Evidence for a discrete evolutionary lineage within Equatorial Guinea suggests that the tsetse fly Glossina palpalis palpalis exists as a species complex

Tsetse flies of the palpalis group are major vectors of Human African Trypanosomiasis in Africa. Accurate knowledge of species identity is essential for vector control. Here, we combine ribosomal internal transcribed spacer 1 ( ITS1 ), mitochondrial Cytochrome Oxidase 1 ( COI ) and microsatellites to determine the population structure and phylogenetic relations of Glossina p. palpalis in Equatorial Guinea. CO1 sequence data suggest that G. p. palpalis in Equatorial Guinea is a distinct subspecies from previously described G. p. palpalis in West Africa and Democratic Republic of Congo. Glossina p. palpalis in Equatorial Guinea and DRC share a common ancestor which diverged from West African G. p. palpalis around 1.9 Ma. Previous ITS1 length polymorphism data suggested the possible presence of hybrids in Equatorial Guinea. However, ITS1 showed incomplete lineage sorting compared with clearly defined COI groups, and data from 12 unlinked microsatellites provided no evidence of hybridization. Microsatellite data indicated moderate but significant differentiation between the populations analysed (Rio Campo, Mbini and Kogo). Moreover, unlike previous studies of G. p. palpalis , there was no evidence for heterozygote deficiency, presence of migrants or cryptic population structure. Variance effective population size at Rio Campo was estimated at 501–731 assuming eight generations per year. This study of the population genetics of G. p. palpalis in central Africa provides the first estimate of genetic differentiation between geographically separated G. p. palpalis populations.     

Estimating tsetse population parameters: application of a mathematical model with density-dependence

A density-dependent model is used to describe the dynamics of an open population of tsetse flies (Diptera: Glossinidae). Immigration (or emigration) takes place when the total population is below (or above) a biologically determined threshold value. The population is also subjected to birth and death rates, as well as to the risk of being trapped (continuously or intermittently). During trapping the population decreases toward a 'low' equilibrium population and when trapping ceases the population starts recovering and increases toward a 'high' equilibrium. The model is fitted using data collected on trapped flies in four experiments. The first one was conducted with 'intermittent trapping' (i.e. several trapping-recovery cycles) on Glossina fuscipes fuscipes Newstead in the Central African Republic (Bangui area). In the other experiments, trapping data on Glossina palpalis palpalis (Robineau-Desvoidy) was collected in 'aggregate' form over several days at a time. Two of these were in Congo-Brazzaville (Bouenza area) and one in the Ivory Coast (Vavoua focus). Estimates are derived for the low and high equilibrium values as well as the trapping rate. The estimated effect of sustained trapping is to reduce the population to low equilibrium values that are 85-87% lower than the levels without trapping. The effects of the natural intrinsic growth and of the migration flows cannot be estimated separately because in the model they are mathematically indistinguishable.     

Seasonal variations in the distribution and abundance of the tsetse fly,Glossina morsitans morsitans in eastern Zambia

The seasonal changes in the distribution of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) and its main host, cattle, were examined in a cultivated area of the plateau of eastern Zambia. During four consecutive years, the tsetse and cattle populations were monitored along a fly-round transect traversing the two main vegetation types in the study area. These were miombo, a one-storied open woodland with the genera Brachystegia and Julbernardia dominant, and munga, a one- or two-storied woodland where the principal tree genera were Acacia, Combretum and Terminalia. Concurrently, a capture/mark/release/recapture (CMRR) exercise was conducted along two other transects also traversing both vegetation types. The index of apparent abundance of tsetse (IAA) in miombo increased at the beginning of the rainy season (November), reached its peak at the end of the rainy season (April) and was low during the cold season (May to late August), but especially the hot dry season (September to late October). The IAA of tsetse in munga showed a pattern that was the reverse of that in miombo. The seasonal changes in the IAA of tsetse in both vegetation types were in accordance with changes in the movement patterns of tsetse between the two vegetation type as observed using CMRR. The distribution and abundance of cattle along the transect also showed a seasonal trend. This was especially so in munga, during the first three years of observations, where cattle abundance increased gradually from June onwards, reached a maximum at the end of the hot dry season (October-November) and declined steeply at the start of the rainy season (November-December). In both vegetation types, the monthly mean IAA of tsetse was positively correlated with the abundance of cattle in the previous month. It is concluded that the distribution of tsetse in cultivated area of the eastern plateau of Zambia undergoes substantial seasonal changes, which can partly be attributed to changes in the distribution of cattle. The implications of these observations for the control of tsetse are discussed.     

A comparison of the feeding behaviour of tsetse and stable flies

In Zimbabwe, observations were made of the behaviour of individual stable flies (Stomoxys spp.) (Diptera: Muscidae) and tsetse (Glossina spp.) (Diptera: Glossinidae) feeding on cattle during the wet (Stomoxys and tsetse) and dry (tsetse only) seasons. For Stomoxys landing on adult cattle, only 27% took a full meal (mean feeding time = 147 s). Most Stomoxys left the host before completing their meal, largely due to disturbance by the host's defensive behaviour (24%, mean time = 59 s) or other flies (44%, 71 s). The probability of a Stomoxys leaving the host progressively increased with time. Simultaneous observations of tsetse showed that, compared to Stomoxys, their feeding success was lower (15%), feeding was interrupted earlier (33 s) and the time taken to complete a meal was shorter (109 s). Further studies of tsetse across different seasons and hosts showed that feeding success varied according to host age (adult = 7%; calf = 3%) and was negatively correlated with the frequency of host defensive behaviour and the relative abundance of non-biting Diptera. Disturbances were more often caused by host behaviour (69%) than other flies (31%) and the probability of tsetse leaving decreased with time on the host. Overall, these results suggest that tsetse and Stomoxys have different feeding strategies. In particular, tsetse appear to be more responsive to host defensive behaviour, which reduces their feeding success relative to Stomoxys. These behavioural differences are consistent with the respective life-history characteristics of Stomoxys and tsetse.     

Interactions between cattle and biting flies: effects on the feeding rate of tsetse

In Zimbabwe, studies were made of the effect of host behaviour on the feeding success of Glossina pallidipes Austen and G. morsitans morsitans Westwood (Diptera: Glossinidae) attracted to cattle of different age and sex. The mean feeding rates for male and female G. pallidipes attracted to oxen were 60% and 58%, respectively, compared to 33% and 53% for male and female G. m. morsitans. The feeding rate of G. pallidipes varied between oxen and was inversely correlated with a host's rate of defensive leg movements, which, in turn, was positively correlated with the density of Stomoxys spp. (Diptera: Muscidae) caught in the vicinity of the host. Tsetse were significantly less successful in feeding from young cattle. For G. pallidipes, the feeding rate on calves (<6 months) was 11%, whereas for male and female G. m. morsitans the rates were 12% and 20%, respectively. Significantly lower feeding rates were apparent for cattle aged up to 2 years, when the feeding rate for G. pallidipes (31%) was still significantly less than that on mature oxen (68%). Feeding rates for G. pallidipes on adult female cattle were lower than those on oxen (45% vs. 61%). The lower feeding rates in young animals were attributed to higher rates of defensive movements. The results suggest that higher rates of defensive activities by young cattle reduce the risk of them contracting trypanosomiasis.     

Abundance and distribution of the tsetse flies, Glossina austeni and G. brevipalpis, in different habitats in South Africa

The distribution and abundance of Glossina austeni Newstead and Glossina brevipalpis Newstead (Diptera: Glossinidae) were studied in the three main vegetation types in Zululand, KwaZulu-Natal, South Africa. During a period of 12 months, a trap transect consisting of 38 H-traps traversing the three vegetation types was monitored. The Index of Apparent Abundance (IAA) for G. brevipalpis was high in indigenous forest and open grassland but lower in exotic plantations. Glossina austeni, on the other hand, was captured mainly in or adjacent to indigenous forest. The seasonal trend in the IAA did not differ between vegetation types. The findings on the distribution of G. brevipalpis are in contrast with the historic records. Historically, this species was considered to be restricted to areas with a dense overhead canopy and high relative humidity. The repercussions of these findings for the epidemiology of livestock trypanosomiasis and the control of tsetse in Zululand are discussed.     

Effects of gamma-irradiation on the mating and insemination capabilities of male Glossina palpalis palpalis

Mating ability, sexual aggressiveness and competitiveness of irradiated males subjected to 12-krad dose in air as compared to untreated males of Glossina palpalis palpalis (Rob.-Desr.) were evaluated in relation to the time required for initiation and completion of copulation with the transfer of viable and motile sperm into the female spermathecae. Males of G. p. palpalis can inseminate up to eight females. The minimum age for successful insemination is 4-days-old while the minimum copulatory time for optimal insemination is 40 min. However, sterile males require at least a day longer to mature (day 4 instead of day 3) and remained in copula for longer period but they become aspermic at the same rate as fertile males. There was significant reduction (P<0.05) in female fecundity during a mixed mating test involving a ratio of one sterile to one fertile male. Therefore 12-krad-treated males could satisfy the mating requirements of female G. p. palpalis.

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Résumé L'aptitude à l'accouplement, l'agressivité sexuelle et le succès dans la compétition ont été comparés chez des mâles soumis à 12 krad dans l'air et des mâles témoins. Ces paramètres ont été évalués en fonction du temps nécessaire pour provoquer l'accouplement et l'accomplir avec le transfert de spermatozoïdes viables et mobiles dans la spermathèque de la femelle. Un mâle de G. p. palpalis peut inséminer jusqu'à 8 femelles. L'âge minimum pour obtenir une insémination réussie est 4 jours, tandis que le temps de copulation minimal pour une insémination optimale est 40 mn. Cependant les mâles stériles demandent au moins 1 jour supplémentaire pour atteindre leur maturité (4ème jour au lieu du 3ème); ils restent accouplés pendant plus longtemps, mais deviennent aspermiques au même taux que les mâles fertiles. Il y a eu une réduction significative de la fécondité des femelles (P>0.05) après accouplements mixtes impliquant autant de mâles stériles que de mâles fertiles. Ainsi des mâles traités avec 12 krad pourraient satisfaire les exigences sexuelles de femelles de G. p. palpalis.

     

Salivary gland hyperplasia in wild caught tsetse from Zimbabwe

Hypertrophied salivary glands were collected from wild populations of the tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen in the Zambezi valley, Zimbabwe. Examination of the glands by electron microscopy showed that the associated virus was identical in form in both species. The incidence of salivary gland hypertrophy was significantly higher in G. pallidipes than in G.m.morsitans, and 83% of the enlarged glands of the former were, in addition, found to be infected with Rickettsia-like organisms. There was no indication from this small sample that viral infections predisposed flies to infections with trypanosomes.

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Résumé Des glandes salivaires hypertrophiées ont été extraites de Glossina morsitans et de G. pallidipes provenant de populations sauvages capturées dans la vallée du Zambèze au Zimbabwe. L'examen des glandes au microscope électronique a montré que la forme du virus associé était identique dans les deux espèces. La fréquence d'hypertrophie des glandes salivaires était significativement plus élevée chez G. pallidipes que chez G. morsitans et la fréquence des glandes hypertrophiées de G. pallidides contaminèes par des organismes ressemblant à des Rickettsia était de 0,02. Rien n'indique à partir de ce petit échantillon que la contamination virale favorise l'infection par des trypanosomes.

     

Abortion by the tsetse fly Glossina palpalis palpalis after treatment with insecticides

Batches of wild-caught adult females of Glossina palpalis palpalis R.-D. were treated topically (0.36 l per fly) with different doses of 14 chemical insecticides in acetone and subsequent abortion (if any) and death of each fly were recorded. Control flies received acetone alone. No fly was fed after treatment. Abortion rates of treated fly-batches and the corresponding control batch were compared by Fisher's exact test. The rate of abortion for 274 control flies rose from 1.1% on day 1 to 6.3% on day 5, then declined to the last abortions on day 9. The increase of rate probably resulted from increasing period of starvation. Significantly increased rates of abortion of eggs or immature larvae, compared with the control, were only found with high doses of fenitrothion, propoxur, permethrin and dieldrin and with various doses of sumithrin, sumicidin and deltamethrin. Abortion was most frequent for the last three; synthetic pyrethroids. All significant rates were for rapid abortion, within 24 h after treatment, and many abortions for sumithrin, sumicidin and deltamethrin were within 60 min Pyrethroid-treated flies frequently showed rapid and prolonged knock-down. Especially for pyrethroids, many aborted eggs or larvae were covered with white, sticky fluid. It is suggested that insecticides caused high rates of abortion by general relaxation of maternal muscles, thus opening the vaginal sphincter, smoothing the uterine wall and allowing accumulated milk gland secretions to flow into the uterus. This reduced friction between contents and the uterine wall and the egg or larva escaped passively through the vaginal aperture.

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Résumé Les femelles de G. palpalis palpalis provenaient de captures de populations sauvages près d'Abuja au Nigéria. Les lots de mouches ont reçu des applications locales (0,36 l par mouche) de 14 insecticides chimiques à différentes doses. L'émission d'un oeuf ou d'une larve immature était considérée comme un avortement. Les témoins ont reçu de l'acétone. Les mouches n'ont pas été alimentées après traitement. Les résultats des différents lots ont été comparés par le test de Fisher. Pour 274 mouches traitées à l'acétone, le taux d'avortement croît de 1,1% le ler jour à 6,3% le 5e jour pour diminuer jusqu'au taux final le 9e jour, qui n'excède pas le taux naturel de 7%. On peut penser que le taux croissant est dû à la prolongation du jeûne et que l'acétone n'est pas un abortif. Les avortements se sont produits chez les mouches qui ont reçu chacun des 14 insecticides, les taux variant avec les doses, et pour la plupart des insecticides, augmentant avecle le temps depuis le traitement. Les taux d'avortement étaient particulièrement augmentés pour les lots traités avec des doses élevées de fénitrothion. propoxur ou perméthrine, et étaient très significativement augmentés chez ceux traités avec des doses très élevées de dieldrine ou différentes doses de sumithrine, sumicidine ou deltaméthrine: tous ces taux élevés étaient enrigstrés dans le jour après le traitement.Les avortements étaient particulièrement rapides, dans les 60 minutes, pour la sumithrine, la sumicidine et la delthaméthrine. Aucune mouche traitée à l'acétone ou à l'un des 11 autres produits n'a avorté dans les 60 minutes. Beaucoup de mouches traitées aux pyréthroïdes ont présenté un effrondement rapide et prolongé, suivant la dose. Beaucoup d'oeufs et de larves avortés après traitement aux pyréthroïdes étaient couverts d'un fluide blanc et visqueux. On peut estimer que les insecticides ont provoqué l'avortement par relâchement des muscles maternels, permettant l'ouverture du vagin, le déridement de la paroi utérine et la décharge dans l'utérus des secrétions de lait accumulées dans les glandes. Ceci a réduit la friction entre les parois de l'utérus et son contenu (oeuf ou larve) qui glissait à travers l'ouverture du vagin. Il est peu vraisemblable que les avortements induits par les insecticides puissent réduire les populations pendant les traitements.

     

Horizontal transmission of mycotic infection in adult tsetse,Glossina morsitans morsitans

AdultGlossina morsitans morsitans exposed to wet conidia ofBeauveria bassiana andMetarhizium anisopliae suffered high mortalities ranging from 90 to 100% by 2 weeks post-exposure. Infected ♂ ♂ maintained in the same cages with non-infected ♀♀ throughout the experimental period transmitted the fungal infection to the ♀♀ resulting in mortalities of 65% withB. bassiana and 55% withM. anisopliae. Likewise, infected ♀♀ maintained together with non-infected ♂♂ transmitted the infection to the ♂♂ resulting in mortalities of 75% withB. bassiana and 45% withM. anisopliae. Female tsetse flies infected withB. bassiana andM. anisopliae and maintained in the same cages with non-infected ♀♀ also transmitted infection to the non-infected tsetse resulting in mortalities of 62% and 48% withB. bassiana andM. anisopliae respectively. Infected tsetse exposed to non-infected tsetse of the opposite sex for only 30 min were also able to transmit the fungal infection. Pupae produced by female tsetse infected withB. bassiana andM. anisopliae exhibited higher pupal mortality than those produced by non-infected ♀♀. However, pupae exposed directly to dry spores ofB. bassiana andM. anisopliae had no increase in pupal mortality but adults emerging from theB. bassiana-exposed pupae had markedly reduced longevity.