AN INTRODUCTION EXPERIMENT OF AN HERBIVOROUS LADY BEETLE - CHARACTERISTICS OF THE ADULT-POPULATION

In May 1971, 45 adults of an herbivorous lady beetle Epilachna niponic a (Coleoptera: Coccinellidae) from Asiu Experimental Forest were intro duced into a botanical garden of Kyoto University, where is 10 km sout h of the southern limits of its distribution with being 3-5-degrees-C warmer than the original site. The introduced population of the lady b eetle was thus investigated from 1975 to 1981. Mark-release-recapture experiments were applied to individual adult beetles, to estimate popu lation size and daily survival rate. Overwintering adults emerged from hibernation around early April, reaching peak numbers in late April t o early May, then gradually declined to late June. No adults remained at the end of June. Adult survival was maintained at a high level to e arly May, and declined consistently until late in the reproductive sea son. New adults began to emerge in late June and quickly reached a pea k in early July; thereafter they decreased in number and had entered h ibernation by late October. In spite of seasonally deteriorating food resources and heat stress in summer, new adults showed moderately high survival during the inimical period. New adults which emerged later i n the season tended to be smaller in body size than those that emerged early. The proportion of females in the new adult population graduall y increased throughout the pre-hibernating period, suggesting that mal e-biased mortality occurred during this period. When compared to the s ource population, the introduced population had a higher rate of popul ation growth. Coupled with the improved population growth, heavy leaf damage during the larval period suggested that intensive intraspecific competition was most likely to occur among larvae in the introduced p opulation.