Seasonal natural history of aphidophagous Syrphidae (Diptera) attacking the balsam twig aphid in balsam fir (Pinaceae) Christmas tree plantations Richard Berthiaume, 1 Christian Hébert, Georges Pelletier, Conrad Cloutier Abstract—The balsam twig aphid, Mindarus abietinus Koch (Hemiptera: Aphididae), is one of the most important pests of balsam fir(Abies balsamea (Linnaeus) Miller; Pinaceae) grown as Christmas trees in eastern North America. Aphid feeding on the current-year shoots results in needle distortion and shoots stunting, which reduces the aesthetic value of balsam fir trees and can have substantial economic impact. Syrphidae (Diptera) fly predators attacking this aphid are poorly known. We identified four species attacking the balsam twig aphid. Syrphus torvus Osten Sacken (Diptera: Syrphidae) was the most abundant species followed by Eupeodes lapponicus (Zetterstedt) (Diptera: Syrphidae) and Eupeodes americanus (Wiedemann) (Diptera: Syrphidae). We described the seasonal natural history of the two most abundant syrphids feeding on this aphid and we studied their synchrony with their prey. Syrphid larval density on balsam fir shoots increased rapidly and closely followed density of the balsam twig aphid. We also reported heavy parasitism of syrphid pupae by two Hymenoptera on tree foliage but lower parasitism on pupae in the soil under trees. Several cases of multiple parasitism and hyperparasitism were observed in syrphid pupae under trees. Abundance of predaceous syrphid species was higher in untreated balsam fir plantations compared with treated plantations. More research is needed to understand the role and the impact of these predators on balsam twig aphid population dynamics. Introduction The balsam twig aphid, Mindarus abietinus Koch (Hemiptera: Aphididae), is one of the most important pest of balsam fir(Abies balsamea (Linnaeus) Miller; Pinaceae) and other firs grown as Christmas trees in eastern North America (Bradbury and Osgood 1986; Kleintjes 1997; Berthiaume et al. 2000). The balsam twig aphid has a monoecious life cycle and goes through three or four generations in southern Québec, Canada from late April to mid-late July (Cloutier and Jean 2002). This species overwinters as eggs, which are laid mainly on the current-year shoots of their host trees (Deland et al. 1998). Eggs hatch in late April into fundatrices, which initially feed on previous year needles before bud break, without causing any visible damage. After bud break, mature fundatrices move towards newly elongating shoots and begin parthenogenetic reproduction. Their progeny grow rapidly and form colonies that feed on current-year shoots during the elongation period. Their feeding on the current-year shoots results in needle distortion and stunting of shoots (Varty 1966; Saunders 1969; Nettleton and Hain 1982; Bradbury and Osgood 1986). Such damage is usually of little concern in natural forests but in Christmas tree plantations, it reduces the aesthetic value of balsam fir trees and can have substantial economic impact (Berthiaume 1998; Deland et al. 1998; Berthiaume et al. 2000). Cloutier and Jean (2002) reviewed biological control of balsam twig aphid in Canada. Numerous Coccinellidae (Coleoptera) are known as predators R. Berthiaume, 1 Département des Sciences du bois et de la forêt, Faculté de foresterie, de géographie et de géomatique, Université Laval, Ville de Québec, Québec, G1K 7P4, Canada C. Hébert, G. Pelletier, Centre de foresterie des Laurentides, Service canadien des forêts, Ressources naturelles Canada, C.P. 10380, 1055 rue du P.E.P.S., Stn. Sainte-Foy, Ville de Québec, Québec, G1V 4C7, Canada C. Cloutier, Département de biologie, Faculté des sciences et de génie, Pavillon Alexandre Vachon, 1045 Ave de la Médecine, Ville de Québec, Québec, G1V 0A6, Canada 1 Corresponding author (e-mail: richard.berthiaume@sbf.ulaval.ca). Subject editor: Jon Sweeney doi:10.4039/tce.2015.84 Received 17 April 2015. Accepted 7 September 2015. Can. Entomol. 00:1–10 (2016) © 2016 Entomological Society of Canada 1