Many people have been complaining of bites in northeastern Illinois over the past couple of weeks. The reports are primarily in the suburban communities in western Cook and the eastern half of DuPage counties, just west of Chicago. A considerable amount of effort has been expended in searching for the source of these bites. This has included the placement and analysis of sticky traps, searching galls, dissecting periodical cicada egg-laying sites, sifting leaf litter, and observing attacked skin surfaces. Specimens have been found, but positive identification of them and association with the bites are still in progress. It appears that the most heavily involved areas are those where periodical cicada was numerous this spring, which is why twigs containing periodical cicada eggs are being examined.
Due to the nature of the bites, the lack of associated specimens, and several other factors, it appears that the most likely cause is an itch mite in the genus Pyemotes. The oak leaf gall itch mite, a member of this genus, was discovered a few years ago in Nebraska and has subsequently been found in Kansas, Missouri, Texas, and other locations. There have been reports for at least a couple of years from southern Illinois of similar bites. This mite has not been found previously as far north as northeastern Illinois. The mite apparently is a recent invasive species into North America from Eurasia, with it being first reported in the United States in 1955.
Previously in the United States, the mite has been associated with the oak marginal fold gall maker. This gall appears on pin oak as an approximately 1/8-inch-wide lapping over of the leaf underside onto its upperside. It appears on the inside curves of the leaf’s lobes and is typically about 1/2 to 1 inch in length. The gall is green and not obvious during most of the growing season, being noticeable only by the lighter green leaf underside along the edge of its upperside. At this time of year, the galls have turned brown, making them much more obvious against the green leaf uppersides. The fold is easily opened by prying back the folded leaf margin. The oak marginal fold gall is caused by a gall midge, a very small fly. Through much of the growing season, one can find the transparent to white, spindle-shaped fly larvae inside the galls.
The oak leaf gall itch mites attack and feed on the gall midge larvae, killing them. The mature female mite develops a large, rounded abdomen and is found inside the empty gall. Mites are produced in very large numbers, which leave the galls and fall from the trees. They are blown on the wind to new areas. The number of these mites is so large that this dispersal is commonly referred to as “mite showers.” The mites are elongate, whitish, and exceeding tiny, at the lower edge of being visible to the unaided eye. They are about 2/10 of a millimeter long, about the size of the leg of a twospotted spidermite.
These mites try to feed on anything that they land on. They take at least 4 hours to bite people. When they bite the skin of people, the result after 10 to 16 hours is a roundish, red area about one inch across, with a small, whitish swollen area in the center. This bite typically takes 10 to 14 days to subside. Most people are being bitten on the face, neck, arms, chest, and back. Because we and other animals other than insects are not the correct host, the mites are unable to survive on us and no reproduction occurs. Unfortunately, these mite showers can last from early to mid-August into early December.
Bites can be reduced by wearing long-sleeved shirts, long pants, hats, and other clothing that covers as much of the skin as possible. Apply insect repellents containing DEET to exposed skin and clothing to reduce the number of bites. Change clothes and shower or bathe after being outdoors in infested areas. Landscapers and other professional horticulturists who need to spend extended time outdoors in infested areas should change clothes and shower every 4 hours, if practical, to reduce the number of bites. Soap kills on contact any mites on clothes or skin. There have been numerous hospitalizations due to bites in northeastern Illinois. Because these mites blow easily on the wind and are tiny enough to go through the mesh openings in window screening, it is advised to keep doors and windows shut as much as is practical.
It is not recommended to spray trees with pesticides to reduce mite numbers. Not only do we not know if these mites are to blame, but they have blown onto all surfaces. If the cause is the oak leaf gall itch mite, the females are protected from contact sprays because they are inside the galls. The gall tissue is brown and dead, so it does not transmit systemic miticide into the gall. Even if insecticide moved into the gall tissue, itch mites feed on the gall midge larvae, not leaf tissue, so the mites would still not be killed. Surviving gall midge larvae have dropped to the ground and pupated, so their control is not practical at this time. If it is determined that the oak marginal fold gall maker is involved with this situation, control recommendations will be made in this newsletter by next spring when any preventive treatments can be made.