A Comparison of Rhopalosiphum padi (L.) and R. fitchii (Sand.) as Vectors of Barley Yellow Dwarf Virus

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Abstract
Although Rhopalosiphum padi L. was common on cereals and Gramineae in Ontario, only a small number of R. fitchii (Sand.) were found on one grass plant in 1961. Fundatrigeniae of R. padi from Prunus virginiana L. and of R. fitchii from Crataegus spp. and Malus spp. were transferred to oats and wheat on which alienicolae of R. padi multiplied rapidly, but R. fitchii very slowly. R. fitchii transmitted all but one of the isolates of barley yellow dwarf virus (BYDV) that R. padi transmitted to Clintland 60 oats, but fewer test plants became infected, the incubation period for symptom development was longer and symptoms were less severe when R. fitchii was the vector. Both species infected more test plants after a 5-day than after a 2-day acquisition feed on the virus source. It appears that R. padi, a common and efficient vector of BYDV, has frequently been mistaken for R. fitchii which is not common on cereals and not a highly efficient vector. A dosage concept is proposed to explain differences in vector efficiencies of R. padi and R. fitchii. It is also suggested that strains of BYDV become ’adapted’ to transmission by a vector species.