Ophiomyia congregata Malloch

Ophiomyia congregata (Malloch)

(Figs. 83–85)

Material examined. MASSACHUSETTS: Franklin Co., Northfield, 276 Old Wendell Rd., 31.v.2016, em. 14.vi.2016, C.S. Eiseman, ex Prenanthes altissima, #CSE2577, CNC634787 (1♂); near 276 Old Wendell Rd., 14– 26.v.2016, em. 4–26.vi.2016, C.S. Eiseman, ex Prenanthes, #CSE2543, CNC654088–654094 (4♂ 3♀).

Host. * Asteraceae: Prenanthes altissima L.

Leaf mine. (Figs. 83–85) In summer and autumn, young larvae produce narrow, whitish, linear mines in the leaf blade, which appear fragmented because they undulate between the upper and lower leaf surface (Fig. 85). Eventually, the larvae descend into the petiole and apparently overwinter in the crown of the host plant. In spring, they resume feeding in the new leaves, mining primarily up and down the petiole (Fig. 84) but occasionally making excursions along the midrib and lateral veins on either leaf surface (Fig. 83). The most extensively mined leaf blade we observed was attached to an unusually short petiole, just 4 cm long. Brown, liquidy frass is deposited in spots and long trails within the petiole; mines in the leaf blade typically appear clean and whitish.

Puparium. Whitish; formed in the petiole (one found in a vein at the base of the leaf blade). The puparium is often externally visible, either with the anterior spiracles protruding through the epidermis or behind a thin “window” of transparent epidermis; however, some are formed deeper within the petiole and can only be detected with bright backlighting.

Distribution. USA: AZ, CO, *MA; Canada: AB, BC.

Comments. Prenanthes is recorded from neither Arizona (the type locality) nor either of the Colorado counties where this fly has been found (USDA, NRCS 2016), so clearly other hosts remain to be discovered.

We pieced together the life cycle of Ophiomyia congregata after first finding a leaf mine with an associated puparium (which proved to be parasitized), in Heath, Franklin Co., Massachusetts on 27 May 2015. At two locations in northern Vermont on 5–6 September 2015, we collected young mines (on Prenanthes altissima and P. trifoliolata (Cass.) Fernald), and when immature larvae began to vacate the leaves on 21 September, we surmised that this species overwinters in the plant before completing development. One of our reared series came from individual plants on which we had first noted leaf mines on 7 October 2015, then monitored in spring for new feeding activity. Fresh mines in the blades were first observed on 26 May, but by this time the larvae had already fed extensively in the petioles. The last larva was observed on 30 May; it had pupariated by the next day. The earliest date we have seen mines of young larvae at this locality is 23 August (2016); we found similar mines in Ohio on 6 August 2016, and others were photographed in Iowa on the same date (van der Linden 2016).