Biological characteristics and rearing conditions of the stick insect, Ramulus irregulariterdentatus (Phasmida: Phasmatidae)

Abstract

Public interest in Ramulus irregulariterdentatus as a pet insect in Korea is increasing although it is also considered as a potential forest pest. This study was carried out to investigate the biological characteristics and mass rearing technique of R. irregulariterdentatus.<br/> In 2016, seasonal occurrence of R. irregulariterdentatus was investigated at Guksabong Peak in Goyang, Gyeonggi-do. The first instar nymphs appeared from April 20. The population of the species was peak on May 19 and then declined in late July. No more individual were observed after September. The 62.5 % of individuals were observed from black locust trees and 21.5 % oak trees such as oriental chestnut oak, mongolian oak and chestnut. The amount of daily leaf consumption by one adult was about 4.2 cm2 (0.09 g). R. irregulariterdentatus propagates mainly by parthenogenesis, thus male occurs very rarely in nature, but a male was discovered which was mating in Goyang, 2013. The color of 1st and 2nd instar nymphs was always light brown and light green, respectively, while 3rd-6th instar nymphs and adults showed green or brown according to their living conditions. <br/> Several unique survival strategies of R. irregulariterdentatus were investigated. The duration of death-feigning of nymphs and adults was 336 sec. and 515 sec., respectively. R. irregulariterdentatus showed color change from the 3rd instar nymph. After autotomy of one leg, the leg was regenerated in the next or 2nd next molting. At the first phase, the leg was malformed with the symptoms of twisted, thin and short shape. At the next phase, the leg was unfolded normal shape but very short, compared to normal one. The short leg became longer as molting was repeated, and was recovered by being over 90 % of length of the normal leg after 3 times of molting after regeneration of the twisted leg.<br/> Development rates were fitted with a nonlinear Briére model which estimated optimal temperatures to be 24.5 and 26.2 °C with upper development thresholds of 29.3 and 31.4 °C for egg and nymph, respectively. In a linear model, lower development thresholds were estimated as 7.6 and 5.2 °C for egg and nymph, respectively. Survivorship was the highest at 21.0 and 22.2 °C for egg and nymph, respectively. Mean fecundity per female life ranged from 14.4 eggs at 17.5 °C to 32.0 eggs at 23.5 °C. It was fitted to an extreme value function. Adult survival and cumulative oviposition rate of R. irregulariterdentatus were fitted to a sigmoid function and a two-parameter Weibull function, respectively. These models can be used to forecast phenology and population dynamics of R. irregulariterdentatus in the fields and optimize environmental conditions for rearing R. irregulariterdentatus. <br/> The egg hatch rate of R. irregulariterdentatus was more than 80 % at 20-25 °C, but 0 % at 30 °C. In nature, R. irregulariterdentatus overwinters as an egg, but it appears that cold temperatures are not required for termination of egg diapause. When several treatments were taken to improve the egg hatch rate, in the treatment using floral foam, fermented sawdust and leaves, the rate was 98.2 %. Clover (Trifolium repens) was an excellent diet as it was similar to the host plant and could be used as an alternative diet. The rate from 1st instar nymph to adult of R. irregulariterdentatus was 88.3 % by using an artificial diet of 25 % black locust leaf powder and 1 % agar. In the density experiments for mass rearing, the survival rate from 1st instar to adult for 60 days was 82.0 % at 50 insects/pot (59 × 18 × 13 cm, planted clover) and the number of oviposition was 710 at 20 adults/cage (49 × 34 × 27 cm). <br/>