Dynamics of the Photo-Trophic Termination of Reproductive Diapause in Females of the Multicolored Asian Ladybird Harmonia axyridis (Pallas) (Coleoptera, Coccinellidae)

Abstract

Females of certain aphidophagous ladybirds in the absence of natural protein food (aphids) enter reproductive diapause. Reactivation of diapausing beetles is possible only after consuming the food which is suitable for oogenesis of females and development of larvae. The influence of diet and photoperiod on the dynamics of weight and on the rate of reproductive maturation of reactivating females of Harmonia axyridis was studied under laboratory conditions. The experiments were conducted at combinations of two day lengths (12 and 18 h) with 6 trophic regimes with the following mean numbers of daily consumed aphids: 0 (aphids were absent over the whole experiment), 0.1 (1 aphid was provided every 10 days), 0.5 (1 aphid every second day), 1 (1 aphid every day), 10 (10 aphids every day), and 50 (about 50 aphids every day). Judging from the state of ovarian development estimated by dissection 20 days after the beginning of the experiment, the threshold of the trophic termination of reproductive diapause under the long day conditions lies between the regimes of 0.1 and 0.5 aphids per day, which is approximately equal to the earlier investigated threshold of the trophic induction of diapause. Short day slowed reactivation and has increased the threshold of the trophic response up to 1-10 aphids per day. Based on these data, we conclude that under natural conditions females which entered reproductive diapause because of the absence of aphids are capable of reactivation in the presence of even a minimal amount of natural protein food. Moreover, in spring and summer (when the probability of the increase in aphid abundance is relatively high) reactivation starts at lower prey population density than in autumn, when the appearance of the stable aphid colony (which is necessary to complete the development of the emerged larvae) is less probable.