Top: Crops: Corn: Corn Blotch Leafminer:

DATE:            July 22, 2002

TO:                 Idaho Corn Growers

FROM:          Juan Alvarez, University of Idaho Research and Extension Entomologist, and Gale Harding and Reed Findlay, UI Extension Educators

RE:                 New Corn Pest in Idaho  

A new corn pest, the corn blotch leafminer (Agromyza parvicornis), was discovered last week in a northeastern Idaho field near Rigby. To the best of our knowledge, this is the pest’s first occurrence in Idaho, although it has been reported previously in Southeastern and Midwestern states and, in 1995, in Nebraska. The current scientific literature indicates that corn is its only known host plant.

The first symptoms of the presence of A. parvicornis are linear mines on plant leaves that merge to produce what are called blotch mines (Fig. 1a and b). The adult is a small, black or brown, clear-winged fly about 1/10 of an inch (2.5 mm) long (Fig. 2). Wing lengths are about 1/8 inch (3 mm). The adult female fly deposits eggs on the upper or lower surface of corn leaves, and eggs can hatch in three to four days under summer conditions in northeastern Idaho.

Maggots (the larval stage) emerge from eggs directly into leaf tissue and begin feeding, forming the large, blotchy, burned-looking mines that can seriously injure and even kill leaves. Larvae are minute, 1/8 to 1/5 inch (3-5 mm) long and a translucent greenish-yellow color and are located inside the leaf mines (Fig. 3). They consume the interior of the leaf and leave the epidermis looking like a silvery film over the leaf mines. In the Rigby-area field, we noted that plants in the whorl stage averaged five lower leaves that were heavily damaged by this leafminer.

As with similar leafminers, the larvae are protected within the leaf during their feeding cycle. Leaf mines weaken the leaves and provide easier access for bacterial and fungal diseases. Larvae complete their development in one week and then pupate within their mines. The pupa is light- to dark-brown and about 1/8 to 1/6 inch (3-4 mm) long (Fig. 4). Adults emerge after seven days to repeat the cycle.

Reports from other states indicate that field damage is usually very limited but that large populations can occasionally lead to economic losses. Although no known economic thresholds have been established for this pest, a University of Nebraska publication compares its damage to hail damage and associates 50 percent destruction of leaf area with 6 percent potential yield loss.

So far, the only known infestation in Idaho is in the Rigby-area field, but we know of no other cornfields that have been checked. Although the leafminers in the infested field were quite numerous, we do not expect this insect to become a serious pest here in Idaho this year. We believe this outbreak was related to the elimination of this insect’s parasitoids by insecticides applied against other pests. In fact, few wasp parasitoids were found in the attacked field where our photographs were taken. Because A. parvicornis is usually not an economically important pest, it has not been studied and information on its control is very limited. No insecticides are currently registered for use against it.

Growers who suspect they have this leafminer in their cornfields should contact their UI Extension educator in their county.

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