The yellow dwarf disease in winter cereal crops is caused by species of Barley yellow dwarf virus (BYDV) and Cereal yellow dwarf virus (CYDV) (Luteoviridae). These viruses are transmitted to grasses (Poaceae) by aphids (Aphididae) and the frequency of virus population is affected by oscillations in the vector and host populations. Seasonal fluctuations of BYDV-PAV, BYDV-MAV, and CYDV-RPV in aphids and grasses were analyzed in corn in the summer, and wheat and oat plots in the winter in Coxilha, RS, Brazil. Among the aphids collected, 12.7% transmitted B/CYDV, and 92.6% of those aphids were Rhopalosiphum padi while 7.4% were Sitobion avenae. The viruses that R. padi transmitted were BYDV-PAV (95.4%), CYDV-RPV (2.3%), and BYDV-MAV+PAV (2.3%), while S. avenae only transmitted BYDV-PAV. Among the wheat and oat plants collected, 65.8% were seropositive, all of which were infected with BYDV-PAV and 0.7% of which were also infected with BYDV-MAV. The population dynamics of the virus was similar in aphids and plants, with peaks in the winter crop season. The 35 isolates of BYDV-PAV analyzed were able to infect wheat and oat, being transmitted by R. padi (EF=94.4%), S. avenae (EF=76.1%), and M. dirhodum (EF=63.4%). They were not transmitted by S. graminum or S. maydis. Since several common vector species efficiently transmit BYDV-PAV, this may explain why it is the dominant virus species in the "yellow dwarf pathosystem" in Southern Brazil.

Cereal aphids (Hemiptera: Aphididae) are economically important pests in all wheat-producing regions of Brazil and about eight species can be found in wheat crops. Aphids acquired the condition of severe pest in wheat in the 1970s, due to the emergence of Metopolophium dirhodum and Sitobion avenae. A successful biological control program of wheat aphids was introduced in southern Brazil in 1978. This study describes temporal changes in aphid populations in a wheat producing area in the northern region of Rio Grande do Sul (RS) state, Brazil. Data from weekly samples (water traps of Moericke type) for the current (May to November 2008November , 2009November and 2010 population frequency of each species was compared with 1970s populations. In addition, their seasonal fluctuations and the influence of rainfall and temperature on their populations were estimated. The species collected, and their frequencies, were as follows: Rhopalosiphum padi (57.6%), Sitobion avenae (31.0 %), Schizaphis graminum (7.7%), Metopolophium dirhodum (1.4%), Rhopalosiphum rufiabdominalis (0.9%), Rhopalosiphum maidis (0.6%), Sipha maydis (0.4%) and Sipha flava (0.4%). Aphids populations in winter cereals in northern RS have changed radically. The high populations of M. dirhodum in wheat in 1970s have been reduced to such levels that rarely have been found. Nowadays, R. padi was the predominant species. When the rainfall remained between 0-20 mm and temperatures between 15-20 ºC, aphid populations were more numerous than the average for the sampling period.