Physical and Biological Perturbations: Their Effect on the Movement of Apterous Rhopalosiphum padi (Homoptera: Aphididae) and Localized Spread of Barley Yellow Dwarf Virus
Apterous adult and nymphal Rhopalosiphum padi (L.), previously reared on plants infected with barley yellow dwarf virus (BYDV), were subjected to eight perturbations (wind, rain, herbicide, coccinellid predators, crowding, mechanical disturbance, drought, and virus-infected plants) to determine effects on aphid dispersal and pattern of local spread of BYDV to oats in greenhouse experiments. Viruliferous aphids, caged overnight on oat plants in a 25-plant area (infection focus) in the center of each 625-plant plot, were subjected to a given perturbation after cages were removed and then allowed to move freely within the plot for 2 d. Presence of virus-infected plants outside the infection focus provided indirect evidence of aphid movement and direct evidence of virus spread. All types of disturbances except rain caused movement of apterous aphids and spread of BYDV. For all except the rain perturbation, controls were pooled and compared by paired t-tests with means of three indices of infection pattern (incidence, total distance, and average distance) for each experimental treatment. Incidence of infection (number of infected plants located outside the infection focus) in treated plots was significantly greater than pooled controls for the crowding, predator, wind, and herbicide perturbations. Total distance of infection (the sum of distances from the central plant in the infection focus to each infected plant outside the focus) was significantly greater in mechanical disturbance, crowding, predator, drought, wind, and herbicide perturbations than in controls. Average distance of infection (total distance divided by incidence) in treated plots was significantly greater than in the pooled controls for the drought, wind, and virusinfected oat treatments.