Effects of temperature on life histories of three endangered Japanese diving beetle species

To elucidate population-increasing factors in the diving beetle Cybister tripunctatus lateralis (Fabricius) (Coleoptera: Dytiscidae) in Japan in recent years, life histories and oviposition patterns were compared among three endangered diving beetle species, Cybister brevis Aube (qualified by the Japanese Red Data List as 'near threatened'), Cybister chinensis Motschulsky (vulnerable), and C. tripunctatus lateralis (vulnerable). Oviposition in C. brevis, C. chinensis, and C. tripunctatus lateralis was observed from late April to mid-June, from late April to early July, and from late May to mid-August, respectively, under semi-outdoor conditions. There were no interspecies differences in total hatchling production during the reproductive season. In rearing experiments at various temperatures (20, 23, 25, 28, and 30 degrees C), the mortality of C. tripunctatus lateralis larvae was higher at 20 degrees C, and gradually lower with increasing temperature up to 30 degrees C. Adult body size of females in C. tripunctatus lateralis is larger than that of males but there were no significant differences among temperatures (25-30 degrees C). Cybister brevis had a higher emergence rate at 23-28 degrees C than at 20 and 30 degrees C. In C. brevis, the body size of adults reared at 25 or 28 degrees C was significantly larger than at other temperatures. Cybister chinensis did not differ in emergence rate and adult body size among the five temperature conditions. The developmental zero (i.e., the lower developmental threshold) from the first instar to adult emergence was 11.1 degrees C for C. brevis, 8.7 degrees C for C. chinensis, and 16.8 degrees C for C. tripunctatus lateralis. We speculate how the influence of global warming may have a positive impact on the growth and survival of C. tripunctatus lateralis.