The influence of the larval food plants on microsporidia (Nosema mesnili P.)infection in diapausing Pieris brassicae L. pupae

Abstract: The Large White Butterfly, Pieris brassicae L., is a cosmopolitan insect, and is foundwherever cruciferous plants are grown. The P. brassicae is one of the most important pest insectsin cruciferous plants of Estonia. The experiments were carried out in the experimental laboratoryof the Estonian University of Life Sciences. In our experiment six foodplants were used: whitecabbage (Brassica oleracea L. var. capitata f. alba), german turnip (B. oleracea var. gongylodesf. gongylodes), ornamental cabbage (B. oleracea var. acephala), broccoli (B. oleracea var.italica), cauliflower (B. oleracea var. botrytis) and Garden Nasturtium (Tropaeolum majus L.).The aim of the present work was to investigate the effect of larval food plants on natural infectionby mircosporidia (Nosema mesnili Paillot) in diapausing Pieris brassicae L. pupae and to assessthe influence on hibernation of P. brassicae pupae. Physiological state of P. brassicae pupae wasestimated by measuring standard metabolic rate (SMR). The infection by N. mesnili wasdetermined by microscopic analysis after SMR.A statistical analysis of the results indicated that infection by N. mesnili of P. brassicaepupae was the lowest on white cabbage. Infection was highest on german turnip and gardennasturtium. The experiments showed that less pupae were infected by N. mesnili in ornamentalcabbage, broccoli and cauliflower variants. Healthy pupae on nasturtium plants had a higherSMR rate than on white cabbage. Comparing infected and healthy pupae in white cabbage andgarden nasturtium variants revealed that the value of SMR was about three times higher in bothinfected variants. The higher metabolic rate of infected P. brassicae pupae indicated that thosepupae would not have been able to survive winter conditions.