Aphid movement and aphid-borne virus spread in mixed cropping systems of legumes and cereals

Growing legumes in combination with non-hosts of legume viruses is a cultural practice that has been observed to suppress aphid-borne virus epidemics in indigenous farming systems. This study was aimed at unraveling the physical and biological relationships that affect aphid dispersal and the consequent spread of aphid-borne legume viruses in mixtures of snapbean and maize or soybean and sorghum.;Maize rows in maize-snapbean mixtures acted as barriers that reduced wind speed at bean canopy level, leading Uroleucon ambrosiae (Thomas) alatae, collected from compositae and placed manually on bean plants, to take off more readily than alatae on plants in bean monocultures. These findings were further substantiated by simulation modeling of SMV epidemics which showed that movement rates of aphids are higher in mixed crops than in legume monocultures. However, even though plant-to-plant movement rates may be higher in crop mixtures, disease spread was lower than in monocultures because fewer aphids landed in these mixed crops.;In mixed crops, dwarf sorghum provided statistically similar protection from legume virus epidemics than tall sorghum barriers. Therefore, to minimize yield loss by shading and at the same time protect the crop from aphids and aphid-borne virus spread, legumes might best be intercropped with crops (that are not hosts of the virus) of similar height.;Fewer aphids landed in crop mixtures in response to the greater canopy cover (percentage ground covered with vegetation) in these habitats. Height and porosity of sorghum rows did not affect aphid landing rates on soybean significantly, but fewer alatae passed through porous maize rows than through less porous ones. Aphidicide applications in sorghum whorls significantly reduced apterae and alatae densities of sorghum-infesting Rhopalosiphum maidis (Fitch) but did not affect landing rates of alatae on sorghum or soybean, indicating that the majority of the R. maidis alatae caught in pan traps immigrated from outside the plots. Likewise, colony suppression did not affect rate of soybean mosaic virus spread, SMV incidence, or percentage SMV-induced seed mottling in soybean-sorghum bicultures, confirming that transient alatae were mainly responsible for the spread of SMV.