Abstract
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.
Highlights
Differences in Rhopalosiphum padi (L.) infestation between species of spring cereals and different varieties have been reported in Finland by Markkula and Roukka (1972)and Rautapää (1970) and elsewhere eg. Hsu and Robinson (1963)
R. padi much less than other spring cereals, and aphids on wheat seldom need to be controlled in Finland
The observation that a higher number of spring migrants landed on oats than on barley was a surprise, on the basis of information about the poor ability of R. padi to select host plant presented by Ahman et ai
Summary
Differences in Rhopalosiphum padi (L.) infestation between species of spring cereals and different varieties have been reported in Finland by Markkula and Roukka (1972)and Rautapää (1970) and elsewhere eg. Hsu and Robinson (1963). Differences in Rhopalosiphum padi (L.) infestation between species of spring cereals and different varieties have been reported in Finland by Markkula and Roukka (1972). The order of favourability to R. padi is known to be: barley, oats and wheat. R. padi much less than other spring cereals, and aphids on wheat seldom need to be controlled in Finland. On the basis of experiments in cages, Rautapää (1968) has described changes in yield components on cereals infested with R. padi. Nitrogen fertilization is known to promote vegetative growth and the ability of spring cereals to tiller, thereby creating new sites for R. padi to feed on. By extending the period of vegetative development, excessive nitrogen extends the time for R. padi to multiply. It has been claimed that excessive nitrogen increases the need for aphid control
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