Ecological and life-history notes on syrphid predators of woolly apple aphid in Virginia, with emphasis on Heringia calcarata

Abstract

Three species of hover fly commonly prey on woolly apple aphid, Eriosoma lanigerum, in Virginia, USA apple orchards. Larvae of Heringia calcarata are specialized predators of this pest, while Eupeodes americanus and Syrphus rectus are generalist aphid predators. The developmental duration of the immature stages of H. calcarata was determined under laboratory conditions, revealing a generation time of 19–20 d at 25°C. Descriptions of the larval, pupal and adult stages of H. calcarata are reported. Potted apple trees infested with arboreal colonies of woolly apple aphid and deployed in an orchard in Virginia were used as sentinels to measure seasonal changes in the relative abundance of the three syrphid species, based on the number of unhatched eggs deposited during weekly, 48-h exposure intervals from April to October, 2003–2005. Similar trends in the relative abundance of each species were recorded across all years. Eupeodes americanus was recorded first, showing a pronounced peak between mid-April and mid-May, followed by a prolonged period during which it was absent or present in very low numbers and then a much smaller peak in September and October. First records of H. calcarata occurred slightly later than for E. americanus. Early peaks of H. calcarata abundance typically occurred in May and June and tended to be smaller than those of E. americanus. Heringia calcarata eggs were recovered throughout most of each season. The potential role of predation by aphidophagous hover flies on the suppression of woolly apple aphid outbreaks in eastern apple orchards is discussed.