Notes on the Bionomics of Glossina swynnertoni, Austen

Extract

1. G. swynnertoni is a source of danger to man and to cattle. Until reclamation was started it was rapidly advancing in the Shinyanga District and was driving the native population in front of it.

2. Round Shinyanga this species occurs in gently undulating country supporting four main types of vegetation, namely:— (a) a Commiphora Fischeri community found on the well-drained rises with a good grass growth ; (b) a Lannea humilis-Commiphora Schimperi community on low-lying hard-pan areas in which the grass is poor and patchy ; (c) communities in which the grass growth is good and the main trees are Acacia spp. ; (d) a plains community, partly treeless, partly covered with shadeless woods of the small gall acacias, which are swampy during the rains and very dry at other periods of the year ; during the dry season the trees are completely leafless, and very dry and severe conditions prevail.

3. Game is in moderate numbers throughout the year, so that there is no lack of food in any type of vegetation.

4. The majority of flies living in the hard-pan areas obtain and digest their food there. Flies behave similarly in the C. Fischeri association, but some of them whilst searching for food may encounter a hard-pan area, where they will continue their search for food. When they get food they may or may not return to the C. Fischeri association to digest it. If a fly strikes a road or open plain it will patrol this until it finds food, when it will return to more sheltered vegetation to digest it.

5. G. swynnertoni is mainly a thicket-breeder, though pupae are also found under decumbent logs, overhanging rocks and in hollows at the bases of trees. Where the C. Fischeri was much broken up by valleys of hard pan, breeding was found to be taking place to the same extent in the thickets of both of these types.

6. G. swynnertoni is commonest in hard pan, much less common in C. Fischeri, scarce in the Acacia bush types worked in Shinyanga, and least common in the open plains.

7. When flies are numerous, each of the first three types enumerated in (2) is able to support a fly-community. When the flies are scarce, the C. Fischeri and the Acacia communities lose their flies to the hard pan and hence are little infested. The hard pan is to a far greater degree self-contained as regards the requirements of the fly.

8. G. swynnertoni is commonest during the first month or two of the dry season. As the dry season progresses it decreases in numbers and reaches its minimum during the short rains. There is a rise in the short dry season following the improvement of conditions brought about by the early rains ; after this numbers remain fairly constant until the end of the long rains. Hunger and female percentage are least in the beginning of the break of the short rains.

9. There is no indication that the concentrations of fly in the hard-pan areas disperse at the end of the rains.

10. Like Glossina morsitans, G. swynnertoni appears to find food by sight rather than by scent. The part that scent plays in the matter has hardly been finally elucidated.

11. G. swynnertoni is capable of crossing an 800 yards clearing without the assistance of game, irrespective of whether the clearing passes over alluvial or eluvial soil. It is the hungry flies mainly that cross. They are presumably searching for food.

12. The hungrier a fly is the more persistent it is in following man. Males are more persistent followers than females.

13. An experimental attempt was made to exterminate G. swynnertoni by hand-catching off screens in a block of 15 square miles during sixteen months. Reduction in numbers occurred, but flies continued to exist in the block because the isolating clearings were too narrow. Further, female flies do not appear sufficiently readily to catchers, so that breeding is able to continue.

14. A very large rise in female percentage occurred. A similar but lower rise marked an experiment in the catching-out of G. palpalis by hand on Mugasiro Island, near Musoma, by Robert Koch in 1913. In this the catching was off man only.