Effects of applying insecticides to control aphid vectors and cucumber mosaic virus in narrow-leafed lupins Lupinus angustifolius)

Abstract

Summary. Narrow-leafed lupins (Lupinus angustifolius) were sown in 4 field experiments to investigate the effects of applying organophosphorus and carbamate insecticides on the spread of cucumber mosaic virus (CMV), which is transmitted non-persistently by aphids. The seed sown carried 0 or 5% CMV infection and seed-infected plants were the primary source for subsequent virus transmission by aphids. Virus incidence was gauged by calculating the percentage of plants per plot showing typical current-season CMV symptoms. At final assessment in plots sown with 5% infected seed, recorded CMV incidence was always significantly smaller (up to 3-fold less) when they received sprays of pirimicarb and/or methamidophos every 2 weeks than when they received no insecticide. When single or double strategic sprays of methamidophos or a mixture of pirimicarb/thiometon were applied in late August or early September, they either decreased recorded CMV incidence (up to 2-fold less infection) or had no significant effect on it. Recorded CMV incidence was not significantly diminished by dressing seed with disulfoton or by single- or double-foliar applications of pirimicarb used alone. However, pirimicarb sprays applied every 2 weeks improved grain yield (24% increase), as also did single or double applications of pirimicarb (14%). In one experiment, applications of methamidophos every 2 weeks to plots originally sown with 5% infected seed resulted in increased grain yield (35%) and yields were increased in plots sown with healthy seed whether sprayed every 2 weeks with methamidophos or left unsprayed (42–61%). Pirimicarb suppressed numbers of colonising Acyrthosiphon kondoi and Aphis craccivora to a greater extent than those of Myzus persicae, especially when sprayed every 2 weeks. Single-, double- and sprays of methamidophos every 2 weeks also decreased numbers of Acyrthosiphon kondoi, but had less effect on M. persicae. When different insecticides were tested for their abilities to kill colonising, insecticide-resistant M. persicae, methamidophos was the most effective. This work suggests that single or double strategic foliar sprays of organophosphorus or carbamate insecticides cannot be relied on to control CMV adequately to be recommended for use in increasing grain yields in CMV-infected lupin grain crops.