Conclusions from

“2007 Outbreak of Human Pruritic Dermatitis in Chicago, Illinois Caused by an Itch Mite,
Pyemotes herfsi”

Illinois Natural History Survey Technical Report 2008 (17)

May 20, 2008

Edmond R. Zaborski, Ph.D.
Division of Biodivesity and Ecological Entomology
Illinois Natural History Survey
1816 South Oak Street
Champaign , Illinois 62761

1. The causative agent of the outbreak of bite rashes in the Cook and DuPage Counties in Chicago area of Illinois during August-September 2007 was Pyemotes herfsi (Oudemans, 1936), a polyphagous obligate ectoparasite of insects. This is the same species of mite that caused the Kansas-Nebraska bite outbreak in 2004-2005.
   
   
2. Identification of the causative agent was confirmed by Dr. John Moser, Emeritus Research Entomologist, U.S.D.A. Forest Service.
   
   
3. Voucher specimens are deposited in the insect collection of the Illinois Natural History Survey and in the U.S.D.A. Forest Service Southern Research Station collection of mites associated with bark beetles at Pineville, LA.
   
   
4. In the 2007 Illinois outbreak, the only insect found to be parasitized by P. herfsi in the field was the developing eggs of 17-year periodical cicadas, Magicicada spp. Brood XIII periodical cicadas emerged earlier in the year and laid large numbers of eggs in the terminal branches of many deciduous tree species in the outbreak area. It appears that this abundance of cicada eggs served as the host upon which populations of P. herfsi mites amplified to outbreak levels, unlike the Kansas-Nebraska outbreak in which the amplification host for P. herfsi was oak leaf gall midges.
   
   
5. The proposed common name “oak leaf itch mite” for P. herfsi is misleading and contributed to the delay in identifying the causative agent of the 2007 Illinois outbreak. Insects from at least 5 insect orders and 9 families occupying a range of habitats have been recorded as hosts for P. herfsi.
   
   
6. This is the first confirmed report of P. herfsi parasitizing cicada eggnests.
   
   
7. Contrary to recent reports, P. herfsi is not new to North America; it has been found in Utah in 1982 and in Colorado in 1956. A species of Pyemotes that, based on available data is indistinguishable from P. herfsi, was reported to be “the mite most commonly found” with periodical cicada eggnests among samples collected in 1885 in Michigan, Virginia and the District of Columbia. The possibility that P. herfsi has been in North America for more than one hundred years, and may even be indigenous to North America, cannot presently be excluded.