Host plant preference of Rhopalosiphum padi (Hemiptera: Aphididae) and its role in selecting crop border plants to reduce Potato virus Y (PVY) in seed potatoes

Winged Rhopalosiphum padi in search of good quality host plants transmit the non-persistent Potato virus Y (PVY) to seed potatoes. Planting a non-virus host plant as a border around the main crop can reduce PVY incidence, because aphids tend to land in high numbers at the edge of a field and the crop border acts as a virus sink. Using a trap crop could increase the number of aphids landing in the border. During host-plant searching behaviour, alate (winged) aphids respond to short (UV) and long (green - yellow) wavelength stimuli and plant volatiles. The present study evaluated R. padi preference for maize and wheat cultivars in comparison to potato cultivars to identify a potential crop border plant that is attractive to aphids for landing but does not support a high aphid population density. The study further served to develop selection criteria for potential crop border plants. To this end the landing and settling preference and reproductive rate of R. padi on three cultivars each of maize, potato and wheat were compared in choice and no-choice laboratory trials. The attractiveness of different shades of green, relative to differences in spectral reflectance of the crop cultivars, to R. padi was determined as well as olfactory responses of this species, to the plant cultivars tested. Landing and reproduction of R. padi suggested maize cultivars ‘6Q-121’ and ’78-15B’ are potential crop border plants. In choice and no-choice experiments, R. padi produced the highest number of offspring on wheat than on maize. In contrast, wheat cultivars had the greatest potential to be used as crop border plants based on percentage wavelength reflection in the green-yellow wavelength region. Rhopalosiphum padi preferred to land on yellow and lime colour targets with a maximum wavelength reflectance of 46% and 26%, respectively. The peak light reflectance of the crop plants ranged between 12% and 20%, with wheat reflecting the highest percentage of light. However, olfactory responses of R. padi indicated that maize ‘6Q-121’ may prove to be a suitable crop border plant. The volatiles emitted by maize ‘6Q-121’ did not contain compounds such as -farnasene, (E)-2- hexenal, indole and TMTT which are known to repel R. padi. On the other hand, R. padi did not distinguish between maize and wheat cultivars prior to landing, which indicates that both visual and olfactory cues are important in host plant selection behaviour and thus for selecting crop border plants. The results of the present study suggest that maize ‘6Q-121’ could be used as a crop border plant in seed potato production regions where R. padi is abundant, due to high aphid landing and low reproduction rates. In conclusion, a suitable crop border plant should be a preferred host plant and cultivar of the dominant aphid vector species in a seed potato production region. Potential plant cultivars should reflect a higher percentage of light in the green-yellow wavelength region than the main crop and the volatile profiles should preferably not contain compounds that are known to repel the aphid vector.