Determination of fitness traits of Orius strigicollis Poppius (Hemiptera: Anthocoridae) on Pectinophora gossypiella (Lepidoptera: Gelechiidae) using two-sex life table analysis

Insect rearing

Collection of P. gossypiella

Pupae and adults of the pink bollworm P. gossypiella (strain resistant AZP-R) were supplied by the Institute of Plant Protection and Soil Fertility, Hubei Academy of Agricultural Sciences, Wuhan, China, and were placed in cages (50 × 50 × 50 cm) and provided with 10% honey solution for egg laying as a diet for adults. Each cage was covered with white gauze and filter paper for oviposition. Egg masses were harvested or collected daily on the filter paper during the total oviposition period following previously described methods (Wan et al., 2012), and fresh eggs were provided as a food source for the flower bugs. All experiments were conducted at 75 ± 5% RH and a 16:8 (L:D) photoperiod in climatic controlled chambers (HP350GS, Ruihua Instrument & Equipment Co., Ltd., Wuhan, P.R. China) equipped with fluorescent lighting controlled by an automatic timer.

Feeding potential of O. strigicollis

The feeding potential of O. strigicollis predatory stages, that is, third, fourth and fifth nymphal instars and adult stages (male and female), was recorded on P. gossypiella eggs and first instar larvae on the basis of choice experiments. The feeding efficiency of all predatory stages with P. gossypiella eggs after 24 h, and for first instar larvae after 12 and 24 h was determined in three temperature treatments (24, 28 and 32 °C), in plastic Petri dishes (9 cm diameter and 2 cm depth), lined with filter paper. There were 30 replicates for each predatory stage with 20 prey (eggs and larvae). Owing to zero percent mortality of fresh and healthy P. gossypiella eggs at three different temperatures in our main experiments, we did not take them into account while analyzing the data and there was no control treatment for O. strigicollis potential on P. gossypiella eggs. Although there were 30 replicates of the control treatment without predators at three temperatures after 12 and 24 h for O. strigicollis potential on P. gossypiella first instar larvae. The control treatment was considered as a prediction of P. gossypiella first instar larvae mortality with or without predators to check the actual feeding potential of O. strigicollis predatory stages on P. gossypiella first instar larvae by following previously described methodology (Calixto et al., 2013).

Two-sex life table traits

The O. strigicollis eggs in bulk quantity (100) at each temperature were selected and the development time was checked from egg hatching to second instar nymph in groups (≤10 in each box) when P. gossypiella eggs were provided as diet and cotton balls drenched with water to maintain the moisture level and source of water for immature nymphs. Beginning with the third instar nymphal stage (N₃), individual bugs were isolated in plastic Petri dishes lined with filter paper to avoid cannibalism (Tuan et al., 2015), and each individual was considered as a replicate at each temperature until death to check the biological parameters of O. strigicollis. Each nymph was fed until adult emergence, and the stadial duration of each stage was noted after every 24 h. Fresh P. gossypiella eggs were provided daily, and after every 24 h, consumed or damaged eggs were counted under a stereomicroscope (1309 LED 40X Binocular Stereo Microscope; Jiangsu Victor Instrument Meter Co., Ltd., Taizhou, JS, China), and magnifying lens, and dead individuals of O. strigicollis were counted to check the mortality of O. strigicollis.

Moulting or shredded skin of O. strigicollis nymphs in the round plastic Petri dishes were checked daily to assess the next nymphal stage (Ahmadi, Sengonca & Blaeser, 2007). From last instar nymph (fifth) to adult emergence was checked and male/female sex was identified under a stereomicroscope as documented by (Ali et al., 2020), and kept separately and starved for 24 h for the reproductive study.

For the adult longevity and egg laying capacity of O. strigicollis, both females and males were shifted to new cylindrical glassy vials (2.6 cm diameter and 15 cm length) enclosed with a fine mesh nylon screen. The pairs were noted to ensure that mating occurred, and the females that continued copulation for >1.5 minutes were considered to have been mated (Butler & O’Neil, 2006). The pairs at each temperature were placed in Petri dishes lined with filter paper, and P. gossypiella eggs were provided as food source. The small fragile stem of Vitex negundo L. was provided as a substrate for female O. strigicollis oviposition (Zhou et al., 2006) at each temperature (24, 28 and 32 °C). The stem was covered with moistened cotton as described above.

The stems were investigated daily under a stereomicroscope to count the number of eggs laid by O. strigicollis. Consumed or damaged eggs of P. gossypiella were counted and replaced with fresh eggs daily. The following biological parameters were recorded at each temperature: adult pre-oviposition period (APOP = the time period between the female adult emergence to its first egg laying), total pre-oviposition period (TPOP = the time interval between birth to the start of egg laying), oviposition days (taking only those days (time units) with fecundity >0 into account), and daily fecundity following previously described methods (Huang & Chi, 2013; Chen et al., 2017). The stems with eggs laid by O. strigicollis were kept in plastic containers at each temperature, 70% ± 5% (RH%) and L16:D8 photoperiod. The number of O. strigicollis hatched eggs were recorded daily under a stereomicroscope. The percentage of egg hatchability and mating pair success was recorded per pair. All O. strigicollis bugs (female) were kept and observed until death following the methodology of Zhang et al. (2012).

Prey preference

The predatory stages of O. strigicollis third, fourth, fifth instars and males and females were exposed to 10 prey/host stages of both P. gossypiella eggs and first instar larvae in the same plastic Petri dishes at three different temperatures (24, 28 and 32 °C). There were 10 replicates for each predatory stage at each temperature. Each replicate was provided with moistened cotton placed inside the Petri dish to maintain the moisture and a water source for all predatory stages in the above-mentioned controlled chamber. The preferred or consumed host/prey were examined after 12-and 24-h intervals at each temperature under a stereomicroscope.

Statistical analysis

In this study, the data of feeding potentials of O. strigicollis on P. gossypiella eggs and larvae at three different temperatures were statistically analyzed using one-way analysis of variance (ANOVA), and their mean values were compared using least significant difference (LSD) test at the p = 0.05 level of significance. The correlation between different temperatures and feeding potential of predatory stages was determined by linear regression analysis (Saeed et al., 2016). All statistical analyses were carried out using statistics 8.1 software (Analytical Software, Tallahassee, FL, USA). Different biological parameters (each stage developmental period, survival rate, adult longevity, age-specific fecundity, APOP and TPOP) were statistically evaluated using age-stage two-sex life table theory (Chi & Liu, 1985; Chi, 1988) and the TWOSEX-MS Chart computer program (Chi, 2015; Hafeez et al., 2019). Means and standard errors (SE) of all biological and population life table parameters were determined using 200,000 bootstrap replicates to obtain stable SE estimates (Huang & Chi, 2012; Akca et al., 2015). Bootstrap and paired bootstrap tests were figured in TWOSEX-MS Chart, and all results of the treatments were associated using the paired bootstrap test based on the confidence interval of difference (Efron & Tibshirani, 1993; Akköprü et al., 2015). The age-stage-specific survival rate (s_xj), age-stage-specific fecundity (f_xj), age-specific survival rate (l_x), age-specific fecundity (m_x), age-stage life expectancy (e_xj), age-stage reproductive value (v_xj), and life table parameters (r, intrinsic rate of increase; λ, finite rate of increase; R₀, net reproductive rate; and T, the mean generation time), were designed in sequence according to previously described methods (Chi & Liu, 1985; Tuan et al., 2015). Sigma Plot 12.0 was used to construct the curves for all population or life table parameters, including s_xj, f_xj, m_x, v_xj and e_xj.

The age-specific survival rate (l_x) and age-specific fecundity (m_x) were calculated as follows: (1) $$l_x=\sum _{j=1}^k{\mathrm{s}}_{xj}$$ (2) $$m_x={\displaystyle \frac{{\sum }_{j=1}^k{s}_{xj}{f}_{xj}}{{\sum }_{j=1}^k{s}_{xj}}}$$where s_xj is the age-stage specific survival rate, that is, the possibility that an individual (newly hatched) will live or exist to age x and stage j. The intrinsic rate of increase (r) was then predicted iteratively following the Euler–Lotka equation with age indexed from 0 (Goodman, 1982) as follows: (3) $$\sum _{x=0}^{\mathrm{\infty }}{\mathrm{e}}^{-r\left(x+1\right)}{l}_x{m}_x=1$$

The net reproductive rate R₀ was calculated as follows: (4) $$R_0=\sum _{x=0}^{\mathrm{\infty }}{l}_x{m}_x$$

The net reproductive rate (R₀) and mean female fecundity (F) relationship was calculated as follows: (5) $$R_0=F{\displaystyle \frac{N_f}{N}}$$where N indicates the total number of individuals used for the life table study and N_i represents the number of female adults (Chi, 1988). The gross reproduction rate was defined as follows: (6) $$GRR=\sum _{x=0}^{\mathrm{\infty }}{m}_x$$

The finite rate (λ) was recorded as follows: (7) $$\mathrm{\lambda }=e^r$$

The mean generation time (T) represents the time span that a population needs to increase to R₀-fold of its size, that is, $e^{rT}=R_0$ or ${\mathrm{\lambda }}^T=R_0$ at the stable age-stage distribution. The value of T was calculated as follows: (8) $$T={\displaystyle \frac{\mathrm{l}\mathrm{n}{R}_0}{r}}$$

Age-stage life expectancy (e_xj) is defined as the length of the duration or time that an individual or insect of x and j is predicted to live, calculated by the method of (Chi & Su, 2006) as follows: (9) $$e_{xj}=\sum _{i=x}^{\mathrm{\infty }}\sum _{y=j}^k{s}_{iy}^{{}^{\prime }}$$Where $s_{iy}^{{}^{\prime }}$ is defined as the probability that individuals of x and j will survive to age i and stage y, and is found by assuming $s_{iy}^{{}^{\prime }}$ =1 (Tuan, Lee & Chi, 2014).

The age-stage reproductive value (v_xj) was defined as the contribution of individuals of age x and stage j to the future population (Yang et al., 2015). In the age-stage, two-sex life table, it was calculated as follows (Tuan, Lee & Chi, 2014): (10) $$v_{xj}={\displaystyle \frac{e^{-r\left(x+1\right)}}{S_{xj}}\sum _{i=x}^{\mathrm{\infty }}{e}^{-r\left(x+1\right)}\sum _{y=j}^k{s}_{iy}^{{}^{\prime }}{f}_{iy}}$$

Prey preference between two prey types for example, P. gossypiella eggs and first instar larvae was determined using a paired t-test.

Feeding potential of O. strigicollis

The feeding potential of O. strigicollis predatory stages, that is, third, fourth, fifth nymphal instars and adult stages (male and female) and correlations between different temperatures, when fed on P. gossypiella eggs (Fig. 1), and first instar larvae were recorded (Table 1). Among all predatory stages on P. gossypiella eggs, females showed significantly more feeding capability at 28 °C (F_2,87 = 8.89; P < 0.001) than those in the two other treatments at 24 °C and 32 °C after 24 h. Similarly, males showed significantly more feeding capability at 28 °C (F_2,87 = 8.33; P < 0.001) than those in the other two treatments at 24 °C and 32 °C after 24 h (Fig. 1).

Similarly, all predatory stages fed with P. gossypiella larvae showed different feeding capability with varying temperature after 12-and 24-h intervals. Females showed significantly more feeding at 28 °C (F_2,87 = 11.80; P < 0.0001) than those in the other two treatments at 24 °C and 32 °C after 12 h, and males showed significantly more feeding at 28 °C (F_2,87 = 41.00; P < 0.0001) than those in the other two treatments at 24 °C and 32 °C after 12 h (Table 1). Meanwhile, females showed significantly more feeding at 32 °C (F_2,87 = 14.40; P < 0.0001) than those in the other two treatments at 24 °C and 28 °C after 24 h, and males showed significantly more feeding at 32 °C (F_2,87 = 18.20; P < 0.0001) than those in the other two treatments at 24 °C and 28 °C after 24 h (Table 1). There were no differences recorded in male and female average predation on P. gossypiella first instar larvae after 12-and 24-h intervals. For the control treatment (F_2,87 = 11.30; P < 0.0001) after 12 h, the mortality of P. gossypiella first instar larvae was significantly higher at 32 °C than that in the other two treatments at 24 °C and 28 °C after 12 h. Similarly, for control treatment (F_2,87 = 7.79; P < 0.001) after 24 h, the mortality of P. gossypiella first instar larvae was significantly higher at 32 °C than that in the other two treatments at 24 °C and 28 °C after 24 h. Overall, predatory stages feeding after 12-and 24-h were significantly different as compared to control treatment (Table 1).

Biological parameters of O. strigicollis

Longevity, oviposition and fecundity of adults

The mean total longevity of male adults considerably declined with increasing temperature (26.00 d to 21.25 d), whereas the mean total longevity of female adults was the highest at 28 °C (26.12 d) as compared with the other temperatures at 24 °C (23.83 d) and 32 °C (23.33 d) (Table 2). However, there were no differences recorded in the total longevity of female adults among the three treatment temperatures. The TPOP of female adults differed at 24 °C (21.50 ± 0.42 d), 28 °C (17.50 ± 0.50 d), and 32 °C (15.56 ± 0.30 d) when fed on the P. gossypiella eggs. Similarly, the APOP of female adults differed at 24 °C (3.00 ± 0.46 d), 28 °C (0.25 ± 0.11 d) and 32 °C (0.11 ± 0.08 d) when fed on the P. gossypiella eggs. The oviposition period of female adults differed at 28 °C (7.88 ± 1.92 d) as compared with the other temperatures at 24 °C (3.75 ± 0.31 d) and 32 °C (5.89 ± 0.78 d). There were differences recorded in fecundity (total number of eggs per female), and the highest fecundity was recorded at 28 °C (66.12 eggs/female) compared with the other two temperatures. The mating pair success (the percentage of pairs of adults that successfully laid eggs or were eligible to continue in the next generation) of O. strigicollis at three different temperatures was different, and the highest percentage of MPS was recorded at 28 °C (92.59%). Similarly, the hatching percentage of eggs laid by O. strigicollis females at three different temperatures was different, and the highest percentage of hatched eggs was recorded at 28 °C (54.37%) (Table 2).

Population parameters

The effects of different temperatures on population or fitness parameters of O. strigicollis fed on the P. gossypiella eggs were calculated by using bootstrap technique with 200,000 resamplings (Table 3). The intrinsic rate of increase (r) and finite rate of increase (λ) of O. strigicollis fed on the P. gossypiella eggs differed and were higher at 28 °C (0.14 ± 0.01 d⁻¹ and 1.15 ± 0.02 d⁻¹) compared with those at 24 °C (0.0052 ± 0.002 d⁻¹ and 1.00 ± 0.02 d⁻¹) and 32 °C (0.12 ± 0.02 d⁻¹ and 1.13 ± 0.02 d⁻¹). In addition, the net reproductive rate (R₀) was the highest and different at 28 °C (17.63 offspring) compared with those at the other two temperatures: 24 °C (1.13 offspring) and 32 °C (10.23 offspring). Conversely, the mean generation time (T) was the highest and different at 24 °C (23.79 days) compared with those at the other two temperatures: 28 °C (20.89 days) and 32 °C (19.60 days). Gross reproductive rate (GRR) was different and the highest at 28 °C (90.32) compared with those at the other two temperatures: 24 °C (6.18) and 32 °C (30.76) (Table 3).

Survival rate

From the detailed age-stage survival rate (S_xj) of O. strigicollis fed on the P. gossypiella at different temperatures (Fig. 2), our results indicate the probability of a newly hatched predator living to age x and stage j. Significant difference was also observed in overlapping plotted curves for different developmental stages because developmental rate varied among individuals at different temperatures. The projected curves revealed entirely diverse patterns for each developmental stage at each temperature. As the age increased, the survival rate of individuals gradually decreased and showed an inverse relation at each temperature. The developmental time of O. strigicollis male and female was longer and survival rate was lower at 24 °C, compared with other two temperatures (28 °C and 32 °C). The survival peak was higher for male (18 d, 0.21) and female (20 d, 0.23) at 28 °C and 32 °C (male; 16 d, 0.20 and female; 16 d, 0.22) than at 24 °C (male; 19 d, 0.06 and female; 19 d, 0.20) when fed on P. gossypiella eggs.

Figure 3 shows the age-specific survival rate (l_x), the age-specific fecundity of total population (m_x), age-stage specific fecundity (f_xj) and the age-specific maternity (l_xm_x) curves plotted at different temperatures. The curves of l_x (basic form of the age-stage survival rate S_xj) declined or showed inverse relation at 24 °C and 32 °C, but it was directly proportional to 28 °C. The peak recorded values of age-stage specific fecundity (f_xj) was (22 d, two eggs), (17 d, 19.85 eggs) and (19 d, 7.4 eggs) appeared at 24, 28 and 32 °C, respectively. The curves of m_x showed that reproduction began relatively earlier at 32 °C (age; 12 d), than at 28 °C (age; 14 d) and 24 °C (age; 19 d). But, the highest recorded m_x peak was (seven eggs per individual; age 22 d) at 28 °C than 24 °C (1.33 eggs per individual; age 23 d) and 32 °C (4.00 eggs per individual; age 19 d).

Life expectancy

The effects of the different temperatures on the predicted average life expectancy (e_xj) of the population at every stage of O. strigicollis (Fig. 4) were determined. The longevity of the newly hatched eggs of O. strigicollis at age zero was 15.92 d at 24 °C, 16.18 d at 28 °C and 15.41 d at 32 °C when fed on P. gossypiella eggs. With variation in other developmental stages, an increasing trend in the female adult expectation was observed (16.5 d, age 23 days) at 28 °C than those at the other two temperatures: 24 °C (5.75 d, age 20 days) and 32 °C (9.80 d, age 17 days). Overall, the highest life expectancy was recorded at 28 °C compared with those at 24 °C and 32 °C.

Reproductive value

The age-stage reproductive value (V_xj) describes the part of an individual of age x and stage j toward the upcoming population (i.e., the scale of population forecasting). The results reveal that the curves for V_xj significantly increased (8.48 d⁻¹ on 20 d to 51.68 d⁻¹ on 15 d) at 24 °C and 28 °C, respectively, but decreased (22.65 d⁻¹ on 13 d) at 32 °C when female emerged.

In addition, the V_xj is exactly the same as the finite rate, that is, 1.00 d⁻¹ at 24 °C, 1.15 d⁻¹ at 28 °C and 1.13 d⁻¹ at 32 °C when fed on P. gossypiella eggs. The results reveal that the V_xj was higher at 28 °C, and showed that the P. gossypiella had a more positive effect on O. strigicollis reproduction than 24 °C and 32 °C (Fig. 5).

Prey preference

The prey preference among P. gossypiella eggs and first instar larvae was evaluated by paired t-test (Figs. 6A and 6B). Male and female preferences were not different, and they preferred first instar larvae of P. gossypiella than other predatory stages. The other predatory stages (third, fourth and fifth instars) preferred P. gossypiella eggs rather than first instar larvae after 12 h (Fig. 6A) and 24 h (Fig. 6B) at each temperature.