Predatory functional response and fitness parameters of Orius strigicollis Poppius when fed Bemisia tabaci and Trialeurodes vaporariorum as determined by age-stage, two-sex life table

Nymphal development

Approximately sixty freshly laid healthy eggs of O. strigicollis were isolated from the insectaria and incubated until hatched. All collected eggs of O. strigicollis were equally distributed to feed on B. tabaci and T. vaporariorum third instar nymphs separately. After hatching, O. strigicollis neonates (≤24 h) were isolated in small Petri dishes (diameter: nine cm; depth: two cm) firmed with filter paper. From the results of functional responses, we supplied fifteen third instar nymphs of B. tabaci and T. vaporariorum separately to each individual of the predatory bug as food. Dead/empty nymphs of whitefly were replaced every day. A stem of V. negundo was placed in each Petri dish to provide shelter and moisture to the predatory bugs. The end of each stem was wrapped with wet cotton to keep them moist. Thirty nymphs were used in the experiment with three biological replicates for each prey species. Developmental time for each nymphal instar was measured. Individuals that died before reaching the adult stage were also recorded. Sex was confirmed as soon as the adults emerged.

Adult longevity and fecundity

Newly emerged male and female O. strigicollis adults were paired for mating. Females that mated for more than 1.5 min were considered to have been mated (Butler & O’Neil, 2007). Each mated female was placed separately in a new cylindrical translucent vial (2.5 × 14 cm diameter and length respectively) enclosed with an adequate mesh nylon screen. A small section of Vitex negundo stem was offered to each O. strigicollis female as an oviposition substrate. Each stem was wrapped with moist cotton at the end to provide moisture to the stem as well as the bugs using the method of Zhou et al. (2006). B.tabaci and T. vaporariorum nymphs (n = 15) were supplied into each vial as a source of food for female O. strigicollis. Stems of Vitex negundo were examined under a stereomicroscope (15 ×) to confirm egg laying. The stem was changed every day after the female laid the first egg. The total number of eggs laid by each female was counted under a stereomicroscope (15 ×). All predatory bugs were observed until they died. Development period, survival rate, pre-oviposition and oviposition period, fecundity, and longevity of female and male adults of O. strigicollis were recorded.

Functional response

The shape of the curve was find using the polynomial logistic regression following the method describe by Pervez (2005), Butt & Xaaceph (2015) and Varshney, Budhlakoti & Ballal (2018). The polynomial function described the relationship between the proportion of prey consumed (N_a) in relation to the density of prey offered (N₀) (Holling, 1959). Hence, a cubic model was applied in a logistic regression analysis (Juliano, 2001). ${\displaystyle \frac{N_a}{N_0}=\frac{exp\left(P_o+P_1{N}_o+P_2{N}_o^2+P_3{N}_o^3\right)}{1+exp\left(P_o+P_1{N}_o+P_2{N}_o^2+P_3{N}_o^3\right)}}$

In this equation, P₀, P₁, P₂, and P₃ represent the intercept, linear, quadric, and cubic coefficients, respectively. The negative and positive values of linear coefficient (P₁) define type II and III functional responses, respectively. The cubic expression will often provide a good fit to a type II and III responses and will provide a good starting point for fitting a logistic regression (Trexler, McCulloch & Travis, 1988; Trexler & Travis, 1993; Juliano, 2001). Modifying the Holling disc equation through reciprocal linear transformation, handling time (T_h) and the attack rate (a) were calculated (Livdahl & Stiven, 1983). This method is famous because of its simplicity and accuracy (Veeravel & Baskaran, 1997; Pervez & Omkar, 2003; Omkar & Pervez, 2004). The modified Holling equation, after reciprocal linear transformation is as follows:

1 / N_a = 1 / αTN₀ + T_h / T + ϵ

where N_a is the number of prey killed by predators during time (T: 24 h). N₀ is the density of prey, α is the attack rate and T_h is the predator handling time for one prey item. Hence, the variables used in the regression were y = ax + b + ϵ. The inverse mathematical function for curve estimation was used to estimate the values of α and T_h (Ali, Naif & Huang, 2011). For each prey density we calculated total handling time (Th_total= T_h x N_a), search time (Ts = T - T_htotal), attack rate (α = N_a/ (N₀ x Ts)), and search efficiency (E = N_a/N₀) (Hassell, 2000; Rocha & Redaelli, 2004). All statistical analyses were performed in MINITAB 19.

Life table analysis

The data obtained from experiments were analyzes based on the age-stage, two-sex life table theory using Twosex-MSChart, a computer program (Chi, 1988; Chi & Liu, 1985; Huang & Chi, 2011; Qi Chen et al., 2017; Farooq et al., 2018). The different biological traits and demographic parameters such as eggs, nymphs and adult development, oviposition, pre-oviposition, fecundity and r, λ, R ₀, GRR and T were calculated by following the methodology of Chi (2015).

In the age-stage, two-sex life table, the age-specific survival rate (l_x) and fecundity (m_x) were calculated using the Eqs. (1) and (2) respectively.

(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 this equation, S_xj is the age-stage specific survival rate of O. strigicollis (x = age which is in days and j = stage) which describe the probability of survival of neonate to age x and stage j, meanwhile, f_xj describe age-stage specific fecundity of adult female (Chi & Liu, 1985). The Euler–Lotka equation was used to estimate the intrinsic rate of increase (r) following the method 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 (R ₀) which described the ability of the female to produce a mean number of offspring in his lifetime, was calculated as (4)${\displaystyle \sum _{x=0}^{\infty }{l}_x{m}_x=R_0}$

The relation between female and R₀ can describe as (5)${\displaystyle R_0=F\frac{N_{\mathit{f}}}{N}}$

Here N and N_f represent the total number of O. strigicollis used in the experiment and the number of female adults respectively (Chi, 1988). The gross reproduction rate GRR and rate of increase (λ) were estimated as

(6)${\displaystyle GRR=\sum _{x=0}^{\infty }{m}_x}$ (7)${\displaystyle \lambda =er}$

The mean generation time (T) that a population required to rise to R ₀-fold of its size i.e., e^rT = R₀ or λ^T = R₀ at the stable age-stage distribution was intended using the equation (8)${\displaystyle T=\frac{ln{R}_0}{r}}$

The bootstrap test method (100,000 bootstrap) was used to estimate the standard errors of developmental time of each stage, fecundity, adult longevity and population parameters (Akca et al., 2015; Akköprü et al., 2015; Tuan et al., 2016). To compare the values, a pair bootstrap test based on the confidence of the interval of difference was used (Chi, 2015; Pru et al., 2015). To obtain the curves of survival rate, fecundity, life expectancy and reproductive vales, Sigma Plot 12.0 was used.

Life expectancy and reproductive values

The curves for life expectancy (e_xj) of O. strigicollis at each stage when presented with B. tabaci and T. vaporariorum are shown in Fig. 4. It describes the estimated survival of an individual of age x and stage j at a later age x. The life expectancy of a newly hatched egg of O. strigicollis was 28.57 and 24.91 d on B. tabaci and T. vaporariorum respectively. The age-stage reproductive values (v_xj) of adult female O. strigicollis when exposed to different species of whitefly are shown in Fig. 5. Age-stage specific reproductive value is a measure of the contribution of an individual (from age x and stage j) to a future population. When adult female O. strigicollis fed on B. tabaci, the highest peak was observed at 21 days, which was greater than when offered T. vaporariorum (19.79 days). The Age-stage specific reproductive curve (V_xj) indicated that the presence of B. tabaci had a more positive effect on O. strigicollis reproduction than T. vaporariorum.