Demography and predatory potential of Orius strigicollis on eggs of Plutella xylostella at two temperatures

Insects

A few individuals of O. strigicollis were collected from the experimental cotton fields of Huazhong Agricultural University (Wuhan, China). A further stack culture was setup following the same method that we used in our previous study (Rehman et al., 2020), described by Zhou et al. (2006) with modifications. The rearing arenas were made of small plastic containers with lids, each measuring 23 cm × 22 cm × 5 cm. Both young and adults individuals of predatory bugs were supplied with Aphis fabae to eat and two to three stems of Vitex negundo (Lamiaceae), commonly known as Chinese chaste, were wrapped in wet cotton at the end to facilitate the oviposition. Moreover, the leaves of V. negundo provide a microclimate that can be suitable for the development of Orius eggs and nymphs, offering optimal humidity and temperature conditions (Ali et al., 2020; Zhou et al., 2006). All the bugs were reared in an insectary located on the campus with the following environmental conditions; temperature 26 ± 1 °C, relative humidity (RH) 70 ±  5%, and a photoperiod of 16 L: 8 D h at a light intensity of 1,400-1,725 lux.

Eggs of P. xylostella were collected from Hubei Key Laboratory of Insect Resources Utilization and Sustainable Pest Management, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China. The newly hatched larvae were provided with Chinese cabbage leaves that were sown in small plastic pots and placed inside large screen cages (65 × 65 × 65 cm) until pupate. The pupae then transferred to new cages for adult emergence. After emergence, adult were provided with honey solution (10%). The radish (Raphanus sativus: Brassicales: Brassicaceae) were potted in small plastic boxes (6 × 10 × 20 cm) and provided to adults for oviposition. The temperature and relative humidity for P. xylostella mass rearing were set at 25 ± 1 °C and 75 ±  5% (RH) respectively with 16 L: 8D light and dark photoperiod.

Experimental protocols

The mean temperature in the area of the Yangtze River is subtropical (Zhang et al., 2012). The results obtained from the study conducted by Cocuzza et al. (1997) on O. albidpennis, showed that Orius spp. can adapt to a range of high temperatures. Hence, based on the previous studies conducted on O. strigicollis in this region (Ali et al., 2020; Amer, Fu & Niu, 2018; Zhang et al., 2012; Zhou et al., 2006), two temperatures (i.e., 28 and 32 ±  1 °C) were selected for our study. The artificial climate controlled chamber (HP250GS, Ruihua Instrument & Equipment Co., Ltd., Wuhan, China) with 70 ±  5% R.H and 16: 8 L & D photoperiod was used in all experiments.

Bioassay

The biological characteristics of O. strigicollis were determine using the same method we used in the previous study (Rehman et al., 2020). In total, 80 fresh eggs of predatory bugs were collected from the insectary, incubated, and after hatching the individuals of O. strigicollis (≤ 24 h) were separated in small Petri dishes (diameter: nine cm and depth: two cm) containing filter paper (i.e., experimental unit). A small stem of Vitex negundo was provided in each arena to avoid desiccation. Based on the preliminary experiment, 30 eggs of P. xylostella were introduced to each experimental unit as food. The number of eggs consumed by predatory bugs in whole or part was counted after every 24 h. Nymphal development and mortality were also recorded. The gender was confirmed immediately after adult emergence. Once the adults had emerged, the male and female gender of O. strigicollis were paired for mating. The females of predatory bugs that remained in mating for >1.5 min were considered as mated (Butler & O’Neil, 2006). After mating, each pair was transferred into small limpid boxes as described above in the mass rearing of O. strigicollis. Each box was supplied with a small tender stem of Vitex negundo as oviposition substrate at each temperature (28 °C and 32 °C). The healthy eggs of P. xylostella (n = 60) were provided to each pair of O. strigicollis for feeding every day. The number of eggs consumed by O. strigicollis adults was counted and replaced by new eggs every day. To confirm the oviposition, the stem was examined every 24 h under a stereomicroscope (15 ×). After the first egg was laid by a female, the stem was replaced every day. The total number of eggs laid by a female was counted under a stereomicroscope (15 ×) until her death. The daily egg consumption and life table traits such as developmental time, pre-oviposition and oviposition, fecundity, longevity, and survival of male and female adults of the predatory bug were recorded at each temperature.

Statistical analysis

To calculate daily and total egg consumption, the data were analyzed by independent samples t-test without transformation (P < 0.05). Data were analyzed using SPSS (version 23, SPSS Inc., Chicago, IL, USA).

The data obtained from life table study was analyzed following the same method that was used in our previous study (Rehman et al., 2020). This method based on the age-stage, two-sex life table theory using a computer program (Twosex-MSChart). (Butler & O’Neil, 2006; Chi, 1988). The population and demographic parameters (eggs, nymphs and adult development, oviposition, pre-oviposition, fecundity, and r, λ, R₀, GRR and T) were calculated using the method described by Chi (2015). Equations (1) and (2) was used to calculate the age-specific survival rate (l_x) and fecundity (m_x). (1)${\displaystyle l_x=\sum _{j=1}^k{\mathrm{s}}_{xj}}$ (2)${\displaystyle m_x=\frac{\sum _{j=1}^k{s}_{xj}{\mathit{f}}_{xj}}{\sum _{j=1}^k{s}_{xj}}.}$ In the equation, S_xj represents age-stage specific survival rate of predatory bug (x = age in days, and j = stage) which indicates the probability of survival of infant to age x and stage j. similarly, f_xj donates age-stage specific fecundity of adult female (Chi & Liu, 1985). To calculate the intrinsic rate of increase (r), the Euler–Lotka equation was used, following the system of iterative division with the age index from 0 using Eq. (3) (Goodman, 1982). (3)${\displaystyle \sum _{x=0}^{\infty }{\mathrm{e}}^{-r\left(x+1\right)}{l}_x{m}_x=1.}$

The values of (R₀) (the potential of a female to produce a mean number of progenies during her life), was determine using Eq. (4). (4)${\displaystyle \sum _{x=0}^{\infty }{l}_x{m}_x=R_0.}$ The relation between female and R₀ can describe as (5)${\displaystyle R_0=F\frac{N_{\mathit{f}}}{N}.}$ In the equation N and N_f donate the total number of O. strigicollis used in the experiment and the number of adult female respectively (Chi, 1988). The gross reproduction rate (GRR) and rate of increase (λ) were calculated using Eqs. (6) and (7). (6)${\displaystyle GRR=\sum _{x=0}^{\infty }{m}_x}$ (7)${\displaystyle \lambda =er.}$ The mean generation time (T) that a population stand in need to rise to R₀-fold of its size i.e., e^rT = R₀ or λ^T = R₀ at the stable age-stage distribution was determine using the Eq. (8). (8)${\displaystyle T=\frac{ln{R}_0}{r}.}$ To calculate the stander errors (SEM) of population and demographic parameters, a bootstrap test method (100,000 bootstrap) was used (Akca et al., 2015; Akköprü et al., 2015; Tuan et al., 2015). To compare the means, a pair bootstrap test method built on the confidence of the interval of difference was used (Chi, 2015; Pru et al., 2015). To get the peaks of survival rate, fecundity, life expectancy and reproductive vales curves, Sigma Plot 12.0 was used.

Predation rate

The effect of two temperatures on daily and total egg consumption of P. xylostella by O. strigicollis is given in Fig. 1. The daily egg consumption of the first and second instars nymph were significantly higher at 28 °C (F: 0.024, t: 5.071, df: 69, P: 0.0001) than at 32 °C (F: 0.564, t: 4.317, df: 42, P: 0.0001). However, the rest of the nymphal instars showed no significant difference in daily predation at both temperatures. While considering the entire pre-adult stage of life, the total egg consumption of third and fifth nymphal instars were highest (F: 33.262, t: −2.126, df: 53.630, P: 0.038; F: 0.039, t: 2.880, df: 30, P: 0.007), with 36.3 eggs per individual at 32 °C compared to 25.8 eggs at 28 °C. Whereas, the consumption of the total eggs by an adult O. strigicollis was statistically higher (68 eggs; F: 1.633, t: 2.872, df: 62, P: 0.006) at 28 °C. The maximum daily predation (15 eggs; F: 6.529, t: −4.523, df: 62, P: 0.000) at the adult stage was recorded at 32 °C.

Nymphal development

The effect of two temperatures on growth and development of O. strigicollis, when fed on eggs of P. xylostella, is shown in Table 1. The obtained results showed that the mean developmental time for eggs of the predatory bug was significantly shorter at 32 °C (2.40 days) compared to 28 °C (2.83 days). The developmental time for 1st, 2nd, and 5th instar nymphs of O. strigicollis was significantly increased at 28 °C. Similarly, the female adult longevity was statistically different and higher (17.33 ± 2.30 days) at 28 °C. However, no significant difference was found in male adult longevity (9.12 ± 1.59 and 7.29 ± 1.90 days) at 28 and 32 °C, respectively. The highest survival rate (42%) of individuals from the eggs to adult stage of life was recorded at 28 °C than at 32 °C (28%).

Fecundity and oviposition

A significant difference was also noticed in the adult pre-oviposition period (APOP), total pre-oviposition period (TPOP), oviposition period, and fecundity of female adult O. strigicollis (Table 2). The highest pre-oviposition (5.0 days) and oviposition period (9.38 days) was recorded at the 28 °C. Similarly, the total fecundity per female adult of O. strigicollis was highest (42 eggs/female) at 28 °C, whereas, it was 17.3 eggs/female at 32 °C. Moreover, the total longevity of adult females was longer (33.44 ± 1.32) at 28 °C. However, no significant difference was observed in the male adult total longevity at both temperatures.

Life table and population parameters

The highest intrinsic rate (r: 0.087 d⁻¹) was observed at 28 °C (Table 3). Thus, our results showed that r decreased significantly with the increase in temperature. Whereas, the rate of increase significantly decreased to λ: 1.09 d⁻¹ at 28 °C compared to at 32 °C (1.28 d⁻¹). The net reproductive rate was R₀: 9.45 and 1.28 offspring/individual at 28 °C and 32 °C respectively. The mean generation time (T) and gross reproduction rate (GRR) significantly increased from 25.71 days, 38.70/offspring and 19.15 days, 8.12/offspring with the increase in temperature from 28 °C to 32 °C, respectively.

The age-stage specific survival rate (S_xj, x = age in days while j = stage which describes the probability of survival of a neonate to age x and stage j) under two different temperatures when fed on eggs of P. xylostella is given in Fig. 2. The results showed variations in the developmental rate and overlapping occurred at the start and end of each stage at both temperatures. The probability of survival of a newly laid female egg to the adult stage of life was 22.5% and 7.5% at 28 °C and 32 °C temperatures respectively. Similarly, for male eggs, the probability of surviving to adulthood was 20% and 17.5% at 28 °C and 32 °C, respectively.

The curve of age-specific survival rate (l_x), the age-specific total fecundity of the whole population (m_x), the age-stage based female fecundity (f_x), and age-specific maternity (l_xm_x: formed on the basis of l_x & m_x) at two temperatures when Anthocorid spp. fed on eggs of P. xylostella is shown in Fig. 3. In the life table study, (l_x) and (m_x) are considered essential parameters. Age-specific survival (l_x) decreased with age and showed an inverse relation with low and high regimes. However, f_x, m_x and l_xm_x first increased and then decreased as age increased at both temperatures. The highest value of m_x and f_x (3.62 eggs and 5.1 eggs) was calculated at 28 °C.

The curve of age-stage specific life expectancy (e_xj) for O. strigicollis, when fed on eggs of P. xylostella, affected by different temperatures is shown in Fig. 4. Results showed an inverse relation for life expectancy with temperature as it decreased from 16.96 days to 10.04 days at 28 °C and 32 °C respectively. The age-stage reproductive value (v_xj i.e., the forecasting scale for future population individuals of O. strigicollis at age x and stage j) under two temperatures is given in Fig. 5. The peak of age-stage specific curve (V_xj) for newly laid eggs was 1.51 eggs and 1.53 eggs at 28 °C and 32 °C, respectively. The curve of reproductive values significantly increased as the age and stage increased at each temperature. The highest peak for an adult female of O. strigicollis was obtained on the 20th and 15th day at 28 °C and 32 °C respectively.