Published online by Cambridge University Press: 06 April 2009
Hymenolepis citelli caused a marked mortality in Tribolium confusum during the first 14 days of infection. The mortality was higher when the beetles were starved for 6 days immediately prior to infection (73–93%) than when they were starved for 1 day (51%). Regardless of the starvation time, the majority of beetle deaths occurred between days 8 and 11 post-infection (p.i.) and the precise survival time for individuals tended to vary inversely with the number of parasites they contained. With few exceptions beetles that survived until day 15 p.i., when the experiments were terminated, contained 14 or fewer cysticercoids, whereas those that died on or before day 14 p.i. contained 20 or more parasites. Mortality was equally high in both sexes but the mean survival time was significantly shorter for female beetles (8·8 days) than for males (9·6 days). Also, parasite development rate varied with the sex of the host. When parasite populations recovered from beetles that died on the same day were compared, the degree of development attained by those from males was significantly higher than in the populations grown in female hosts. The line of H. citelli used in these experiments has been maintained in the laboratory for the past 5 years. The results obtained with the current generation of this line were compared with those obtained with it 2 years earlier (approximately 16 generations ago). Mortality was lower with the current generation (67%) than with the earlier generation (93%). However, with the latter generation, the mean parasite population size in beetles that survived to day 15 p.i. was significantly higher (14·1 cysticercoids/beetle) than in those infected with the recent generation (7·3–7·9) cysticercoids/beetle). These findings suggest that relatively high selection pressures have been applied by T. confusum to H. citelli, thereby reducing the number of parasite lines with high infectivity and consequently improving host survivorship. The host also responded to artificial, directed selection for reduced cysticercoid number (heritability estimates of 0·30 ± 0·12 and 0·26 ± 0·12) and increased survival (heritability estimates of 0·28 ± 0·12 and 0·15 ± 0·083). The realized heritability was 0·37 for cysticercoid number and 0·17 for increased survival.