SEASONAL ABUNDANCE OF CINARA-CRONARTII (HOMOPTERA, APHIDIDAE) AND THE EFFECT OF AN INTRODUCED PARASITE, PAUESIA SP (HYMENOPTERA, APHIDIIDAE)

Population levels of the aphid Cinara cronartii Tissot & Pepper, which arrived in South Africa in 1974 from the United States, were observed for 3 yr (1983-1985). Studies were done in an isolated 6-yr-old plantation of Pinus patula at Buffelspoort, South Africa, after a parasite (Pauesia sp., also from the United States) had been released there in 1983. Infestations by C. cronartii started at the beginning of June in 1983 and at the end of April in 1984 and 1985. In 1983 and 1984, aphid populations peaked in early July. They collapsed early in August and remained small until the end of the aphid season, which lasted until September-October. In 1985, the aphid population remained low without any distinct peaks; the season lasted until the middle of November. Most aphid colonies were small and medium in size. All trees sampled were infested for at least 1 wk in each season. The aphids were anholocyclic, and colonies changed feeding sites on the trees continually throughout the season. The percentage of trees infested was a sigmoid function of the population density. This relationship remained constant from season to season. Pauesia sp. was temporarily established in winter 1983 but could not be found in the following summer and most of the 1984 aphid season. The parasitoid reappeared late in the 1984 season (first part of August) and became permanently established. In 1983 and 1984, aphid populations increased when the parasite was absent and collapsed later in the season when Pauesia sp. became established. In the 1985 season, however, when the parasite was well established, the aphid population remained small without distinct peaks throughout the season. This study indicates that Pauesia sp. may have been responsible for the changes in C. cronartii population levels.