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 subject ed to eight perturbations (wind, rain, herbicide, coccinellid predator s, crowding, mechanical disturbance, drought, and virus-infected plant s) 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 c enter of each 625-plant plot, were subjected to a given perturbation a fter cages were removed and then allowed to move freely within the plo t for 2 d. Presence of virus-infected plants outside the infection foc us provided indirect evidence of aphid movement and direct evidence of virus spread. All types of disturbances except rain caused movement o f apterous aphids and spread of BYDV. For all except the rain perturba tion, controls were pooled and compared by paired t-tests with means o f three indices of infection pattern (incidence, total distance, and a verage distance) for each experimental treatment. Incidence of infecti on (number of infected plants located outside the infection focus) in treated plots was significantly greater than pooled controls for the c rowding, predator, wind, and herbicide perturbations. Total distance o f infection (the sum of distances from the central plant in the infect ion 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 infe ction (total distance divided by incidence) in treated plots was signi ficantly greater than in the pooled controls for the drought, wind, an d virus-infected oat treatments.