Morphometrics and reproductive parameters of Rhynchophorus morphotypes(Coleoptera: Curculionidae) in Ghana

doi:10.21203/rs.3.rs-4919892/v1

Palm weevils are widely distributed throughout Ghana causing severe economic injuries to palm plantations. However, the description and categorisation of palm weevil species based on the morphology in Ghana are lacking. This study sought to identify and determine the morphometrics and reproductive parameters of different Rhynchophorus morphotypes in Ghana. One thousand and two hundred (1200) larvae each weighing 5 grams were collected by handpicking from infested palm trunks from palm plantation site in the Middle Belt (Kobedi, Diabaa, Asuhyiae, Nsuta, Tanoso, Bomaa, and Akrodie) and Southern Ghana (Fumesua, Kubease, and Donyina). These larvae were allowed to cocoon and pupate, and the adults that emerged were described and categorised into different morphotypes using morphological description keys of Rhynchophorus species. Three Rhynchophorus morphotypes were identified and described; rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). The fecundity and female longevity of morphotypes of BWS (FF = 102.6 ± 38.85, FL = 43.6 ± 12.07) and BW (FF = 100.1 ± 39.45, FL = 43.6 ± 12.10) were superior to mature RRW female morphotype (FF = 39.1 ± 28.33, FL = 38.6 ± 9.21). The RRW morphotype may require more generations to adapt to the conditions in Ghana to improve on its life history parameters. The lowest fecundity of the RRW morphotype could be an advantage to limiting its spread across palm plantation sites in Ghana.

Rhynchophorus species

fecundity

pronotum spots

morphotype

Rhynchophorus species are major pests of palm across many countries in Africa, Asia, and South America, infesting over twenty palm tree species belonging to sixteen different genera (Egonyu et al. 2024). Nine Rhynchophorus species are highly destructive to palm plantations worldwide (Giblin-Davis et al. 2013). Among the nine species, Rhynchophorus ferrugineus which is endemic in South-Eastern Asia has spread to North Africa due to the exchange of host planting materials between research institutions during the last two decades (Rugman-Jones et al. 2013).

According to Rugman-Jones et al. (2013), R. ferrugineus has been recorded in Egypt, Thailand, Philippines, Vietnam, Cambodia, and several states in Malaysia. In contrast, R. phoenicis and R. palmarum are the dominant species in West Africa and South America, respectively (Egonyu et al. 2024). In Ghana, research on Rhynchophorus species is still at the infant stage (; Debrah 2016; Anankware et al. 2016; Quaye et al. 2018; Debrah et al. 2019; Anankware et al. 2021). Furthermore, studies on diversity among and within Rhynchophorus species in Ghana are lacking because there are insufficient publications describing the pest status of any morphotypes in the last decade (CABI 2021). Three Rhynchophorus species; R. phoenicis, R. quadrangulus (Chebbi 2011; CABI 2019), and R. ferrugineus (Wattanapongsiri 1966) have been recorded in Africa. In June 2021, CABI’s participatory rural approach (PRA, number P05557) speculated about the accidental introduction of R. ferrugineus in Ghana with a high degree of infestation to palm plantations in Southern Ghana.

Furthermore, identifying Rhynchophorus species using colour characteristics of a rusty red and black body with a high variability in the case of R. ferrugineus and R. phoenicis was inconclusive in the case of R. palmarum in Colombia (Lohr et al. 2015). Although previous studies by Rugman-Jones et al. (2013) found a wide range of phenotypic variations in R. ferrugineus and R. phoenicis, the use of colour without the typologies and morphometrics may not aid the descriptions within or among the species. Several functions have been assigned to insect coloration, including regulation of body temperature (Forsman et al. 2002), mimicry (Turner 2005), success in mate choice (Breuker and Brakefield 2002), and habitat choice can drive the evolution of body coloration patterns to optimize detection by conspecifics (Théry et al. 2008).

Morphometrics of the species is highly recommended for efficient morphological descriptions of insects (Leonard et al. 2021). Additionally, it is crucial to differentiate between local species and those that are obtrusive, and this might be done by employing morphological description. Although, R. phoenicis have been observed in Southern Ghana, the detailed fecundity and the life history parameters of the morphotypes within the species have not also been reported in Ghana. This invasive species has over the years threatened the coconut and other palm trees, which are key economic growth drivers in Ghana especially among the rural folks (Egonyu et al. 2022). In the current study, R. phoenicis morphotypes were described using morphometrics, and the life history parameters were assessed on peel sugarcane under laboratory conditions. The study aimed to morphologically describe Rhynchophorus species and assessed the reproductive performance of each group.

Insect sampling

The Rhynchophorus species were sampled from the Middle belt (Kobedi, Diabaa, Asuhyiae, Nsuta, Tanoso, Bomaa, and Akrodie) and Southern Ghana (Fumesua, Kubease, and Donyina) (Fig. 1 and Table 1). These areas were selected based on the availability of palm weevil host plants; oil palm (Elaeis guineensis Jacq.), date palm (Phoenix dactylifera L.), raffia palm (Raphia spp.), and coconut palm (Cocos nucifera L.) (Gries et al. 1994; Bong et al. 2008). The annual mean temperature of these selected areas ranges from 25 ^oC to 29 ^oC throughout the year. These areas lie in the forest vegetation zone with annual rainfall of 2,000 mm (https://www.fao.org/3/ab567e/AB567E02.htm).

Table 1: Morphotypes of R. phoenicis recorded in the Middle belt and Southern Ghana

Study area	Locations	RRW		BWS		BW
Middle belt		No	%	No	% s	No	%
	Kobedi	0	0.0	108	90.8	11	9.2
	Nsuta	0	0.0	112	92.6	9	7.4
	Tanoso	0	0.0	88	85.4	15	14.6
	Bomaa	8	11.9	49	73.1	10	14.9
	Akrodie	17	28.8	26	44.1	16	27.1
	Diabaa	11	12.4	71	79.8	7	7.9
	Asuhyiae	9	13.0	29	41.0	31	44.9
Southern Ghana	Fumesua	42	37.8	57	51.4	12	10.8
	Asotwe	16	22.9	39	55.7	15	21.4
	Donyina	21	24.7	40	47.1	24	28.2
Total Average			15.2		66.2		18.7

Rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). (N= 25) (N= 893)

Experimental conditions

The larvae were inoculated at a constant temperature (28 ± 2°C) and relative humidity RH (70 ± 5%) with a photoperiod of (L: D) 12: 12 h.

Larval culturing

One thousand and two hundred larvae were sourced from infested palm trunks from palm plantations site in the middle belt and Southern Ghana. The larvae were collected from the field during the major rainy season between June to July 2020 when there was a high incidence of Rhynchophorus species. The larvae were inoculated in plastic containers measuring 8 cm ×5 cm (width × height) with four half pieces of coconut coir measuring (4 cm × 5cm) (width × height). The containers were covered with nylon cloth and tightened with rope and rubber bands. The inoculated containers were tagged with the respective names of the locations where the larvae were sourced. The cocoons were sorted thirteen days after larval inoculation. The sorted cocoons were placed in an airtight black polyethylene bag to provide a stable temperature and humidity to enhance adult emergence.

Identification of the morphotypes

The adults that emerged were separated based on the colour phenotypes and pronotal marking pattern. They were designated into colour phenotypes as described by, and Wattanapongsiri (1966)’Ditjenbun (2010) and Monzenga et al. (2017): rusty red weevil with black spots on the pronotum (RRW), black weevil with a red stripe on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW) (Fig. 2). The diversity of pronotal marking patterns of all the adults that emerged was observed, and the pattern types were recorded. Percentage incidences of pronotal marking were recorded to determine the diversity of different Rhynchophorus morphotypes in the various locations (Mizzi et al. 2009; Tambe et al. 2013; Ul Hag et al. 2018). The differences in typologies on the protonum of the identified morphotypes were recorded (Wattanapongsiri 1966; Mizzi et al. 2009; Tambe et al. 2013; Ul Hag et al. 2018). The adults that emerged were sexed into males and females based on the presence of setae on the frontal part of the male’s rostrum, which is absent in the females (Al-ayedh 2008). The geographical positioning system (GPS) data were collected from the study sites using Geographical Information System (GIS) software version 10.3 and used to construct the distribution map (Fig. 1).

Morphometrics

Fifty pairs of each identified Rhynchophorus morphotypes were sampled for morphological descriptions. The adults were killed by freezing and were mounted by direct pinning to measure the different body parts. The adults’ body parts were measured in millimetres using Image J software and a rule (Girish and Vijayalakshmi 2004). Morphological characteristics included male abdominal length, male abdominal width, female abdominal length, female abdominal width, length of male’s rostrum, length of the female’s rostrum, length of the male’s pronotum, the width of the male’s pronotum, length of the female’s pronotum, the width of the female’s pronotum, and length of the female’s ovipositor of the identified morphotypes were studied for morphometric differences.

Evaluation of fecundity of identified morphotypes

Twenty-five pairs of each identified Rhynchophorus morphotype were cultured at the insectary. Peeled sugarcane measuring 5 cm in length was soaked in water for three days to increase the moisture content. The soaked peeled sugarcane pieces were removed from the water and placed in a sieve for excess water to drain. The adults were separated into males and females based on the presence of setae on the frontal part of the male’s rostrum which is absent in the females (Al-ayedh 2008). A one-day old adult pair (a male and a female) was placed in different containers (3 cm × 5 cm) (width × height) with well-drained sugarcane measuring 5 cm for oviposition. The eggs were counted by splitting the oviposited sugarcane and removing the eggs using a camel hair brush. New substrates were replaced for the female to continue laying till it died. Pre-oviposition duration, oviposition duration, fecundity, and male and female longevity were recorded for each morphotype. Pre-oviposition was calculated as the days elapsed before the first egg was laid. Oviposition duration was calculated as the days elapsed between pre-oviposition and the last day the female egg was laid. Female fecundity was calculated as the total number of eggs laid whiles male and female longevity was calculated as the days elapsed from emergence to the day the mortality of the adults was recorded.

Statistical analysis

The statistical analysis of the studied parameters was carried out using Genstat software (version 2018) for the analysis of variance (ANOVA), and general linear model procedure (α = 0.005). Tukey’s test of significance difference was used to separate the means.

Characteristics of identified Rhynchophorus morphotypes

From the total of one thousand and two hundred larvae that were inoculated, eight hundred and ninety-three adults emerged from the cocoons. Using the morphological description keys of Rhynchophorus species; three morphotypes with distinct morphological features were identified in the Middle belt and Southern Ghana: These were black weevil with longitudinal red stripes on the pronotum (BWS; Fig. 2A), rusty- red weevil with two black spots on the pronotum (RRW; Fig. 2B) and black weevil without a longitudinal stripe on the pronotum (BW; Fig. 2C) based on the previous morphological identification of species by Wattanapongsiri (1966); Ditjenbun (2010) and Mozenga et al. (2017). Phenotypic colour variation of Rhynchophorus species was recorded in both study areas. The morphotype of RRW populations were recorded in Bomaa, Akrodie, Diabaa, Asuhyiae, Fumesua, Asotwe, and Donyina whilst BWS and BW morphotypes were recorded in all the locations in the Middle belt and Southern Ghana (Table 1).

Composition of the different Rhynchophorus morphotypes

Of the total of eight hundred and ninety-three adults that emerged comprising three Rhynchophorus morphotypes, 66.2% were categorised as BWS, 15.2% of the population were RRW whilst 18.7% were grouped as BW (Table 1). Southern Ghana recorded the highest RRW frequency with a mean of 22.16% whilst 8.14% were captured in the Middle belt. The BWS morphotype was the dominant species in Southern Ghana and Middle Belt with a mean population of 77.2% and 55.2%, respectively. Furthermore, the three morphotypes of Rhynchophorus were found in both sexes.

Morphometrics of identified Rhynchophorus morphotypes

Tables 2 and 3 indicated eleven morphometric characteristics with means and standard deviation of the identified morphotypes. All the measured body parts of the morphotypes showed significant differences, except the length of the male’s rostrum. The morphometric analyses indicated that ten characteristics could potentially be useful for the separation of the Rhynchophorus morphotypes. Among the morphotypes recorded, there was a variation in the pronotal markings on the pronotum of all morphotypes (Fig. 2). Within each lineage, there were no differences between males and females in terms of the pronotal markings recorded on the pronotum of the three morphotypes, Females from lineage BWS and BW were bigger than those from RRW.

Table 2

Mean dimensions (mm) of identified male adults of the different morphotypes recorded in the Middle belt and Southern Ghana.
Species type	Male Abdominal length (mm)	Male Abdominal width (mm)	Length of male’s rostrum (mm)	Length of male’s pronotum (mm)	Width of male’s pronotum (mm)
BWS	14.15 ± 1.76^a	8.97 ± 1.22^a	8.56 ± 0.90^a	9.52 ± 0.88^a	7.18 ± 0.83^a
BW	13.70 ± 1.57^a	8.88 ± 1.77^a	8.56 ± 0.91^a	9.44 ± 0.86^a	7.04 ± 0.78^a
RRW	11.09 ± 1.09^b	7.07 ± 0.73^b	8.89 ± 0.74^a	8.98 ± 0.88^b	6.64 ± 0.65^b
F_{(2, 98)}	97.69	86.59	2.99	7.01	11.39
P value	< 0.001	< 0.001	> 0.055	< 0.001	< 0.001
Rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). Means with different superscripts in the column showed a significant (P < 0.005) difference between the morphotypes (BWS, RRW, and BW). (N = 50)

Table 3

Mean dimensions (mm) of identified female adults of the different morphotypes in the Middle belt and Southern Ghana.
Morphotypes	Female Abdominal Length (mm)	Female Abdominal Width (mm)	Length of female rostrum (mm)	Length of female’s pronotum (mm)	Width of female’s pronotum (mm)	Length of female’s ovipositor (mm)
BWS	14.70 ± 1.10^a	9.24 ± 1.00^a	10.48 ± 0.87^a	10.28 ± 1.02^a	7.74 ± 0.61^a	5.87 ± 0.42^a
BW	14.50 ± 1.09^a	8.98 ± 0.90^a	10.40 ± 0.76^a	10.06 ± 0.81^a	7.54 ± 0.51^a	5.79 ± 0.54^a
RRW	12.47 ± 0.81^b	7.46 ± 0.93^b	9.60 ± 0.84^b	9.70 ± 0.75^b	7.24 ± 0.45^b	5.10 ± 0.34^b
F_{(2, 98)}	17.66	40.91	8.45	6.91	9.48	55.71
P value	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). Means with different superscripts in the column showed a significant (P < 0.005) difference between the morphotypes (BWS, RRW, and BW). (N = 50)

Reproductive parameters of the identified morphotypes

The pre-oviposition period of BWS (2.50 ± 0.48) and BW (2.70 ± 0.49) was significantly shorter than RRPW (3.60 ± 0.36). However, the oviposition period of BWS (4.10 ± 0.70) and BW (4.30 ± 0.42) was longer than RRPW (5.40 ± 0.34). The fecundity of BWS (102.6 ± 38.85), BW (100.1 ± 39.45), and RRPW (39.1 ± 28.33) and longevity of both sexes of Rhynchophorus were significantly different among the morphotypes (Tables 4 and 5). All of the morphometric variables did not correlate with each other, indicating that they are independent (Table 6).

Table 4

Means weight of identified adult morphotypes recorded in the Middle belt and Southern Ghana.
Morphotypes	Male weight (g)	Female weight (g)
BWS	1.06 ± 0.50^a	1.07 ± 0.49^a
BW	1.04 ± 0.41^a	1.06 ± 0.47^a
RRW	0.67 ± 0.39^b	0.88 ± 0.42^b
F_{(2, 98)}	20.16	13.42
P value	< 0.001	< 0.001
Rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). Means with different superscripts in the column showed a significant (P < 0.005) difference between the morphotypes (BWS, RRW, and BW). (N = 25)

Table 5

Means parameters of identified adult morphotypes recorded in the Middle belt and Southern Ghana.
Species type	Pre oviposition period (days)	Oviposition period (days)	Female Fecundity (FF)	Female Longevity (FL)	Male Longevity (ML)
BWS	2.50 ± 0.48^a	4.10 ± 0.70^a	102.6 ± 38.85^a	43.6 ± 12.07^a	48.8 ± 12.27^a
BW	2.70 ± 0.49^a	4.30 ± 0.42^a	100.1 ± 39.45^a	43.6 ± 12.10^a	48.4 ± 11.69^a
RRW	3.60 ± 0.36^b	5.40 ± 0.34^b	39.1 ± 28.33^b	38.6 ± 9.21^b	40.4 ± 10.71^b
F_{(2, 72)}	65.59	76.92	24.07	0.24	1.06
P value	< 0.001	< 0.001	< 0.001	< 0.001	< 0.001
Rusty red weevil with black spots on the pronotum (RRW), black weevil with two red stripes on the pronotum (BWS), and black weevil without a longitudinal stripe pronotum (BW). Means with different superscripts in the column showed a significant (P < 0.005) difference between the morphotypes (BWS, RRW, and BW). (N = 25)

Table 6

**Correlation Matrix of Morphometric characteristics** Based on the correlation matrix on morphometric characteristics, the observable pattern of all the variables did not correlate with each other. Therefore, we can summarise that all the variables are independent of the other.
Morphometric characteristics	Male abdominal length	Male abdominal width	Female abdominal length	Female abdominal width	Length of male rostrum	Length of female rostrum	Length of male pronotum	Width of male pronotum	Length of female pronotum	Width of female pronotum	Length of female ovipositor	Female longevity	Male longevity
Male abdominal length	1
Male abdominal width	0.385	1
Female abdominal length	0.341	0.067	1
Female abdominal width	0.420	0.460	0.047	1
Length of a male rostrum	-0.154	-0.101	0.047	-0.134	1
Length of a female rostrum	0.209	0.194	0.278	0.080	0.593	1
Length of male pronotum	0.017	0.219	-0.128	0.331	-0.058	0.075	1
Width of male pronotum	0.064	0.153	0.091	0.230	-0.047	0.066	0.682	1
Length of female pronotum	0.257	0.129	-0.138	0.122	0.107	0.162	0.257	0.056	1
Width of female pronotum	0.108	0.133	-0.025	-0.080	0.041	0.159	0.168	0.241	0.489	1
Length of female ovipositor	0.376	0.417	0.128	0.298	0.153	0.352	0.147	0.185	0.493	0.422	1
Female longevity	0.211	0.159	0.169	0.327	-0.114	0.021	0.022	0.088	0.032	0.042	0.058	1
Male longevity	0.268	0.064	0.146	0.146	0.196	0.194	0.026	0.085	0.272	0.166	0.233	0.249	1
Note. p < 0.00

Three morphotypes of Rhynchophorus were recorded in the Middle Belt and Southern Ghana with distinct morphological features. Interestingly, 15.2% of the morphotypes that emerged were grouped as RRW. This morphotypes had similar characteristics to R. ferrugineus morphotypes reported in Malta and Indonesia (Mizzi et al. (2009). Rhynchophorus ferrugineus species was reported in Eastern part of Asia (Wattanapongsiri 1966), Northern Africa (CABI 2019; Wattanapongsiri 1966), and Europe (Ul Hag et al. 2018) and Australia (Ul Hag et al. 2018) and later discovered in Western parts of Northern America in 2010 (Ul Hag et al. 2018). According to Cole (2009), R. ferrugineus was identified in Malta in 2007 whilst Mizzi et al. (2009) confirmed that 28% of the population was found in the same country. Despite what preceded in those countries, species such as R. ferrugineus and R. vulneratus are rare in West Africa, whilst Monzenga et al. (2017) reported a 100% incidence of R. phoenicis adults in Tshopo and Lubunga. Although the R. phoenicis reported by Monzenga et al. (2017) had some topologies similar to what was reported on R. vulneratus and R. ferrugineus, Wattanapongsiri (1966) concluded that the shape of R. vulneratus and R. ferrugineus pronota were more appropriate when differentiating the two species. Compared to R. ferrugineus, which has a pronotum and gular suture that is curved at the sides and has a more uniform anterior curve, R. vulneratus has a pronotum that is firm and narrowly curved anteriorly (Wattanapongsiri 1966). Furthermore, Monzenga et al. (2017) described R. phoenicis as having black body with two longitudinal stripes on the pronotum whilst Wattanapongsiri (1966) described R. vulneratus as a species having a black body without a longitudinal stripe on the pronotum. Mizzi et al. (2009) and Longo (2006) reported that R. ferrugineus has a reddish body with black spots on the pronotum, However, the description provided by Wattanapongsiri (1996), Monzenga et al. (2017), and Mizzi et al. (2009) and Longo (2006) were insufficient because Rhynchophorus species displayed high levels of polymorphism and phenotypic plasticity over a wide geographic range which resulted in taxonomic ambiguity (Lannino et al. 2016).

According to the study on the taxonomy of Rhynchophorus species (Wattanapongsiri 1966), three species were recorded in Indonesia. Santosa (2016) and Sukirno et al. (2018) reported that there has been ambiguity regarding the identification of R. vulneratus in Indonesia and surrounding countries. For instance, in Malaysia, this taxon has occasionally been mistaken for R. ferrugineus and R. palmarum (Wattanapongsiri 1966). According to Whitman and Ananthrakrishnan (2009), this phenotypic plasticity describes changes in phenotypes and morphology caused by various environmental factors.

The morphometrics of the pronotum, abdominal length, and width of both sexes of all morphotypes were significantly different. Contrarily, Monzenga et al. (2017) reported an insignificant difference between both sexes using the abdominal length and width of R. phoenicis. This could be attributed to the variations in the experimental population used for the study. The ovipositor of the morphotypes was significantly different (P < 0.005) among morphotypes. The ovipositor of BWS and BW was longer than RRW. This could affect egg survivability as shorter ovipositors cannot bury eggs deeper into the substrates exposing them to external factors that may hamper their hatchability. The fecundity of BWS and BW morphotypes was superior to RRW, maturing and laying significant number of eggs within the duration of its life span under the same experimental conditions. This could be attributed to the fact that the BWS female morphotype were bigger than the females of RRW. This corroborates with the findings of Kaakeh (2005) who reported that bigger females with significant weight mature and lay more eggs than smaller females, irrespective of the experimental conditions. However, the longevity of females did not affect the fecundity of the identified species, buttressing the hypothesis by Kaakeh (2005) that a physiologically mature adult may not require several days to deposit all its eggs within its life cycle.

Generally, morphometrics can be integrated into pest management a tool for monitoring and assessing the fecundity of the individual species in the field. The RRW and BW morphotypes differed from those reported by the Ministry of Food and Agriculture in Ghana. The sector can take advantage of the lower fecundity of the RRW to manage and control the species to limit its infestation in Ghana. Morphometrics can be used as a continuous tool to identify genetic divergence among Rhynchophorus populations and provide evidence of adaptations and phenotypic variance in relation to environment and genetics. Furthermore, developing comprehensive predictive frameworks is a crucial and proactive approach to answering questions regarding the evolutionary outcomes of these morphotypes in agricultural environments. By putting such frameworks in place, we might be better equipped to predict and manage the effects of human interventions in agroecosystems that may lead to the evolution of new species.

Declaration of Competing Interest

The authors report that they have no conflict of interest.

Authors Contribution

JPA conceptualisation, writing first draft, and funding. SKD (formal analysis, conceptualised, methodology, and project management), AS (writing seconding draft and visualization), and DOO (writing second draft and visualization).

Acknowledgment

This research was financially supported by HEALTHYNSECT Project (DFC File No. 19-08-KU), DANIDA, and the University of Copenhagen, Department of Nutrition, Exercise and Sports Science. We are thankful to all who provided expertise that greatly assisted the research and improved the manuscript significantly. We thank Raphael Mongare for producing the distribution map of the sampling sites.

Data availability

Data will be available upon request

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Morphometrics and reproductive parameters of Rhynchophorus morphotypes(Coleoptera: Curculionidae) in Ghana

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Introduction

Materials and Methods