EFFECT OF CHANGES IN THE THERMOPERIOD ON REPRODUCTIVE DIAPAUSE IN BRUCHIDIUS-ATROLINEATUS PIC (COLEOPTERA, BRUCHIDAE)

Bruchidius atrolineatus (Pic) is a tropical bruchid developing in Vigna unguiculata (Walp) pods. Adults are in reproductive diapause during the dry season. In conditions of 40:25-degrees-C or 35:25-degrees-C, diapause is induced when larvae develop in thermophases longer than 14 h, in thermophases shorter than 10 h and in continuous darkness. Thermophases of 12 h cause a reduction of the proportion of diapausing beetles. This proportion also depends on the thermoperiod temperatures. Development at low temperatures (23:16-degrees-C, 12:12 h) induces a high percentage of diapausing beetles. However, at low temperatures, the developmental times of the beetles are long and show important interindividual variability. When larvae develop in decreasing temperatures, small changes in thermoperiod temperatures (35:25-degrees-C to 30:20-degrees-C) have a strong effect on diapause induction. When larvae develop at low temperatures (25:15-degrees-C) and then at high temperatures (40:25-degrees-C), the proportion of diapausing beetles depends on the duration of development at low temperatures. Whatever the developmental conditions, both sexually active and diapausing beetles always emerge from the seeds. This interindividual variability is important in this species which has a wide geographical distribution in Africa.