Japanagromyza viridula (formerly Agromyza viridula)

The oak shothole leafminer is considered native to the United States, although very little is known about this species and other agromyzid leaf miners. It is reportedly found throughout the eastern U.S. from Maine to Georgia, and likely the entire eastern half of the United States. Adult female flies are active when red and white oak buds are just opening and until the newly developing leaves are approximately 5 cm (~2") in length. Females create damage to the leaves with their ovipositors, which allows them to drink host plant fluids from the leaf. Occasionally, the female will also lay an egg in the leaf. The eggs hatch and mining from the larvae of the oak shothole leafminer is reported to occur in the second week of June in Maine, while in Ohio there are reports that the blotch mines created by this insect are most evident in early to mid-May in that state. Reportedly, fully grown larvae emerge from their mines to drop to the soil to pupate. Johnson and Lyon (1991) states that although adults are present the following spring, they may also reappear later in the same season “if a second flush of growth takes place or water sprouts (epicormic shoots) develop”. The oak shothole leafminer is not usually considered a significant pest of oak, and the damage it causes typically does not have a significant impact on the overall health of the tree. However, in a 1967 paper detailing observations from the 1960’s in Maine, there is a question about the economic importance of this species on ornamental oaks. In 1964, an “abundance of neat, prominent holes in foliage of ornamental red and white oaks (Quercus borealis var. maxima and Q. alba) in southern Maine” was observed. This was again observed in 1965. The cause, at the time determined to be Japanagromyza viridula, was identified by the State Entomologist with the Maine Forest Service and partners in the USDA. Their observations stated that many more “punctures” are made by the female for feeding than for oviposition (egg laying) and this is a common pattern of flies in this family. This might explain why the damage from the feeding adult females seems so extensive, whereas the mines from the feeding larvae of this insect are not present in the same proportion and may even be difficult to find.

In 1965 in Maine, the first discovery of adult flies was made in mid-June on red oak sprouts. Feeding punctures from female flies was described as numerous, while egg laying was relatively rare. “Mines were of a blotch type.” Larvae emerged from their mines before pupating. Not much is known about the life cycle or its timing during the season for this insect. This 1967 paper suggests that multiple hosts may occur for the flies, although that is not certain either. Based on what occurred in Maine in 1964 and 1965, the authors suggest that the injury caused by the peculiar feeding habits of these flies may be economically important on ornamental oaks. At the time, ornamental white and red oaks were widely affected by the shot-hole injury in much of southern Maine. However, only circumstantial evidence was available to suggest that, under certain conditions, the emergence of agromyzid flies and the co-occurring appearance of juvenile foliage of oak may allow extensive attack. Following the widespread activity of this insect in 1965, “the infestation expected in the spring of 1966 did not materialize for some unknown reason” (LaBonte and Lipovsky, 1967).