new sources of maize weevil resistance for
commercial deployment and further breeding.