Effects of sucrose supplementation and generation on life-history traits of Bracon cephi and Bracon lissogaster, parasitoids of the wheat stem sawfly

Abstract

Knowledge of the life‐history traits that influence the reproductive success of parasitoids could provide useful information to enhance their effectiveness as biological control agents. The wheat stem sawfly Cephus cinctus Norton (Hymenoptera: Cephidae) is a major pest of wheat in the Northern Great Plains of North America. The bivoltine, sympatric and specialist parasitoids Bracon cephi (Gahan) and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae) are the only species known to attack C. cinctus in wheat. In the present study, we quantify the body size, longevity, egg load and egg volume of B. cephi and B. lissogaster aiming to test whether these life‐history traits differ among adult females developing from overwintering and summer generation larvae. We also investigate the effect of sucrose supplementation on the same reproductive parameters. The results obtained show no differences between life‐history traits of B. cephi developing from the overwintering and summer generations. By contrast, the egg load of B. lissogaster is significantly different between generations, with overwintered females producing 1.2‐fold more eggs than summer individuals. Sucrose feeding has a strong positive effect on life‐history traits of both parasitoid species and generations. The longevity of sucrose‐fed females of B. cephi is increased three‐fold, whereas the survival of B. lissogaster is increased almost nine‐fold. Females of B. cephi treated with sucrose have four‐fold more mature eggs, whereas the egg load of B. lissogaster is increased 1.15‐ to 1.25‐fold with sugar feeding. Only B. cephi females increase egg volume with sucrose. The present study provides valuable insights on the life‐history and nutritional requirements of these two important parasitoid species.