The use of gamma irradiated Glossina austeni females as sentinel insects for entomological monitoring in tsetse control programmes

Abstract

A 60 Gy gamma treatment administered to female Glossina austeni on day 2 or 9 following emergence, and likewise, a 50 Gy gamma treatment given to pupae on day 33 following larviposition, induced complete sterility in the female flies without altering their mating behaviour. Treated females remained receptive to mating with untreated males up to 15 days following emergence (mating response of 84 %). The timing of treatment (on day 33 post larviposition, on day 2 and 9 following emergence) influenced significantly the dynamics of the follicle development. Females, irrespective of their age when treatment was received, showed a normal development pattern of the follicles in position A₁ and C₁i.e. normal vitellogenesis, maturation and ovulation. Females treated as pupae however, revealed no visible signs of a development of follicles in position B₁ and D₁. From day 15 on, females displayed inactive ovaries characterized by atrophied oocytes and nurse cells. Treating females on day 2 or 9 of their adult life, resulted in various degrees of development of the B₁ and D₁ follicles. During laboratory cage tests, untreated males exposed to equal numbers of virgin untreated and treated females, showed no significant preference to mating with either type of female. A high degree of multiple mating was observed when untreated and treated females were offered several mating opportunities. On day 9 following emergence, 24,0 % of untreated and 23.8 % of females treated with 60 Gy (on day 2), accepted a male during a fourth mating occasion. The receptivity to remating decreased with a higher radiation dose (120 Gy) and when treatment was given later in the female life (day 5). The results of the laboratory experiments are discussed in view of deploying gamma sterilized female Glossina austeni for entomological monitoring in those areas where low fly densities exist and especially, to expose potential relic fly pockets after control operations have been completed.