Beware of Oak Leaf Gall Mites; They May Be Coming Back 5/6/05

MANHATTAN, Kan. – The tiny mites that made headlines last year in the Midwest because of their fierce, itchy bites may be staging an encore, a Kansas State University scientist said.

Midwesterners in Kansas, Nebraska, Missouri, Oklahoma and Texas in 2004 endured itchy bites caused by the oak leaf gall mite (Pyemotes herfsi), a species of itch mite introduced from Europe, said K-State Research and Extension entomologist Alberto Broce.

Facts About Oak
Leaf Gall Mites

MANHATTAN, Kan. – Oak leaf gall mites (Pyemotes herfsi) are a species of itch mites that appeared in 2004 in several Midwestern states. Kansas State University and University of Nebraska scientists have monitored the pest since it was first reported last August in southeastern Kansas. Following are some facts about those mites:

* Typical bites on humans are raised, reddened, 1/2 to 3/4 inch (diameter) welts, each with a vesicle in the center -- itchy, but painful when scratched. Last year the bites often resulted in secondary bacterial infections. Apparently, the mites must be in contact with a person’s skin for four to five hours for the bite to be felt some 10 to 16 hours later. Because of the delayed reaction, many people don’t know they’ve been bitten until the next day.

* The mite is closely related to the straw itch mite, which for hundreds of years has pestered farmers handling straw and stored products.

* The oak leaf gall mite is a tiny, 0.2 mm (1/125 inches) long mite. It is barely visible to the naked eye.

* The mite’s life cycle is unusual. A mated female searches for a host on which to feed. She is small enough to be carried by the wind. If she finds a leaf marginal roll gall on an oak tree, she enters the gall. If she finds the midge larva responsible for the gall formation, she inserts her mouthparts into the host larva. Within minutes, a potent neurotoxin in her saliva paralyzes the midge larva, who will die of starvation.

(The toxin is potent. One mite’s bite can kill an insect larva 170,000 times its own weight. That toxin is what causes itching when the mites bite humans.)

Once the female starts to feed, she can develop as many as 250 offspring. In seven days, her “abdomen” is fully distended, and her progeny -- of which five to 10 percent are males -- are ready to emerge as fully developed adults. Males emerge ahead of the females, mate with the females as they emerge from the mother and die shortly after. The females complete the cycle by dispersing in search of new hosts.

* First reports of bite outbreaks in Kansas and Nebraska in 2004 were in mid-August and persisted until mid-November. Early reports last year were from people bitten during recreational outdoor activities, but reports late in the season were linked mainly to leaf raking.

Broce has been working with entomologists at K-State and the University of Nebraska this spring, to determine if an outbreak could occur again this year. They found that midges (tiny insects that serve as hosts to the oak leaf gall mite) overwintered as larvae or pupae in the soil or among fallen leaves from oak trees, and they recently witnessed a mass emergence of midges from lawns in neighborhoods seriously affected in 2004 by the mites in Lincoln, Neb.

Those midges, which feed on a wide variety of other insects, cause the oak leaf galls that give the mite its common name. Leaf galls are the small, raised lumps sometimes apparent on tree leaves. They’re a nursery for immature midges. Apparently, itch mites feed on the midge larvae inside of those galls – primarily on pin oak trees, but also on red and black oaks, Broce said.

“We saw midges in the thousands hovering about the lawn grass (in Lincoln, Neb.), and then flying straight up to the leaves,” he said. “Pin oaks that were heavily infested with the itch mite last year are now showing 100 percent of their leaves already colonized by the midge’s first instar larvae, and the leaf margins are curling to form the galls.

“We are predicting that the oak leaf gall mites will again be a problem this year, but there is not much we can do to fight them. We don’t want people to start reacting before the mites are here, too. When and if we find them in significant numbers, we will advertise this through Extension channels and the media.”

Meanwhile, the entomologist said, he’s advising people to go out and enjoy the outdoors. If they develop bites on the shoulders, neck and upper torso after being in areas affected by the mites last year, that will be a good indication that the mites are back.

“Unfortunately, because we lack so much information about the biology and behavior of these mites, we do not have specific control methods or miticides to recommend,” he said.

Scientists do not recommend removal of the oak trees that the mites inhabit. They also do not recommend spraying insecticide on trees and lawns.

“We do recommend using a DEET-based insect repellent formulation,” Broce said.

“If you’re working in gardens near infested areas, handle grass clippings and leaves with gloves. Try to stay outdoors less than four hours. Then go inside; discard your clothes in the washing machine; and take a warm, soapy shower,” he said.

Crawford County in southeast Kansas was one of the areas first affected by the oak leaf mite last year. For those bitten by the mites, the Crawford County Public Health Department recommended using calamine lotion or an antihistamine or hydrocortisone product to help reduce the itching.

Broce said that he and other entomologists will continue to monitor the oak leaf galls for the presence of mites.

“We predict that once they appear, their population will rapidly increase, as these mites have one of the fastest population growth potentials among animals,” he said.

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.