blueberry stem gall wasp

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Blueberry stem gall wasp is a tiny, gall-forming, chalcid wasp. It occurs in the United States and southern Canada east of the Great Plains. In Minnesota it is common in the northeast quarter of the state.

Blueberry stem gall wasp forms a multi-chambered swelling (gall) on a highbush or lowbush blueberry stem. It is often found in wild blueberry populations. It has only recently become a significant pest of commercial blueberry crops.

The female lays multiple eggs in a young stem. Most egg laying, about 70%, takes place low in the stem, within leaf litter near the ground. The remaining is higher on the stem. After laying eggs, the female climbs to the stem tip and stabs it several times. This causes severe damage and often causes the stem to stop growing. Before the eggs hatch, the plant forms a soft, red swelling composed of non-vascular (parenchymatous) cells. After the eggs hatch and the larvae begin to feed, stronger, fibrous (sclerenchymatous) cells form walls around each larval chamber. By mid-summer the gall is hard, woody, darker reddish green, kidney-shaped, and ¼″ to 1½″ (7 to 38 mm) long. The size is determined by the number of eggs in the gall.

Most adults by far are female. They are 1⁄16″to ⅛″ (2 to 2.5 mm) in length. The head, thorax, and abdomen are black.

The antennae are bent (elbowed) and have 10 segments. The first segment (scape) is long. The end segments are enlarged, creating a club at the tip. They are mostly orangish yellow, but the club is black.

The rear part of the body (gaster) is coarsely sculptured. On the female, the needle-like egg-laying tube (ovipositor) is short.

The forewings are long, reaching to the end of the abdomen. They are clear and lightly tinged with black. They have no closed cells. The veins reach more than halfway to the tip.

The legs are orangish yellow. On the hind legs, the third segment (femur) is not swollen, and the fourth segment (tibia) is more or less straight. The last part of each leg (tarsus), corresponding to the foot, has 5 segments.

Total length: 1⁄16″to ⅛″ (2 to 2.5 mm)

Highbush and lowbush blueberry

One generation per year:

Adults are active only in the day, and especially on warm days.

The larvae feed in individual cells within the gall. They overwinter in the gall and pupate in the spring. Adults emerge from late May to early June before the host buds break.

Blueberry stem tissue

Sources

24, 27, 29, 30, 82, 83.

Common

This is the only species in the genus Hemadas.

Location: Fairview Twp.