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Journal of the Lepidopterists' Society 35(3), 1981, 256
SUSCEPTIBILITY OF PIERIS NAPI MICROSTRIATA (PIERIDAE) TO APANTELES GLOMERATUS (HYMENOPTERA, BRACONIDAE)
The braconid wasp Apanteles glomeratus (L.) is probably the commonest parasitoid attacking Pieris rapae (L.) in North America, where it was apparently introduced in the 19th Century (Scudder, 1889, Butterflies of E. U.S., Scudder, Cambridge, Mass.). Although Klots (1951, Field Guide to the Butterflies, Houghton Mifflin, Boston) suggested that the decline of Pieris virginiensis Edw. populations might be due to "the parasitic wasps that breed in great numbers in rapae," almost nothing is known of the interactions of this Palearctic parasitoid with the native pierid fauna. Most native pierids, like their crucifer hosts, are active in spring. In North America A. glomeratus is multivoltine, rare early in the season and commonest in autumn. We might thus predict a small impact on the native, vernal fauna. This is particularly true in California, where the Mediterranean climate enforces vernal uni-or bivoltinism and only two species of pierines (P. protodice Bdv. & LeC, P rapae) fly after June at low elevations. In Yolo Co. up to 70% of large collections of rapae larvae may be parasitized by A. glomeratus from September through November; parasitism of P. protodice rarely exceeds 10% at the same season (Shapiro, 1979, J. Res. Lepid., 17: 1-16, and unpublished data) but may reach 30% when populations are low.
In the Vaca Hills, Inner Coast Ranges, Solano Co., the following crucifer-feeding pierids occur in sympatry: P. rapae, P. napi microstriata Comstock, P. sisymbrii Bdv., Anthocharis sara Lucas, and Euchloe ausonides Lucas. Of these, rapae is multivoltine, sisymbrii strictly univoltine, and the others facultatively bivoltine (ausonides strongly, the others weakly). From 1972 through 1979 I collected and reared over 700 wild larvae, representing good samples of all of these species from the Vaca Hills, and reared A. glomeratus only from P. rapae and only after early May. Of the native species, P. sisymbrii has been found to harbor A. glomeratus at 1500 m in the Sierra Nevada.
In 1980 A. glomeratus was exceptionally common in spring. The frequency of Apanteles attack in wild larvae collected in the Vaca Hills in April was: P. rapae, 6/6; P. n. microstriata, 0/16; A. sara, 0/6; E. ausonides, 0/24. Some of the ausonides came from the same individual hosts as some of the rapae. There is a strong suggestion that A. glomeratus had access to, but discriminated against, the native vernal fauna in the Vacas in 1980.
Sato (1976, Appl. Ent. Zool., 11: 165-175) found that in Japan, P. napi nesis Fruh-storfer and P. melete Men. were resistant to A. glomeratus, but P. n. japonica Shirozu was not. S. R. Bowden has suggested (in litt.) that members of the napi complex in the northwest Pacific and California might be related; resistance to Apanteles would bear on this hypothesis. The A. glomeratus culture established from the 6 Vaca rapae was used to test the susceptibility of P. n. microstriata to attack. Lab-reared second-instar larvae from ova laid by a female from Lang Crossing, Nevada Co., 1400 m, on the Sierran west slope, were exposed to gravid female A. glomeratus and were seen to be stung repeatedly. The host plant was Brassica kaber (DC.) Wheeler. The culture was maintained at 25°C under continuous light.
Of the 8 larvae stung 9 May, one died in the 3rd and two in the 4th instars; one pupated and entered diapause; one pupated and eclosed normally 9 June; and three produced parasitoids in.the normal manner, yielding 17, 21, and 24 wasps. The initial Vaca rapae had produced 17, 29, 23, 16, 19, and 38 wasps.
This quite uncontrolled experiment does not establish the relative attractiveness of P. rapae and P. n. microstriata as hosts. It merely demonstrates that there is no absolute barrier to attack by, and development of, A. glomeratus in the native species, and the apparent discrimination against it in the field must reflect behavioral or ecological factors.
Arthur M. Shapiro, Department of Zoology, University of California, Davis, California 95616.
Date of Issue (Vol. 35, No. 3): 28 April 1982