Fruit flies (Diptera: Tephritidae) pose a serious threat to the horticultural sector in sub-Saharan Africa. In Burkina Faso, mango is the cultivated fruit that is the most affected by these insect pests. Ceratitis cosyra (Walker) has long been recognized as the most damaging tephritid fruit fly pest of mango fruits (Mangifera indica L.) in Africa . The damage of fruit flies on mango worsened following the introduction of Bactrocera dorsalis Hendel (ex. Bactrocera invadens), a highly polyphagous species that attacks both cultivated and wild fruits. In Western Burkina Faso,  reported the existence of seven fruit fly species in mango orchards. Among them, two fruit fly species, B. dorsalis detected for the first time in 2005 in Burkina Faso and C. cosyra represented about 95% of the total catches . These two fruit fly species were responsible for about 96% of the damage on mango fruits . The introduction and successful adaptation of a species out of its natural range of distribution produce drastic changes in the abundance and distribution of species  . Invasive species can modify native biodiversity, shaping new interspecific interactions either directly or indirectly. Interspecific competition is among the most common interactions induced after a biological invasion, at least for insect species . Several studies have shown that B. dorsalis has a high potential to displace some native species from their spawning site as well as their natural geographic range  .  indicated the rapid displacement of C. cosyra by B. dorsalis at Nguruman, Kenya, 4 years after its detection in the African continent. This study aims to deepen knowledge on the bio-ecology of B. dorsalis 14 years after its detection in order to improve control methods. Specifically, it was about studying the interspecific competition between the introduced and invasive species, B. dorsalis and the native one, C. cosyra in mango orchards in Western Burkina Faso.
2. Materials and Methods
2.1. Study Sites
This study was carried out in six mango orchards of two regions in Western Burkina Faso. The selected sites in the two-targeted regions included Dindéresso and Péni (Houet province), Badara and Koloko (Kénédougou province), Bérégadougou and Toumousséni (Comoé province). Figure 1 presents the localization of the study sites. This area is the major fruit-producing zone in Burkina Faso. It belongs to the South-sudanian climatic zone and is characterized by an annual average rainfall between 900 and 1200 mm with monthly average temperatures rarely exceeding 35˚C.
Figure 2 shows the average monthly rainfall of the study area during the study period.
Samples of mango fruits at physiological maturity were randomly collected from
Figure 1. Localization of the study sites.
Figure 2. Average monthly rainfall in the three provinces during the study period.
mango trees in six mango orchards. Thirty mango fruits per variety and per orchard were collected every two weeks during the mango period for two consecutive seasons (2017 and 2018). Each sample of mango fruits was packaged in labeled jute bags. On each label, the following information was mentioned: name of the site, name of the variety and collection date.
2.2.2. Mango Fruits Incubation
Each mango fruit was weighed and placed individually in 1.5 liter plastic box (diameter: 25 cm; height: 12 cm) containing sieved and sterilized sand, which was used by larvae as a pupation substrate after leaving the fruit. Each container had a lid to avoid other insects from contaminating the sample.
2.2.3. Handling of Insects
Each rearing box was checked every five days. Each fruit was observed and those that have been infested with fruit fly larvae were counted. The sand was sifted for the presence or absence of tephritid puparia, which were picked up using a pair of soft forceps or by gently sifting. These were counted before being placed in labelled petri dishes (94 mm × 15 mm) with moistened filter paper. Petri dishes containing the pupae were placed in rectangular cages (15 cm × 15 cm × 20 cm) stored in a breeding room with the following facilities: temperature: 25˚C ± 1˚C; relative humidity: 65% ± 2%. The pupae were held in the rearing cages until the total emergence of insects. Emerged adults were collected when full body coloration was reached and stored in pillboxes containing ethanol diluted to 70%.
2.2.4. Fruit fly Species Identification
Fruit fly adults were identified with published identification keys. Bactrocera dorsalis samples were identified using physical key  while other fruit fly species were identified to either genus or species level using an electronic key . The number of individuals of each fruit fly species was recorded in a data sheet per variety according to the collection date.
2.2.5. Data Handling and Analysis
An exact count of the number of infested mango fruits, the number of pupae and the number of adult insects coming from each variety and from each site was carried out at each collection date. The variables evaluated were: 1) percentage of infested fruits (without distinction between fruit fly species) per variety and collection dates; 2) percentage of fruits infested by C. cosyra; 3) percentage of fruits infested by B. dorsalis; 4) percentage of co-infested fruits (fruits simultaneously infested by both species); 5) number of adults of B. dorsalis per kg of fruit (infestation rate); 6) number of adults of C. cosyra per kg of fruit.
Excel 2013 from Microsoft’s Office 2013 was used for data capture and preparation of the various graphics. Statistical analysis of the data was performed using Statitisca 7.1 program. The statistical analysis was based on an analysis of variance (ANOVA) followed by a Student Newman-Keuls test using a probability threshold of 5%.
3.1. Species Diversity of Mango Fruit-Infesting Fruit Flies
A total of 4502 mango fruits belonging to the varieties Amelie, Mango vert, Sabre (early-varieties), Kent, Lippens, Smith, Valencia (intermediate-varieties), Brooks, Keitt (late-varieties) with a weight of 1384.51 kg were collected and incubated for the emergence of insects. We obtained the emergence of 18,521 adult fruit flies from the 23,797 pupae, representing an overall emergence rate of 77.82%. The fruit fly species associated with mango fruits in order of numerical importance were: Bactrocera dorsalis Hendel represented by 12,280 individuals (66.30% of emergences); Ceratitis cosyra Walker with 6209 individuals (33.52%); Ceratitis silvestrii Bezzi represented by 15 individuals; Ceratitis quinaria Bezzi (11 individuals) and Ceratitis fasciventris Bezzi (6 individuals). The B. dorsalis and C. cosyra species accounted for 99.82% of the fruit fly adults emerged from the mango fruits during the current study.
3.2. Evolution of the Average Attack Rates of Mango Fruits during the Study Period
Table 1 presents the average attack rates of mango fruits by fruit fly species. It can be seen that the rate of damaged fruits depended on the collection dates as well as on the variety considered (Table 1). The attacks were very low at the beginning of the mango season, but they became higher at the end of the season according to the variety considered. Indeed, the average attack rates ranged from 0 to 0.43% at the beginning of the season (early April) and from 20.65% to 86.67% at the end of the season (early July) depending on the variety.
Analysis of variance revealed highly significant differences in average attack rates between mango varieties (F = 6.04 and P < 0.000) on one hand and between collection dates (F = 69, and P < 0.000) on the other hand.
3.3. Co-Infestation of Mango Fruits by Fruit Fly Species
A total of 727 mango fruits were attacked by fruit flies. The results showed that 53.50% wereinfested only by B. dorsalis, 22.14% only by C. cosyra, 20.35% were infested by both species and 3.98% infested by other fruit fly species. In the co-infested mango fruits, 3274 fruit fly adults were emerged, of which 54.03% belonged to B. dorsalis and 45.96% adults were for C. cosyra.
3.4. Evolution of Infestation Rates of B. dorsalis and C. cosyra on Mango Fruits during the Study Period
3.4.1. According to the Sampling Dates
Figure 3 shows the evolution of the average number of adults of B. dorsalis and
Table 1. Average attack rates of the mango fruits according to the varieties and collection dates.
C. cosyra per kg of fruit during the two consecutive mango seasons (2017 and 2018). The infestation rates of both fruit fly species evolved inversely. Indeed, the infestation rate of mango fruits by the species B. dorsalis was zero at the beginning of the mango season and then evolved gradually until the end of the season while the average number of individuals of C. cosyra per kg per fruit decreased as the mango season draws to a close.
3.4.2. According to the Study Sites
Figure 4 shows the infestation rates of mango fruits according to the study site.
Figure 3. Average infestation rates of mango fruits by B. dorsalis and C. cosyra according to the sampling dates.
N.B. vertical bars followed by the same letter are not significantly different at 5% probability.
Figure 4. Average infestation rates of B. dorsalis and C. cosyra on mango fruits according to the study sites.
From this, it can be seen that the average infestation rates of fruits varied depending on the study sites. During the mango season 2017, the mango orchards of Koloko and Bérégadougou recorded the highest infestation rates of B. dorsalis (80.45 ± 131.08 and 46.20 ± 66.66 flies per kg) and the lowest infestation rates of C cosyra (respectively 17.84 ± 57.03 and 9.51 ± 25.27 flies per kg).
During the mango season 2018, the mango orchards of Péni recorded the highest infestation rate of B. dorsalis (122.49 ± 144.61 flies per kg of fruit) and the lowest infestation rate of C. cosyra (5.17 ± 12.41 flies/kg).
We observed that, in general, the mango orchard that recorded the highest density of adults B. dorsalis recorded at the same time the lowest density of adults of C. cosyra.
Five fruit fly species emerged from mango fruits during the two consecutive mango seasons (2017 and 2018), of which B. dorsalis and C. cosyra accounted for 99.82% of emerged adults. However,  reported that seven fruit fly species of Tephritidae were associated with mango fruits in the same study area. This result could be explained by a probable displacement of certain fruit fly species of the genus Ceratitis by the invasive B. dorsalis. Average attack rates varied significantly between collection dates on one hand and between varieties on the other hand. On April 14, only the varieties Kent and Lippens were attacked by fruit flies with average attack rates of 0.43 ± 0.09 and 0.06 ± 0.04, respectively. We observed that mango fruit infestations by fruit flies became more and more important as the campaign progressed. Thus, the highest average attack rates were recorded at the end of the mango season (late June to mid-July). Depending on the mango varieties, we observed that local cultivars were very lowly or not attacked. This situation could be explained by the fact that agronomists have only considered yield and fruit taste criteria during the breeding process, while local varieties that are well adapted to their environment are under less pressure of insect pests.
Among the improved varieties, Amélie and Lippens (early and mid-cultivars) recorded the lowest attack rates in the current study while the late one, Keitt and Brooks suffered the most attacks of fruit flies. This result could be explained by the fact that at the end of the mango season, these varieties constitute the only breeding sites favorable to a period which coincided with the implosion of the populations of B. dorsalis in the mango orchards . During this study, several mango fruits were infested by both species (B. dorsalis and C. cosyra). Recent studies have reported the co-infestation of guava (Psidium guajava) with an introduced species Bactrocera carambolae and native species of the genus Anastrepha in Brazil  and Ceratitis capitata (introduced species) and Anastrepha fraterculus (native species) in Argentina  .
In the current study, the results showed that B. dorsalis and C. cosyra were responsible for respectively 66.30% and 33.52% of damage recorded on mango fruits.  stated that during the mango seasons 2008 and 2009, B. dorsalis was responsible for almost 65% of the damage while C. cosyra was responsible for about 30% of the damage observed on mango fruits in the same study area. Thus, it appears that there is a relatively stable coexistence between these two fruit fly species in mango orchards in Western Burkina Faso. The study of the evolution of the density of the two main species responsible for mango fruit damage during the two seasons revealed that the number of adults per kg per fruit of C. cosyra was higher during the early mango season (April-May) at the end of the dry season. In contrast, the highest infestation rates of B. dorsalis were recorded at the end of the mango season during the wet season. In addition, our findings revealed that the mango orchards with the highest infestation rates of B. dorsalis recorded at the same time the lowest infestation rates of C. cosyra and vice versa. In fact, some mango orchards consist mainly of early varieties mainly attacked by C. cosyra while others are home to mid and late varieties mainly by B. dorsalis.
The main factors that facilitate the co-existence in mango orchards of these insect pests are the climatic factors such as rainfall and temperatures and the availability of fruit hosts. Ceratitis cosyra is most abundant during the dry season when temperatures were high and its population level decline with the first rains . In contrast, B. dorsalis population level was very low during the dry period when no rainfall was recorded, but increased steadily from the start of the rains. Also, in our study area, the mango fruiting season is spread over 4 to 5 months. There are therefore early mango cultivars which were infested with C. cosyra during the dry period and the mid and late varieties which were infested with B. dorsalis. Previous studies indicated that C. cosyra caused damage on early mango cultivars in Burkina Faso  and Benin .  reported that B. invadens is involved in the majority of mango damage after the first rains in this area. In fact, B. dorsalis caused more than 90% of the damage on mid- and late mango cultivars in both Southern Sudan and Northern Sudan zones  . In the Sahelian zone (in Senegal-Niayes), Bactrocera dorsalis was considered to be the main insect pest associated with mango fruits during the rainy season   . In case of strong pressure, C. cosyra move on indigenous fruit hosts present in plant formations surrounding the mango orchards to avoid competition. According to , C. cosyra found from May to July suitable fruit hosts such as Annona senegalensis Pers., Sarcocephalus latifolius (Sm.) E.A.Bruce, Saba senegalensis A. DC., Sclerocarya birrea (A. Rich.) Hochst in the plant formations around mango orchards.
In the co-infested mango fruits, 54.03% fruit fly adults emerged belonged to B. dorsalis and 45.96% adults were C. cosyra.  reported that the high reproductive capability of B. invadens allows it to achieve enormous population size.  stated that a female B. dorsalis has a fecundity rate of 1056.8 eggs as against 356 eggs for C. cosyra. Over time, the numerical advantage of B. dorsalsis may reduce the probability of C. cosyra individuals to have access to available resources. According to , in under interspecific competition, larval duration in B. dorsalsis was significantly shorter (6.2 ± 0.6 - 7.3 ± 0.3 d) as compared with C. cosyra (8.0 ± 1.2 - 9.4 ± 0.4 d).
Authors are grateful to the technicians Sow Adama and Soubeiga Marcel for their assistance in collecting the data as well as in breeding fruit flies in the laboratory. This work was financially supported by the Regional Plan for the Control of fruit flies in West Africa.
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