Contarinia johnsoni Slingerland

This grape pest was first discovered in the Chautauqua grape region of western New York in 1904, and as far as known is still confined to that locality. A closely related species causes a similar injury to grapes in Europe. Its presence in a vineyard is first indicated by abnormally enlarged, yellowish or dark reddish blossom buds (Fig. 378) which fail to open and drop off about the normal time for blossoming. Opening one of these enlarged buds, it will be found to contain a number of small whitish or yellowish maggots from 1/16 to 1/12 inch in length (Fig. 379). From 10 to 60 per cent of the buds are sometimes destroyed, giving the clusters a very thin and ragged appearance, and thus decreasing considerably the market value of the crop.

The parent insect is a delicate two-winged midge with a yellowish body and straw colored legs (Fig. 380). The inch in length and the male a little smaller. The flies emerge from the ground in the latter part of May just as some of the blossom buds of such early varieties as Moore, Early and Worden begin to show a small opening at the tip caused by a spreading of the petals. The female deposits her minute, grayish, elongate, slightly curved eggs in the interior of the bud by means of an extensile fleshy ovipositor inserted through this opening in the apex of the bud. From a few to seventy maggots may be found in a single bud, but twenty-five is about the average in a year of heavy infestation. The infested buds contain a watery fluid in which the maggots live. Such buds become greatly swollen, often three times as large as normal, and turn yellowish, becoming dark reddish, particularly toward the tip. When full-grown the maggot is about 1/12 inch in length and of a yellowish or orange color. They usually escape by the opening at the apex, fall to the ground, where at a depth of about 6 inches they pass the winter as larvae, curled up in small, ovoid, silken lined, earthen cocoons about 1/25 inch in length. Pupation takes place the last of April and the adults emerge about a month later.

- Mark Slingerland, Cyrus Crosby: (1915) Manual of fruit insects^©

Reference: https://www.biodiversitylibrary.org/page/30873865#page/459/mode/1up