ESTABLISHMENT OF A CHINESE STRAIN OF COTESIA RUBECULA (HYMENOPTERA: BRACONIDAE) IN THE NORTHEASTERN UNITED STATES

Cotesia rubecula (Marshall) is a braconid parasitoid of Pieris spp. larvae that is relatively specific to Pieris rapae (L.) (Lepidoptera: Pieridae), a pest of cabbage and related cole crops. The establishment of this parasitoid in the eastern United States to help suppress this garden pest has been long sought. Efforts to establish it in North America have a complex history. A self-introduced population of uncertain origin was discovered on Vancouver Island in British Columbia in 1963 (Wilkinson 1966), and the range of this population has extended as far south as Oregon (Biever 1992). This strain was later released in Missouri, New Jersey, South Carolina, and Ontario (near Ottawa) (Puttler et al. 1970; Williamson 1971, 1972). This strain appears not to have established in Missouri (Parker & Pinnell 1972), but may have established in Ontario (Corrigan 1982). Poor establishment of this strain was attributed to an improperly timed diapause induction response (Nealis 1985). A second population, from the former Yugoslovia, was released in Missouri in the mid 1980s, and subsequently released in Virginia and Ontario. In 1988, the Yugoslavian strain was recovered in Virginia, but this population later appeared to have died out, perhaps due to high level of hyperparasitism (McDonald & Kok 1991). In 1993, C. rubecula, of uncertain origin, was found to be the dominant parasitoid in Quebec, in farming areas near Montreal (about 160 km east of Ottawa) (Godin & Boivin 1998). In 1988, a population of C. rubecula was collected by David Reed of the USDA in Shenyang, China (42 north latitude, 123 east longitude), for release in the eastern United States. This location matched the intended release location in Massachusetts in latitude, and both locations have continental type climates. Parasitized host larvae (P rapae) were shipped to the USDA quarantine laboratory in Newark, Delaware. Adult parasitoids were allowed to emerge and, following confirmation of species identity, 99 female and 49 male C. rubecula adults from this shipment were shipped to the senior author in Amherst, Massachusetts in July of 1988 and all were released in field cages in a pesticide-free, 0.1 ha collard plot in Deerfield, Massachusetts (42 n. 1.). That C. rubecula was not present at this site before the release (through spread, perhaps from some distant source) is demonstrated by the absence of C. rubecula in large numbers of hosts collected at this location and dissected for parasitism rates in a population dynamics study I ran in 1985 and 1986 (Van Driesche 1988). We subsequently reared this strain both in the laboratory and from field-collected larvae between 1988 and 1993 and made 12 other releases in Massachusetts, three in Connecticut, and one in Rhode Island, for 17 release locations in total (Fig. 1, two sets of MA sites overlap on map). Same-year recoveries of the parasitoid were made at seven of these sites, and recoveries were made after one or more years at seven other sites. Among the seven sites at which recoveries were made in subsequent years, we observed the parasitoid at three sites one year after release and at single sites 2, 3, 5, and 8 years after last release. Recovery efforts varied in different years and not all sites were visited yearly. To assess spread away from release sites, we periodically collected groups of P rapae larvae from non-release locations. We have recovered C. rubecula from 13 non-release sites, from just north of Hartford, Connecticut to Craftsbury, Vermont (north of St. Johnsbury) (Fig. 1). Recoveries have been made both along the Connecticut River Valley and in various locations in the Litchfield Hills in Connecticut, the Berkshire Hills in Massachusetts, and the Champlain Valley of Vermont. Towns in which recoveries have been made either at non-release sites or, if a release site, one or more years after the release include Winsor and Falls Village, Connecticut; Williamstown, Lanesboro, Westhampton, Northampton, Amherst, Hadley, Deerfield, Northfield, and Barre, Massachusetts; and Stamford, Rockingham, Hartland, South Royalton, Plainfield, Burlington, and Craftsbury, Vermont (Fig. 1), all of which indicate extensive range expansion in both agricultural valleys and adjacent forested hill country. Recoveries throughout Vermont bring the known range of C. rubecula near the Canadian border. Godin and Boivin (1998)'s report of recovery of C. rubecula of uncertain origin in southern Quebec, seen in the light of the data presented here, may be a further northward extension of the Chinese population, rather than an eastward extension of releases from near Ottawa. This is uncertain, as no molecular markers have been identified to separate these populations. Because establishment of C. rubecula has been associated with declines in density of the other introduced P. rapae parasitoid, Cotesia glomerata (L.), in Oregon and Washington (Biever 1992), we also counted numbers of P rapae larvae and Cotesia parasitoid cocoons (as single cocoons for the solitary species C. rubecula and as cocoon groups for the gregarious species C. glomerata) on entire