All fruit fly specimens caught in traps and those that emerged from fruits were identified using fruit fly identification keys. Voucher specimens were deposited in the IITA-Cameroon insect collection with duplicates at the IITA Biodiversity Center in Benin. The total number of fruit fly specimens from traps and for each of the two genera, Bactrocera and Ceratitis, were used to calculate the relative abundance of the species. Fruit fly species diversity was estimated with the Shannon and Simpson indices using Vegan R 2.0. packages. The Shannon index is a quantitative measure of both species richness and evenness, while the Simpson index measures evenness or species dominance. Due to their non-normal distribution, the Kruskal–Wallis non-parametric test was used to compare diversity indices between orchards and attractants. The seasonality data for the two experimental locations in Foumbot and Nkolbisson were summarized by each of the most abundant fruit fly species—B. dorsalis, C. cosyra and C. anonae—using average monthly trap catches of each species separately for each attractant . A generalized linear model with a Poisson error was used to test for the effects of AEZs and years on the abundance—using weekly means—of fruit flies by species and all species combined. The GLM analysis was conducted separately for Torula yeast, methyl eugenol, and terpinyl acetate.
For Torula yeast, growing blueberries data from both sexes were pooled for the analysis. The relationship between fruit fly catches and weather variables was explored using Pearson’s correlation. The mean number of flies caught per month per sex over the sampling years was used in each AEZ. Species abundance data were transformed with log +1 to correct for statistical errors associated with rare or very common species. Monthly temperature , relative humidity , and total monthly rainfall were calculated over the sampling years and by AEZ before analysis. Food-bait trap catches were compared using a matched-pairs analysis that identified differences in trap catches among the three food baits used in HF-BR. Similar comparisons from WH-MF were not performed since only Torula yeast was used in this AEZ. Comparisons were restricted to the three dominant species, B. dorsalis, C. cosyra and C. anonae, and all fruit fly species combined. Fruit fly infestation level of each fruit sample was calculated as the number of puparia per kg of host fruits, which is a commonly used measure for estimating and comparing fruit fly infestation levels in fruits. GLM with a Gaussian error was used to test for the effect of AEZs and sampling sites on fruit fly infestation levels by fruit and fruit fly species. Only fruit samples that were collected at least 10 times were considered in the analysis. Tukey HSD was used to compare means of fruit species infestations. All the analyses were performed in R software version 3.6.2.This present study was part of the fruit fly management program in mango and other orchard systems initiated in SSA following the invasion of Africa by B. dorsalis.
To our knowledge, it is the first and only study from the Congo basin of Central Africa to present long-term dynamics, spanning 6 years, of several species of frugivorous tephritid fruit flies by simultaneously using male lures, food baits and incubation of fruit fly host fruits. In the course of this study, we discovered four new host plant–fruit fly associations and uncovered new patterns of fruit fly population dynamics from agro-ecologies representing a cross-section of climates and conditions found in Central Africa. Our study also corroborates the findings of several other studies and provides additional information and exceptions on host fruit range, infestation rates, and seasonal dynamics of the encountered fruit fly species. Ten fruit fly species in four genera—Bactrocera, Ceratitis, Dacus, and Perilampsis—were caught in male lure and food bait traps installed in mango and mixed-fruit orchards in the two AEZs covered by our study. Diversity analysis using Shannon and Inverse Simpson indices showed significant differences between AEZs and attractants with a higher number of species in the Nkolbisson HF-BR orchard compared with the Foumbot WH-MR orchard, probably due to higher host plant diversity in the mixed-fruit orchard than in the relatively homogeneous WH-MR orchard. Moreover, food baits, because of their broad species attraction are expected to produce higher diversity indices compared with male lures—with some exceptions. These comparisons are rare, however, because diversity indices are not generally calculated by attractant , which does not allow proper comparisons of diversity indices across studies. In our study, we computed diversity indices by AEZ and by male lures and the food bait Torula yeast which were used across the two AEZs.
As expected, the two diversity indices—Shannon and Inverse Simpson—were larger for Torula yeast than for male lures and were considerably larger than values reported from Western Africa by for pooled data from four food baits . Three studies across four agro-ecological zones in Western Africa found 9–11 Ceratitis species in male lure and food bait traps placed in homogenous and mixed-fruit mango orchards over a period of 4–5 years. The differences in species occurrence and the efficiency of different lures and food baits were discussed by. In our study from two Central Africa AEZs—that do not overlap with the Western Africa AEZ covered by the previous studies, we found six Ceratitis species in similar male lure and food bait traps. Four rare species —C. lentigera Munro, C. pedestris , C. acicularis and C. penicillata —from Western African mango systems were not found in our study. Moreover, three species that are generally restricted to Guinea and Sudan Savanna AEZ of Western Africa—were also absent from our study from Central Africa. Two Dacus species were caught occasionally in traps and probably originated from cucurbit plants in the vicinity of the trapping sites as these species are cucurbit feeders, but D. bivittatus is also known to infest C. papaya, as observed from our papaya fruit samples. Overall, B. dorsalis was the dominant species, particularly in male lure traps, which is likely due to the greater attraction of methyl eugenol for this species compared with the attraction of terpinyl acetate to Ceratitis species. The numerical dominance of B. dorsalis, however, was also reflected in the total number of fruit flies that emerged from incubated mango fruits. These results are consistent with those from Eastern and Southern Africa—Kenya, Mozambique, Tanzania and Western Africa— Benin, Burkina Faso, and Ivory Coast. Bactrocera dorsalis was also particularly abundant in the mixed-fruit orchard at HF-BR where a large diversity of host plants was found, with several wild and cultivated fruits present all year round that could, at the same time, represent both a sink and a source for B. dorsalis. Of the six Ceratitis species collected in our study, C. cosyra and C. anonae were the numerically dominant species in terpinyl acetate and food bait traps, but their abundance patterns were different in the two AEZs covered by our study. Ceratitis anonae was more abundant in the WH-MR than in HF-BR and, therefore, square plant pots appears to be well adapted to the former AEZ with its generally higher altitude and cooler climate compared with HF-BR, where C. cosyra was more abundant than C. anonae. Other studies across altitude gradients in Eastern and Southern Africa have also demonstrated similar patterns of abundance for C. cosyra and C. anonae. The latter also occurs in low frequency in the low altitudes of Western Africa. In comparison, the abundance patterns of C. cosyra, which is a widespread species in Africa, appear to be related more to host plant availability and competition with other species than to climate, although C. cosyra was shown to have a high tolerance to drier conditions based on the effects of relative humidity on survival rates of puparia . Food baits such as Torula yeast, Mazoferm, and under certain conditions, BioLure, are known to have broader species attraction than the specific male lures. While our study focused primarily on the diversity, seasonality, and fruit infestations of fruit flies in mango systems using primarily methyl eugenol, terpinyl acetate, Torula yeast, and fruit incubations, we used the HF-BR orchard to compare the fruit fly trapping capacity of Torula yeast, Mazoferm and BioLure. While Torula yeast is broadly more efficient in trapping fruit flies and has shown consistent efficiency in fruit fly monitoring and suppression across several AEZs, Mazoferm and BioLure have shown inconsistent potential in fruit fly population detection and monitoring. In South Africa, BioLure was more efficient than Torula yeast probably because of the prevalence of C. capitata for which BioLure is an efficient attractant. In our study, Torula yeast consistently caught more B. dorsalis and C. cosyra than BioLure and Mazoferm; however, BioLure was more attractive to C. anonae than Torula yeast which was similar to Mazoferm in attracting C. anonae. It is not known, however, if the same comparative trapping capacity would hold under higher C. anonae and C. cosyra abundance.
While these results are consistent with those reported by [25] and [28] for the comparison of Torula yeast and BioLure, they are inconsistent with similar comparisons from other regions of Africa. In our study, Torula yeast was consistently more efficient than Mazoferm which had broadly similar trapping efficiency to Torula yeast in Western and Eastern Africa. The study also noted that Torula yeast was the preferred bait for detection and monitoring purposes over standard attractants such as Nulure and Mazoferm because its pH remained stable, over time, at 9.2.The abundance of the main tephritids—B. dorsalis, C. cosyra and C. anonae—reported in this study varied between years and AEZs, especially in the WH-BR, where, in all the attractants, fruit fly counts in traps decreased from 2014 to 2015 to nearly 50% of the levels of the previous two years . The decrease in captures may be related to a combination of factors including the low production of mango varieties in the orchard due to ageing, poor tree maintenance, and the use of bait sprays in the fruit trees of the orchard surrounding the experimental block. Long-term trapping conducted in this study established that the large within-year variations in fruit fly catches were linked to climatic conditions, but also to the direct effect of host fruit availability on the population build-up. Research paper indicated that the seasonal fluctuations in most fruit fly species were characterized by high population levels during wet periods and low levels in dry periods. In our case, B. dorsalis and the two Ceratitis cycles corresponded consistently with the rainy season, between the end of April and mid-June in both AEZs. Rainfall has been reported as one of the key factors that can affect plant phenology and nutrient quality, and determine the rapid explosion of various Bactrocera species. Moreover, the catches of B. dorsalis and Ceratitis species males and females were found to be influenced by temperature, especially in HF-BR. Temperature is known to play an important role in the abundance of tephritids through its effect on the developmental rate, mortality, reproduction, and intensity of activity. In HF-BR, the minimum temperature was significantly and positively correlated with male and female catches of B. dorsalis and C. anonae. This result suggested that an increase in temperature causes an increase in B. dorsalis and C. anonae populations, an observation consistent with the findings of [61] on B. dorsalis in South Africa. In contrast, the maximum temperature was negatively correlated with male and female catches of the three tephritids. This relationship between maximum temperature and catches of these tephritids shows the adverse impact of temperature increase on fruit fly captures and supports low populations observed in this agro-ecosystem during the dry season. The low values of the correlation coefficients obtained in the cases of significant influence of temperature, point to the involvement of other factors in this dynamic. A significant and positive correlation was obtained between rainfall and the catches of the three tephritids in the Foumbot orchard, and male catches of B. dorsalis in HF-BR. During this period of high humidity, between April and June, catches of these pests were more abundant, especially B. dorsalis. Populations of B. dorsalis were reported to increase with increasing rainfall. Due to continuous rain in the tropical forest of Central Africa, atmospheric humidity and soil moisture naturally increase, creating suitable conditions for puparia hatching and fruit development and availability that are important for fruit fly population buildup.