Common Syrphidae of Mid-Atlantic Apple Orchards
Syrphid flies, or hover/flower flies, are very common and conspicuously colored flies. Many adults appear as bee mimics and contribute to pollination in various cropping systems. Adults feed on pollen, nectar, and sugars such as honeydew of aphids. Syrphidae can usually be identified by the spurious vein (“false vein”) that runs between the radial and medial veins and typically crosses the r-m cross vein (except some exotic species where the spurious vein is very faint).
Larvae of Syrphidae can be phytophagous, saprophagous, or entomophagous. The aphidophagous (“aphid-eating”) adults deposit their eggs directly next to, or in aphid colonies to limit searching by the blind larvae. The larvae search for prey by a method known as “casting”, where the front of the body is swung side-to-side until contacting prey. Oral hooks are then inserted into the body of an aphid and it is sucked dry. In the mid-Atlantic region many species of syrphid flies can be encountered, but this page will focus mainly on aphidophagous syrphid fly larvae.
Heringia calcarata (Loew)
Heringia calcarata is the most abundant aphidophagous syrphid fly found in mid-Atlantic apple orchards. Unlike many syrphid flies, H. calcarata is black. Adults (Figs. 1 and 2) are about 9 mm in length and complete development in approximately 25 days. The eyes (holoptic, tend to be more reddish) of males touch at some point on the head, whereas those of the female (dichoptic, tend to be burgundy) do not. The egg (Fig. 3) is approximately 0.64 mm long x 0.24 mm wide and has unbroken longitudinal ridges running along its surface. This species is a specialist of the woolly apple aphid (WAA) (Eriosoma lanigerum (Hausmann)). Females lay eggs directly into WAA colonies where the larvae feed for approximately 9 days before pupariating in the soil or on limbs. H. calcarata is a member of the Pipizine tribe of syrphid flies, which are known to prey on colonies of aphids producing waxy, flocculent material and/or galls such as WAA. Edaphic (root) colonies of WAA will continue to be an increasing problem in orchards with the prevalent use of dwarfing rootstocks, not bred for resistance to the WAA. Thus, H. calcarata may become a valuable asset in control of root aphid colonies. H. calcarata is most likely bivoltine and numbers tend to peak right before population increases of the WAA, in early June and early July. In conjunction with Aphelinus mali, H. calcarata can effectively control arboreal WAA populations.
Fig. 1 Adult Male - H. calcarata
Fig. 2 Adult Female - H. calcarata
Fig. 3 Egg – H. calcarata
Fig. 4 Larva – H. calcarata
Fig. 5 Puparia – H. calcarata
Eupeodes americanus (Wiedemann)
Eupeodes americanus is a generalist aphidophagous syrphid. The “American Hover Fly” feeds on WAA, rosy apple aphid (RAA), and spirea aphid (SA) in orchards. It is the most abundant generalist found in the mid-Atlantic orchards, but far less than the specialist H. calcarata. This species is found early in the season in WAA colonies, but then switches to RAA and SA later in the season (mid-June), returning to WAA around the mid to last part of September when WAA populations resurge. It can be grossly differentiated from Syrphus rectus (seen below) by a dark stripe down the front of the face of the adult (Fig. 6a). The egg (Fig. 7) is approximately 0.96 mm long x 0.26 mm wide and has broken longitudinal ridges running along its surface. The larva (Fig. 8) is blackish with orange markings on the body, tends to look somewhat spiky, and is narrower than that of S. rectus. The puparia is grayish-orange and appears mottled, much like the larva (Fig. 8a). First instars are difficult to separate if they were not seen emerging from an egg.
Fig. 6 Adult dorsal – E. americanus
Fig. 6a Adult ventral – E. americanus
Fig. 7 Egg – E. americanus
Fig. 8 Larva – E. americanus
Fig. 8a Puparia- E. americanus
Syrphus rectus (Osten Sacken)
Syrphus rectus is also a generalist aphidophagous syrphid fly that can be found feeding on WAA, RAA, and SA colonies in apple orchards. This species is only prevalent in WAA colonies for 2 weeks in May and then switches to RAA colonies. During those 2 weeks, the population numbers are sparse and are a minor component of the predator complex. However, this species is quite common in RAA and SA colonies. The adult (Fig. 9a) can be grossly differentiated from E. americanus by lacking a dark stripe down the front of the face (Fig. 9b). The egg (Fig. 10) is approximately 1.19 mm long x 0.44 mm wide and has “T-shaped” projections covering its surface. The larvae (Fig. 11) are wider (later instar) than E. americanus and tend to be yellow-pale yellow in color.
Fig. 9a Adult dorsal – S. rectus
Fig. 9b Ventral - S. rectus
Fig. 10 Egg – S. rectus
Fig. 11 Larva – S. rectus
Allograpta obliqua (Say)
A. obliqua is a very sporadic aphid predator. They are not found as often in mid-Atlantic orchards as other generalist predators and even then are primarily found in RAA colonies. The larvae and puparia are quite distinct, in that they are totally green. The adult (Fig. 12) has a fairly narrow abdomen and a characteristic pattern on the fourth abdominal tergite.
Fig. 12 Adult – A. obliqua
Some other common syrphid fly species, though probably not aphidophagous in apple, found in mid-Atlantic apple orchards are Toxomerus marginatus, Toxomerus geminatus, Platycheirus obscurus, and Sphaerophoria spp.