Ovipositional/Numerical response of aphidophagous syrphids (Diptera:Syrphidae) at varying prey densities of Lipaphis erysimi and Brevicoryne brassicae (Homoptera:Aphididae)

Abstract Background: Syrphids are well known generalized predators of aphids having remarkable predation potential and proved as effective aphid controlling natural enemies at larval stage. The adult predatory syrphid flies are strong fliers and have ability to travel a long distance to find suitable oviposition site. They select the ovipositioning site as per the prey density and prey species found on the host plant. The oviposition response is remarkably affected with the prey density. Results: The present study was focused on the affect of varying prey density on the oviposition behaviour of four predatory syrphid species viz. Ischiodon scutellaris (Fabracius), Episyrphus balteatus(Dee Geer), Eupeodes frequens(Matsmura) and Metasyrphus confrater(Wiedemann) on two prey (aphid) species i.e. Lipaphis erysimi L. and Brevicoryne brassicae L. under laboratory conditions. The results depicting that all the four above mentioned syrphid species showed type II numerical response on both the aphid species with significantly (p<0.0001) negative linear coefficient depicting the decreasing proportion of the number of laid eggs with respect to the increasing prey density. The number of eggs laid by single female of syrphids were increased with respect to increasing prey density until it reached to the upper asymptote and then decreased in the condition of overcrowding of prey.From this study it can be concluded that the site selection for the oviposition by the predatory syrphid flies greatly influenced by the population density of prey (aphids). in the absence of aphids they do not prefer to lay eggs whereas as higher aphid density encourage it to lay eggs until it reached to upper asymptote these results may help in decision making for augmentative release of syrphid flies for establishment of optimum population.