Diapause in eastern North American populations of Coccinella septempunctata (Coleoptera: Coccinellidae)

Abstract

Experiments examined the life histories and the effect of photoperiod on the expression of adult hibernal diapause in the Palearctic ladybird beetle Coccinella septempunctata, within 10 years of its discovery in North America. The study used beetles from two populations in eastern United States and focused on two aspects of diapause. First, the effects of daylength on the induction and depth (or duration) of diapause were examined by rearing and maintaining adult beetles from Tompkins County, New York, under six constant photoperiods until oviposition, death, or the passage of 125 days. Second, the seasonal progression of diapause maintenance and termination under natural conditions was evaluated by periodically (between October and April) testing the photoperiodic responses of overwintering adults that had been held out-of-doors; this test used beetles from Bergen County, New Jersey. Despite large variation in responses, the first test indicated that LD 16:8 had a strong, but not complete, diapause deterring effect. About 60% of females reared at LD 16:8 oviposited without delay, whereas photoperiods with shorter daylengths yielded lower incidences of oviposition without delay (18% at LD 14:10, 0% at LD 12:12 and 10:14). Only 27% of the females that experienced daylengths of LD 18:6 and 17:7 reproduced without delay. Females that survived the full length of the experiment (125 days) without ovipositing were considered likely to express an alternate (univoltine) life-history pattern that was unaffected by photoperiod. In the second experiment, results from the naturally overwintering C. septempunctata population indicated that females undergo significant changes in their photoperiodic responses during hibernal reproductive diapause. Based on the beetles' seasonal pattern of photoperiodic responses, we conclude that during autumn, short (and/or decreasing) daylengths play a role in diapause maintenance. By January, the photoperiodic maintenance of diapause diminishes, and by April, photoperiod no longer affects the timing of postdiapause reproduction. In our study, a small proportion of the overwintering adult C. septempunctata was parasitized by the koinobiont parasitoid Dinocampus coccinellae (Hymenoptera: Braconidae); data on the parasitoid's emergence pattern indicated that photoperiod plays a role in maintaining its diapause until the middle of January.

Keywords

• reproductive diapause
• photoperiod
• univoltinism
• seasonality
• biological control
• introduced population