Effects of temperature on the development and fecundity of Thalassodes immissaria Walker

Abstract: 【Objective】Thalassodes immissaria Walker is one of the most important pests in litchi and longan in China. They mainly feed on the young leaves of new shoots of litchi and longan, when they occur in large numbers, the new flowers cannot grow, and they can also feed on flowers so that the trees cannot bear fruits, and the larvae also can feed on the functional leaves during the fruiting period and causes a large number of fruit falling, which seriously threatens the production of litchi and longan. The biological parameters such as effective accumulated temperature and developmental threshold temperature can reflect the adaptability of insects to environmental temperature and can be used to predict the geographical distribution and generation of insects. The experiment was carried out in order to clarify the effect of temperature on survival, growth and reproduction of T. immissaria, and provide basis for prediction and comprehensive control of this insect.【Methods】T. immissaria populations were artificially reared at five constant temperatures (20 ℃, 23 ℃, 26 ℃, 29 ℃ and 32 ℃ ) feeding with litchishoots [14 h∶10 h (L∶D) and (75%±5%) RH], and the survival rates, development duration, adult longevity and fecundity in each temperature treatment group were measured and compared by the agestage two-sex life table. Based on regression analysis and direct optimization, the developmental threshold temperature and effective accumulated temperature of different developmental stages were also calculated. The 1-day-old eggs collected at the same time were placed in a plastic rearing box of 15 cm × 11 cm × 7 cm. The lid of the box was drilled with small holes for ventilation, and a paper towel was placed in the box. Larvae hatched at the same time were reared separately and labeled, which were fed with the young shoots of Feizixiao, a famous variety of litchi. The survival number of each developmental stage, egg hatching time, larval molting and pupation time, and adult eclosion time, were recorded under the different temperature conditions. After pupae emergence, the adults were fed with 10% honey water, and the adults emerged on the same day were fed and mated in groups in accordance with the ratio of female to male 1∶1. The female adults were separated into boxes to lay eggs the next day, and the tender shoots of litch were placed at the bottom of the box for laying eggs, the base were wrapped with wet cotton. The number of eggs laid by the female and the lifespan of the adult were recorded daily until the females died. The parameters of the population life table were calculated according to the following formula: Intrinsic rate of increase (r): ∑x = 0 ∞ e -r(x + 1) l xmx = 1 , net reproductive rate (R0): R0 = ∑x = 0 ∞ l xmx , finite rate of increase (λ): λ = e r , population doubling time (Dt): Dt=(ln2)/r, mean generation time (T): T = (lnR0)/r.【Results】T. immissaria could complete its life cycle under the temperature from 20 ℃ to 32 ℃, and the developmental durations of different stages were negatively correlated with the temperature. The developmental durations of egg, larva and pupa of T. immissaria were the shortest at 32 ℃ (2.29, 12.48, 6.10, respectively). In addition, the temperature had significant effects on the survival rate of this insect. The survival rate of this pest was the lowest at 32 ℃, especially the 1st instar, was greatly affected by the temperature, which was less than 80%. In addition, the longevity of female and the number of eggs laid per female were also the least (125.20±29.38 eggs per female). The experimental population life tables showed that the r values were (0.08±0.01), (0.08±0.01), (0.11±0.01), (0.12±0.01), and (0.10±0.01), and the values of net reproductive rate (R0) were (139.63±7.69), (142.09±10.38), (176.92± 8.52), (107.01±9.11) and (45.57±5.21) at 20, 23, 26, 29 and 32 ℃, respectively. The relationship between the developmental rate of T. immissaria and temperature conformed to the regression quadratic model. Based on the linear regression method, the developmental threshold temperature of the egg, 1st instar, 2nd instar, 3rd instar, 4th instar, 5th instar, prepupa, pupa, and the generation time of T. immissaria were 3.84 ℃, 7.54 ℃, 4.48 ℃, 10.71 ℃, 7.70 ℃, 5.96 ℃, 15.65 ℃, 8.25 ℃ and 7.68 ℃, respectively, and the effective accumulated temperature were 64.30, 43.26, 56.66, 51.51, 66.23, 90.15, 19.70, 135.67 and 531.99 degree-days, respectively.【Conclusion】The development and reproduction of T. immissaria were significantly affected by the temperature, and the optimum temperature range for its growth and development and reproduction was between 23-29 ℃.