Effect of temperatures on predators’ development and fitness
The egg incubation period was significantly affected by temperature in both I. scutellaris and E. balteatus species (F2, 89 = 10.256; P < 0.001). The incubation period decreased with increasing temperature (Fig. 1a). It did not differ significantly between the two species at the lowest temperature (23 °C), but E. balteatus developed faster at the intermediate temperature (27 °C), while I. scutellaris developed faster at the highest temperature (33 °C) (Fig. 1a). A maximum egg incubation period of (4.25 ± 0.09 and 4.50 ± 0.09) days was recorded at 23 °C for I. scutellaris and E. balteatus, respectively, while the minimum periods were (2.37 ± 0.11 and 2.80 ± 0.11) days, at 33 °C. Similar trend in the pupation periods for both species were observed, where I. scutellaris development was significantly faster than E. balteatus under different temperature regimes (F1, 54 = 28.562; P < 0.001) (Fig. 1b). Also, different temperatures alter the pupation periods of both species significantly (F2, 54 = 45.068; P < 0.001). The shortest pupation period (5.70 ± 0.15 days) was observed for I. scutellaris at 33 °C, while the longest (8.70 ± 0.26 days) was for E. balteatus at 23 °C (Fig. 1b).
The population density of the predator is correlated with prey populations (Haenke et al., 2009; Singh and Singh, 2013) as well as to the abiotic conditions including temperature, relative humidity, rainfall, and wind speed in the ecosystem (Kalita and Singh, 2012). The developmental times of both species were greatly reduced at 33 °C. The predator released at a higher or a lower temperature than the average may lead to the quick or slow development, respectively, without synchronizing with aphid population build-up. Hence, releasing the natural enemies at an optimum time is a requisite for biocontrol success in the field. Our results support this hypothesis where the intermediate temperature (27 °C) was the optimum for optimum development of biological control to maximize their chances to sync with aphid population upon release. A considerable number of earlier studies have reported the dependence of syrphid predator success on temperatures (Ankersmit et al., 1986; Tenhumberg, 1995b; Bianchi et al., 2006).
Comparative development and predation rates
Concerning larval and adult durations, insignificant difference between I. scutellaris and E. balteatus was recorded (F1, 146 = 1.433; P = 0.23) and (F1, 36 = 0.266; P = 0.61), respectively. But the females of E. balteatus lived significantly longer than that of I. scutellaris (F1, 146 = 1.433; P = 0.23). However, duration of different larval instars and adult lifespan of male and female flies of each species differed significantly (F2, 146 = 189.399; P < 0.001) and (F2, 36 = 23.527; P < 0.001), respectively (Fig. 2). The 3rd instar larvae of both I. scutellaris and E. balteatus lasted a maximum time (4.70 ± 0.08 and 4.45 ± 0.09 days, respectively), followed by the 2nd instar (3.54 ± 0.10 and 3.53 ± 0.18 days, respectively) and then 1st instar larvae (2.67 ± 0.11 and 2.63 ± 0.11 days, respectively) (Fig. 2a). In general, the female flies lived 30% longer than that of males (Fig. 2b).
The developmental periods expressed as egg, larval, and adult durations exhibited insignificant differences between I. scutellaris and E. balteatus (t = 1.912; d.f. = 32; P = 0.07), (t = 1.078; d.f. = 36; P = 0.29), and (t = 0.458; d.f. = 18; P = 0.65) (Fig. 3). Only the pupation period was found significantly different between both predatory species (t = 5.166; d.f. = 18; P < 0.001), where the pupae of I. scutellaris completed its stage earlier than that of E. balteatus (Fig. 3).
The predation rates expressed in terms of percentage of prey consumption was significantly different between both predatory species and their larval instars (F1, 146 = 9.669; P < 0.001 and F1, 146 = 88.624; P < 0.001, respectively) (Fig. 4a). The 1st and 3rd instar larvae of I. scutellaris had significantly higher predatory potentials (62.83 ± 1.32 and 81.05 ± 1.36%, respectively) than same stage larvae of E. balteatus (58.86 ± 1.66 and 76.12 ± 1.35%, respectively) (Fig. 4), while the 2nd instar larvae of both species did not differ significantly from each other in terms of their predatory potentials (70.85 ± 0.85 and 69.51 ± 0.10%, respectively) (Fig. 4a). The highest predation rates were clearly observed in the 3rd instar of both predator species (Fig. 4a).
The data regarding total predation by larvae (cumulative of all instars) revealed that I. scutellaris larvae consumed significantly higher numbers of aphids as compared to that of E. balteatus (i.e., 438.16 ± 10.76 and 398.37 ± 9.45 aphids) (F1, 36 = 7.715; P = 0.009). The larvae of both species had similar voracity until 10 days after hatching (Fig. 4b), while the older larvae (3rd instar) of I. scutellaris consumed higher numbers of aphids per day as compared with E. balteatus (Fig. 4b). In general, the predation rates on daily basis increased with each successive instar but near the time of molt, this predation declined (Fig. 5a–c). Obtained results demonstrated that the predation rate of both species increased with each successive instar, and they had consumed a total of 30–33 aphids during 1st instar, 110–120 aphids during 2nd instar, and 253–278 aphids during their 3rd instar. The average daily aphid intake was recorded to be increased up to 90% on the 11th day of larval stage (i.e., 4th day since the 2nd molt) (Fig. 5c).
The results have strongly suggested the voracious nature of 3rd instar larvae, and the release of 3rd instar larvae can be a highly efficient approach. The findings are in accordance with the earlier reports suggesting the highest prey-consumption rates of syrphid flies in their 3rd instars (Völkl et al., 2007 and Singh and Singh, 2013). Since both predatory species were found equally efficient against aphids, the use of their mixed cultures for aphid biological control in wheat needs to be studied.
The success of a biological control agents in the field depends upon the reproductive success of the females (Lundgren et al., 2008) and the developmental time of females and their physiological fitness that leads towards greater ovipositional potential (Arnqvist and Nilsson, 2000). In addition, the females of both I. scutellaris and E. balteatus had significantly higher lifespan compared to the males, while the females of E. balteatus had significantly longer lifespan than I. scutellaris, suggesting the competitive advantage to the former species.