Host plants
Cucumber (Cucumis sativus cv. Servis plus) seeds were grown in plastic pots (8-cm diameter and 13-cm height) filled with a soil-cocopeat mix (1:1, v:v) and irrigated every 3 days. The pots were maintained in insect cages (120 × 100 × 60 cm) inside a controlled environment room (25 ± 5 °C, 16 L:8D). After 5–6 weeks from cultivation, the host plants were used for establishment of the whitefly colony and for trials.
Insect rearing
Adults and nymphs of B. tabaci were collected from cucumber fields in Mollasani region, Ahwaz, Khuzestan province, southwest of Iran. The samples were introduced into rearing cages (120× 100 × 60 cm) containing the cucumber pots. The pots were kept inside a controlled environment room (25 ± 1 °C, 65 ± 5% RH and a photoperiod of 16 L:8D). After the establishment of the colony, eggs and third instar nymphs were used in the experiments. To obtain the cohorts, five couples of whitefly adults (each included one male and one female) were collected from the rearing stock and confined on host plant leaves by a clips cage (2 .5-cm diameter). The adults were removed after 24 h. The eggs and third instar nymphs were obtained for the experimentation after 1 and 10 days, respectively.
Adults of O. albidipennis were collected from corn and sunflower fields in Mollasani region, Khuzestan province, southwest of Iran. The female bugs were isolated in a Plexiglas cylinder (18 cm high, 7 .5cm diameter). The top of lid cylinder was covered by a fine net for ventilation as well as the holes placed on its surface. The nymphs and adults of the predatory bug were fed on frozen Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) eggs and pollens of corn and sunflower. The bean pods, Phaseulus vulgaris L., were used as oviposition substrates. The predator was reared at (25 ± 1 °C, 65 ± 5% RH, and 16:8 h (L:D)) in an environmental controlled room. For reduction of cannibalism, the rearing cylinders were lined by crumpled wipe papers.
Experimental design
The experimental arena was a Petri dish (10-cm diameter). A hole (2-cm diameter) was constructed in the fine gauze cover lid for ventilation. Inside each Petri dish, a cucumber leaf disc (5-cm diameter) was placed upside down on a wetted filter paper to keep the leaf discs fresh (Montserrat et al. 2000). Densities of (5, 8, 10, 15, 20, 25, 30, and 35) the preys (eggs or third instar nymphs of B. tabaci) per arena were used. The densities were determined according to preliminary tests. A female predator (1 to 5 days old), kept starved for 24 h before the experiment, was introduced to each arena. Following 24 h of exposure, the predatory bugs were removed and the numbers of consumed eggs and nymphs were recorded, separately. The trials were performed at (25 ± 1 °C, 65 ± 5% RH, and a photoperiod of 16:8 h (L: D)) in an environmentally controlled room. Ten replicates were conducted at each prey density.
Data analyses
Analysis of variance (ANOVA) by SAS 9.1 was used to compare consumed preys among different prey densities. The type of the functional response of predator was determined by logistic regression analysis of the proportion of consumed preys (Na/N0), as a function of prey offered (N0) (Juliano 2001). Data were fitted to a polynomial function that describes the relationship between Ne/N0 and N0 (Eq. 1):
$$ \frac{N_a}{N_0}=\frac{\exp \left({P}_0+{P}_1{N}_0+{P}_2{N}_0^2+{P}_3{P}_0^3\right)}{\left(1+\exp {P}_0+{P}_1{N}_0+{P}_2{N}_0^2+{P}_3{N}_0^3\right).} $$
(1)
where Na is the number of consumed prey, N0 is the initial number of preys, and P0, P1, P2, and P3 are the intercept of the linear, quadratic, and cubic coefficients, respectively. These parameters were calculated, using the method of maximum likelihood (PROC CATMOD, SAS Institute, 2001). When P1 > 0 and P2 < 0, the proportion of consumed preys is positively density-dependent—the data describes a type III of functional response, but if P1 < 0 and P2 > 0, the proportion of consuming preys decreases gradually as the initial number of prey offered increases—the data indicates a type II of functional response (Juliano 2001). In the second step of the analysis, nonlinear least squares regression (PROC NLIN; SAS Institute Inc. 2001) was used to fit Roger’s random attack model, which describes a type II or III functional response (Eq 2 and Eq 3, respectively), for estimation of the functional response parameters, respectively. Because prey density was reduced during the trial, this model, which does not assume a constant prey density, is a suitable choice for the analysis (Juliano 2001).
$$ {N}_a={N}_0\left\{1-\exp \left[- aT{P}_t/\left(1+a{T}_h{N}_t\right)\right]\right\} $$
(2)
where Na is the number of consumed prey, N0 is the initial number of prey, a is the instantaneous attack rate (searching efficiency), T is the total amount time available for searching (in this experiment T = 24 h), Pt the number of predators, and Th is the handling time.
$$ {N}_a={N}_0\frac{1-\mathit{\exp}\left[\left(d+b{N}_0\right)\left( Th{N}_a-T\right)\right]}{1+C{N}_0} $$
(3)
where b, c, and d are constants from the function that relates the attack coefficient (a) and N0 in the type III functional response. The attack coefficient (a) is calculated by Eq 4 (Hassell 1978).
$$ a=\frac{\left(d+b{N}_0\right)}{1+c{N}_0} $$
(4)
The curves of the number of consumed prey by O. albidipennis females to different densities of the eggs and third instar nymphs of B. tabaci were depicted by Excel software.