Title:
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Variation in some geographical populations of the larger grain borer, Prostephanus truncatus (Horn) (Coleoptera:Bostrichidae) : a study of allozyme polymorphisms, morphometrics and feeding behaviour
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This thesis describes studies on population variation in laboratory-reared cultures of the
Larger Grain Borer, Prostephanutsru ncatus( Horn) (Coleoptera: Bostrichidae). Prior to its
introduction into Africa around 1980, P. truncatus was not regarded as being of major
economic importance. However, it has since become a major pest of stored maize and
dry cassava in Africa causing over £500 million of damage per year. In order to gain a
better understanding of the biology of P. truncatus, genetic, morphological and feeding
behaviour variability were studied in six laboratory maintained populations collected
from single locations: four from Africa (Tanzania, Kenya, Togo and Nigeria) and two
from Central America (Costa Rica and Mexico), plus one recently collected field
population from Tanzania. Three techniques were used: 1. Cellulose acetate enzyme
electrophoresis, 2. Morphometrics and, 3. Behavioural observations, in order to explore
potential usefulness of these methods in the study of geographical variation between P.
truncatusp opulations.
Allozyme electrophoresis (using cellulose acetate gel) as performed in this study showed
that genetic variation existed in P. truncates. Of the ten enzyme loci screened, four
(phosphoglucoisseo meraceg,lu tamate-oxaloacetatrtaen saminasep,e ptidarea nd malic) revealed clear
variation among some of the six laboratory populations. The proportion of polymorphic
loci (P) for the six populations was found to be 0.75 and the mean heterozygosity ( H)
0.247 (Togo had the highest- H=0.336 and Costa Rica the lowest- H= 0.141).
Comparison of allele frequencies showed significant differences between some of the six
laboratory populations (P < 0.001). Pairwise comparisons between allele frequencies for
the four polymorphic loci revealed large differences in allele frequencies between the
African and Costa Rican populations and great similarities in allele frequencies between
African and Mexican populations. This suggests that the African populations are more
closely related to the Mexican population and may first have been introduced to Africa
from Mexico. No significant allele frequency differences were found between the field
and laboratory reared populations from Tanzania.
The morphometric study was carried out using seven male characters and nine female
charactersT. he results showed that most characters( exceptf our: hindspur and hindtibia
for males, and forespur and midantennal segment for females) revealed significant
morphological variation between some of the six laboratory populations. All the tests: Univariate Analysis of Variance, Multivariate Analysis of Variance, Discriminant
Function Analysis and Principal Components Analysis showed significant population
differences. The Costa Rican population showed distinctive morphological differences
from the other five populations. The recently collected field population shared similar
morphological characters with laboratory reared population from Kenya.
The behavioural study focused on variation in the relationship between progeny
production and the degree of damage caused to maize grain and dry cassava. After eight
weeks, standardised cultures differed significantly with respect to the total number of
adults and larvae recorded at the end of the experiment (the populations from Mexico
and Kenya had the highest number while the population from Costa Rica had the
lowest). When the degree of stored product damage was considered, significant
differences were again found, and the ranking was similar. The damage done was
significantly correlated with the number of progeny produced (P < 0.001).
Comparison of results obtained from laboratory maintained African populations with a
field sample obtained from Tanzania indicates the potential usefulness of cellulose
acetate electrophoresis of allozymes and morphometrics in the study of geographical
variation in African P. truncatesT. his will lead to a better understanding of the biology of
this economically important insect and provide more solid data on which to base control
measures.
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