Weather and the phenology of some African Tabanidae.

Insect populations were monitored with the aid of light-traps at Ibadan, Western Nigeria, in 1966-68 and for 3 months in 1970 in central Sudan [see RAE/A 61, 4387]. In Nigeria, Tabanus guineensis Wied. was common in all 3 years and T. taeniola P. de B. fairly common at 1 place in 1 year. It is suggested that the most important single factor affecting their phenology at Ibadan was the incidence of rainfall of 10 mm or more in a single fall, which was related to captures of adult females of the 2 species about 28 and 31 days later, respectively. In T. taeniola, there was evidence for a shorter reaction period of 7-10 days also. These species seemed to require alternate wetting and drying of the soil to induce emergence of the adults.T. guineensis was most active (as measured by numbers taken in light-traps) at temperatures of 25-27 deg C, with little or no activity above 31 deg C, and at a saturation deficit of 6-8 mbar. This species was markedly crepuscular, first flight by females being closely defined by the time of the end of civil twilight at a light intensity of about 3.5 lux and crep value of +1.0; no effects of moonlight on catches could be shown. All females of T. guineensis, T. taeniola and several other species caught in light-traps were unfed, and fertilised but with undeveloped ovaries showing no signs of previous ovarian cycles, and it is concluded that they were young females dispersing. Females caught on cattle, including many examples of T. pluto Wlk., were in a similar condition except that the crops of some examples contained varying amounts of fresh blood. It is concluded that females biting cattle were also young examples taking their first blood-meal.In the Sudan, Atylotus agrestis (Wied.) and T. sufis Jaennicke were taken. More examples of both species were taken when the Inter-Tropical Convergence Zone was near the trap than when it was some distance away [see loc. cit., A 63, 413]. A. agrestis became uncommon soon after the final southward movement of the Zone, and T. sufis disappeared altogether 3 days after the last record of southerly winds at the trap site. Trapped females of both the Sudan species included examples that varied from unfed immatures to blood-fed examples with mature eggs; there was some evidence that in both species there were autogenous females. It is shown that the ambient conditions in which African tabanids are active are similar to those reported for high-latitude species in Europe and North America.The survival strategy of T. guineensis and T. taeniola, with a static (diapause) stage awaiting the return of favourable conditions, is contrasted with the dynamic (migratory) strategy of A. agrestis and T. sufis. The implications for the epidemiology of diseases, of insects that may be cyclical vectors migrating in association with the Inter Tropical Convergence Zone is discussed, with special reference to trypanosomiasis in camels (which may be transmitted by migrant tabanids) and anthrax (which is transmitted by tabanids), and diseases carried by other Diptera, such as ceratopogonids.