POPULATION FLUCTUATIONS OF APHIDS AND THEIR NATURAL ENEMIES ON POTATO IN HOKKAIDO, JAPAN

Population fluctuations of aphids and their natural enemies were investigated in a potato field at Hitsujigaoka, Sapporo, Japan (43-degrees-0'N, 141-degrees25'E) during the periods from late May to late September in 1991-1993. The aphid species mainly observed on potato were Myzus persicae (Sulzer), Aulacorthum solani (Kaltenbach), Macrosiphum euphorbiae (Thomas) and Aphis gossypii Glover. Main predators were Orius sauteri (Poppius) (Anthocoridae), Coccinella septempunctata bruckii Mulsant (Coccinellidae) and Harmonia axyridis (Pallas) (Coccinellidae). The numbers of Chrysophidae, Cecidomyiidae, Syrphidae and other predatory arthropods were relatively small, except for spiders. The most abundant parasitoid was Aphidius gifuensis Ashmead (Aphidiidae), which attacked mainly A. solani and M. persicae. Aphids had the first population peak from mid-June to early July, declined rapidly from mid- to late July, and then had a second peak in the first half of August. The first peak was mainly constructed with A. solani and M. persicae and the second one with A. gossypii. The population fluctuation of coccinellids (C. septempunctata bruckii and H. axyridis) closely related to the first peak and that of O. sauteri to the second peak. The first peak of aphids in 1993 was relatively higher than those in other years. This was considered to be caused by the late colonization of natural enemies due to low temperatures and much precipitation in June and early July in 1993. Aphid population dynamics seemed to be influenced by their natural enemies and weather conditions.