Impact of cereal aphids on wheat yields in southern New South Wales, Australia

The effect of naturally occurring aphid infestations on yield was studied using sprayed and unsprayed plots in commercial wheat crops at a number of sites in southern New South Wales, Australia. In 1990, wheat was planted in April or early May following early autumn rains. Aphids were first found in the crops 4 weeks after the wheat was sown. Numbers were generally low and only at 1 site was there a significant effect on yield. In 1991, sowing was delayed by prolonged dry weather during autumn. Most crops were planted in late May and aphids started to appear only in mid July. There was no significant effect of aphids on yield but most crops were badly affected by drought. In 1991, labelled plants at some sites were used to study the impact of varying levels of aphid infestation on yield. There was a significant negative correlation between the number of aphids and yield per plant at 1 site. In both years, aphid numbers peaked in August or September. Rhopalosiphum padi was virtually the only aphid species found. Leaf samples were collected from 3 sites in 1990 and from all sites in 1991 and tested for barley yellow dwarf virus. The incidence of this virus was very high in 1990 but negligible in 1991. The striking difference in the incidence of barley yellow dwarf virus in the 2 years was most likely the result of later sowing times and delayed arrival of aphids in crops in 1991. Aphids may have a considerable impact on yield in early-sown wheat through transmission of barley yellow dwarf virus.