No parasitoids able to complete development on apple aphid, Aphis pomi De Geer, were found in north-central Washington. However, several aphidiid species oviposited in A. pomi and developed to last instars before dying. Of these, Praon unicum Smith and Lysiphlebus testaceipes (Cresson) were found attacking A. pomi in apple orchards. P. unicum was most active in spring and parasitized a high percentage of apple aphids in some orchards. L. testaceipes was more prevalent during summer and apparently contributed little to apple aphid control. Hosts and habitats of these and two other species, Praon occidentale (Baker) and Diaretiella rapae (M'Intosh), were investigated. Winter and spring generations of P. unicum were found on apple, peach, and hawthorne. The summer generation was found mainly on hawthorne, weeds, and grasses. Aphid hosts of P. unicum on apple included Dysaphis plantaginea (Passerini), Rhopalosiphum insertum (Walker), and Nearctaphis bakeri (Cowen). Myzus persicae (Sulzer) was the major host on peaches in winter and spring and on weeds during summer.
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Carroll et al. (1986) studied this question.