Infestations and distribution of Rhopalosiphum padi (L.) on different varieties of barley and oats, effect of nitrogen fertilization and chemical control

Infestations of Rhopalosiphum padi were compared between common varieties grown at nitrogen levels of 50, 100 and 150 kg/ha in 1986 and 1988, and controlled with dimethoate in 1988. Colonization of the aphid was slightly quicker on oats than on barley. The highest peak inside field cages was 305 aphids and in the open field 74 aphids per plant. On barley, the number of aphids was highest on var. Pomo and var. Pokko, six-row type varieties with the longest growth period. On oats the number of aphids remained lower on var. Nasta, an early maturing variety with strong straw, than on other cultivars. Excessive nitrogen either increased or decreased the peak number of aphids per plant, depending on the variety and the year. Over 40 % of aphids on oats fed on the lower base of the plant at or under the soil surface. On barley, aphids lived slightly higher, 30 % in the stem and 30 % on leaves. Dimethoate spray controlled aphids on the top of the plant but not those on the base and lower leaves. Its efficacy against aphids on the upper stem and lower leaves on barley was decreased when an increased amount of nitrogen was used. The percentages of parasitized and diseased aphids decreased after the use of dimethoate