Zoological Studies

Vol. 55, 2016

(update: 2016.5.31)

Population Dynamics and the Effects of Temperature on the Eggs of the Seawater-dispersed Stick Insect Megacrania tsudai (Phasmida: Phasmatidae)

Shun Kobayashi1,2,*, Ryota Usui3, Kouta Nomoto4, Mineyuki Ushirokita5, Tetsuo Denda66, and Masako Izawa6

1Graduate School of Engineering and Science, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa, 903-0213 Japan
2Research Fellow of Japan Society for the Promotion of Science
3Graduate School of Agriculture, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa, 903-0213 Japan
4Itami City Museum of Insects, Koyaike 3-1, Itami, Hyogo, 664-0015 Japan.
5ECO-PLANNING RESEARCH co. Ltd., Sakaemachi 2-28-5, Higashimurayama, Tokyo, 189-0013, Japan
6Faculty of Science, University of the Ryukyus, Senbaru 1, Nishihara, Okinawa, 903-0213 Japan

(Received 2 June 2015; Accepted 4 January 2016)

Shun Kobayashi, Ryota Usui, Kouta Nomoto, Mineyuki Ushirokita, Tetsuo Denda, and Masako Izawa (2016) Although the stick insect Megacrania tsudai cannot fly, its eggs are dispersed by seawater, which allows the species to distribute itself widely via the ocean. The life history of this non-volant insect that establishes settlements by egg dispersal remains poorly understood. We aimed to clarify the population dynamics and effects of temperature on the eggs of M. tsudai through field observation in near the northern limits of the distribution and laboratory experiments. In the wild, all instars appeared over a period of several months, with a larger proportion of first instars emerging from December to March, and a greater number of adults being observed from June to August. Laboratory experiments showed that the hatching rate of M. tsudai was not different between conditions of 25 and 30°C, whereas it was significantly lower at 20°C. The egg-development period was shorter when eggs were maintained under high temperature conditions, and longer for eggs kept at 20°C. The effective cumulative temperature was estimated as 1561.5-2000.0°C, and the developmental zero was estimated as 12.2-13.9°C. The peak appearance of the first instar in the laboratory, which was used to obtain an estimate for the effective cumulative temperature and peak appearance of adults in the wild, was not entirely accurate because the peak appearance of the first instars in the wild was not consistently observed when the estimation indicated that they should appear. Megacrania tsudai is a univoltine in near the northern limit of its distribution, and temperature has a strong effect on its egg development. Our estimation of population dynamics by laboratory experiments did not exactly predict what was observed in the field and it may be controlled by other factors. However, our findings indicate that the limiting factor of the distribution of this species is likely the effect of temperature on egg hatching.

Key words: Developmental zero, Hatching rate, Megacrania tsudai, Non-volant, Northern limit, Population dynamics, Seawater dispersal, Temperature.

Correspondence: E-mail: cheirotonus.jambar@gmail.com

Vol. 55, 2016