a1 East African Institute of Malaria and Vector-borne Diseases, Amani, Tanga, Tanganyika
An account is given of marking and release experiments with Anopheles gambiae Giles in a coastal area of Tanganyika. Laboratory-reared mosquitos were used, labelled either by the topical application of paint or by the introduction of radioisotopes into the larval breeding pans. Two different isotopes were used, 32P and 35S, and recaptures were recognised by autoradiography.
Routine catching stations were established within a circle of radius of 1¼ miles. Releases were made either in the centre or near the periphery of the experimental area, so that recaptures were possible up to a maximum of 2¼ miles.
The following results were obtained:
1. Of 132,000 mosquitos released, 1,019 were recaptured.
2. The mean flight range of females released in the centre was estimated to be 0·64 mile, and of males 0·52 mile. Of females released on the periphery, the mean range of dispersion was estimated to be 0·98 mile. Individuals of both sexes were caught at the maximum range of 2¼ miles.
3. The dispersion of recaptured mosquitos was shown to be non-random and to be related primarily to the distribution of human settlements.
4. In certain series of releases the direction of the prevailing wind had a definite effect on the dispersal of mosquitos. But in general this was a minor factor.
5. Dispersion during the first day or, in many instances, during the first two days after release was more restricted, compared with that of older mosquitos. But no difference in the distribution of catches was detected between those aged three-nine days and those more than nine days old.
6. Marked females were recaptured up to 23 days after release. Apart from the first two days, the regression of density on age amounted to a daily loss of 16 per cent, of mosquitos from the experimental area. The effect of emigration could not be assessed quantitatively, but it was held to be a minor component of the total daily loss. The relatively high level of mortality suggested by these figures is attributed to the use of laboratory-reared mosquitos.
7. The corrected sporozoite rate in marked females at the time of recapture was 0·8 per cent.
8. The survival of males was only slightly lower than that of females.
9. It is concluded from the survivorship curve that the mortality rate remained constant throughout the period in which marked females were recovered.