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Two Connecticut populations of the black-legged tick, Ixodes scapularis (Say), the vector of
Lyme disease spirochetes, Borrelia bugrdorferi sensu stricto Johnson, Schmid, Hyde, Steigerwalt & Brenner, in the northeastern United States, are parasitized by the encyrtid wasp Ixodiphagus hookeri (Howard), formerly Hunterellus hookeri. The wasp was first detected in ticks from a forested site in Bridgeport in 1992. I. hookeri was reared from 18.6% of 148 host-seeking I. scapularis nymphs. In 1993 and 1994, this wasp was found to parasitize 26.0% of 192 engorged nymphs from Bridgeport and 21.8% of 101 nymphs from the Bluff Point Coastal Preserve in Groton. Each parasitized nymph produced an average of 6-8 wasps (range, 3-16) with a female to male sex ratio of 1.9-1. Both study sites are wooded, geographically isolated tracts (Bridgeport, 176 ha and Bluff Point, 326 ha) with high densities (51-72/km2) of white-tailed deer, Odocoileus virginianus (Zimmerman), and superabundant tick populations similar to that of Prudence Island, RI, where I. hookeri also has been reported. We found that I. hookeri emerged from 16.3% of 399 engorged nymphs and 13.7% of 1,081 engorged nymphs collected as unfed ticks from Prudence Island in 1990 and 1991, respectively. No wasps were obtained from nymphs collected in Stamford (n = 38) or Old
Lyme, CT (n = 241). A high proportion (23.1% of 39) of engorged nymphs obtained from Bridgeport deer produced I. hookeri. However, only 2 nymphs (6.7%) recovered from white-footed mice, Peromyscus leucopus (Rafinesque), at Bridgeport were parasitized by the wasp. None of the engorged larvae recovered from deer or mice and fed as nymphs in the laboratory produced I. hookeri (n = 26 from deer and n = 384 from mice). The presence of this wasp in I. scapularis at these 2 insular-like sites on the Connecticut mainland supports the observation that high tick densities are required for the establishment and maintenance of I. hookeri and that the potential role of this wasp in the biological control of I. scapularis is limited.