Biology, predatory potential and growth parameters of the syrphid fly, Scaeva pyrastri (L.) (Diptera: Syrphidae) feeding on the cabbage aphid, Brevicoryne brassicae (L.)

Abstract

BackgroundScaeva pyrastri (L.) (Diptera: Syrphidae) is a large-size Palaearctic region syrphid fly. The larval stage of S. pyrastri is aphidophagous in nature, while the adults are efficient pollinators of various crops. Therefore, this study was carried out to investigate the predation efficacy, biology and population growth parameters of S. pyrastri on the cabbage aphid, Brevicoryne brassicae (L.) (Hemiptera: Aphididae) as prey using age–stage-specific two-sex fertility table.ResultsThe results showed that the mean total larval development duration was 8.31 days and the pre-adult stage was 19.82 ± 0.25 days long. In the adult stage, the mean longevity of female (18.57 ± 0.37 days) was longer than that of the male (15.50 ± 0.17 days). The adult pre-ovipositional, ovipositional and post-ovipositional periods were 4.25 ± 0.25, 8.50 ± 0.46 and 3.25 ± 0.31 days, respectively, and the mean female’s fecundity was 37.75 ± 1.16 eggs per female. The two-sex fertility table indicated that the age-specific fecundity (mx) increased gradually peaking at the 27th day of pivotal age (2.71 eggs per day) and decreased thereafter. The life expectancy of female was observed on the 18th (20.25 days) and 19th day (16.44 days) in the female and male flies, respectively. The female attained the maximum reproductive value (vxj) on the 25th day, which was 26.05 eggs needed to produce one individual. The population growth parameters indicated that the population had a net reproduction rate of 15.10 ± 4.19 offspring per individual and a true generation time of 28.86 ± 0.28 days. The intrinsic rate of increase was greater than zero (0.093 ± 0.011) and the finite rate of increase (λ) was greater than one (1.098 ± 0.012), which indicated that B. brassicae was the suitable prey for S. pyrastri. The values related to the number of aphids consumed by the larvae were: a net predation rate (Co) of 377.06 ± 26.54 nymphs and finite predation rate (ω) of 19.58 ± 0.78.ConclusionsScaeva pyrastri had a considerable predatory potential and fitted well for the biological control of aphids, especially B. brassicae. However the field potential is still unknown; thus, this study will be helpful in determining the field efficacy of S. pyrastri against B. brassicae.