Lower incidence of yellow dwarf disease in autumn-sown wheat crops in New Zealand is linked with sowing dates, insecticide regimes and aerial aphid numbers

Symptoms of yellow dwarf disease caused by Luteoviridae viruses (YDV) in each of 43–75 autumn-sown wheat crops per year were monitored for six years during 1998–2005 in Canterbury, New Zealand. They were related to seed and foliar insecticide applications and the numbers of YDV aphid vectors from one to four regional 7.5 m high suction traps during the crop emergence–GS 30 virus infection-risk period. Over the five years when YDV was assessed in each crop on visual symptoms at flag leaf, the YDV incidence in untreated crops tended to increase with increasing numbers of flying aphids. When aphid numbers were below a relatively low threshold during the risk period for each crop, YDV incidence was low irrespective of insecticide treatments. Above this aphid threshold, the incidence of YDV was lowest in crops with an insecticide programme of an imidacloprid seed treatment followed by one foliar application of lambda-cyhalothrin at 17–24 g ai ha −1 . This regime was estimated to provide protection from aphid-transmitted virus infection for approximately 50 % of the risk period irrespective of sowing date. Wheat crops treated with an imidacloprid-seed treatment plus one insecticide foliar application had 96 % less virus incidence than untreated crops, and one foliar application alone had 70 % less incidence. These results suggest that a tool incorporating aphid flight patterns and wheat growth stages could be developed to optimise insecticide applications for control of secondary aphid incursions in autumn sown wheat crops in New Zealand.