Initial Field Observations in California on Trioxys pallidus (Haliday) a Recently Introduced Parasite of the Walnut Aphid1

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Abstract
The aphidiine beaconed Trioxide pallideas (Holiday) was imported from Europe to California in 1959 and was successfully established on the walnut aphid, Chromaphis juglandicola (Kaltenbach) soon thereafter. The parasite has spread most rapidly in coastal San Diego County and is now distributed over approximately 1,000 square miles in that area. Strong foci also occur in Orange, Santa Barbara, and Ventura Counties. During 1961, study plots were established at three San Diego County localities, Rancho Santa Fe, Escondido, and Pauma Valley. Trends in parasitization of C. juglandicola were followed through most of the walnut-growing season. Parasitization was extremely heavy at Rancho Santa Fe and Escondido in late spring. Parasite activity remained intensive throughout the year at Rancho Santa Fe but slackened perceptibly in the summer at Escondido and Pauma Valley. This reflects, perhaps, adverse effects of more rigorous climatic conditions at these two stations. There was It general uptrend in parasitization at Escondido and Pauma Valley in the fall. Under favorable conditions T. pallidus appears to be an extremely effective parasite. It is fully synchronized with its host both spatially and temporally. Diapause is a key factor in this synchronization. Both aestival and hibernal diapause occur, assuring survival and presence of the wasp through periods of host absence or scarcity, and through climatic adversity.

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