Further studies on the effects of insecticides on aphid vector numbers and spread of cucumber mosaic virus in narrow-leafed lupins ( Lupinus angustifolius)

Abstract

Narrow-leafed lupin ( Lupinus angustifolius) was sown in five field experiments over three years to investigate the effects of applying imidacloprid, alpha-cypermethrin, triazamate and methamidophos insecticides on aphid vector numbers, spread of cucumber mosaic virus (CMV), resulting yield losses and CMV transmission to seed. Of the colonising aphid species, Acyrthosiphon kondoi and Aphis craccivora were most effectively controlled by alpha-cypermethrin and Myzus persicae by imidacloprid. Methamidophos and triazamate had similar effects on colonising aphid numbers to those of alpha-cypermethrin. In one experiment, >82% of the CMV vector M. persicae were insecticide resistant (R1 or R2 E4 esterase resistant types). In two experiments, numbers of colonising aphids were greater in plots in which CMV was spreading from seed-infected plants than in plots where healthy seed had been sown. Visual assessment of plants with CMV symptoms under-estimated actual incidence measured by ELISA tests more than three-fold. Alpha-cypermethrin applied at 25 g a.i./ha decreased CMV incidence by up to 62%, but imidacloprid, methamidophos and triazamate did not significantly diminish it. Increasing incidences of current-season CMV-infected plants were positively correlated with the magnitude of yield losses and extent of CMV transmission through harvested seed. Direct aphid feeding damage did not influence grain yield. However, application of alpha-cypermethrin to control the aphid vectors of CMV in plots sown with infected seed increased yield by up to 60%, but did not give consistent enough control to be recommended for general use. Sowing healthy seed and using recommended integrated cultural management practices that minimise CMV spread should remain the mainstays of successful CMV management in lupins.