The effect of aphid lethal paralysis virus (ALPV) on the biology of grain aphids in the Western Cape

Doctoral Thesis


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University of Cape Town

Aphid lethal paralysis virus (ALPV) could be detected by indirect immunofluorescent technique in dissected aphids. This technique was found to be more sensitive when compared to DAS-ELISA. The choice of a sensitive, low cost .detection method was of importance to test for low levels of virus in infected aphid body tissues where inapparent infection could cause detection problems. ALPV was visualized in ultrathin sections of diseased aphid body tissues by immunocytochemistry utilizing immunogold label. ALPV antigen was detected in the ovariole tissue, tracheocytes, symbionts of the mycetocytes, fat body cells, brain tissue, nerve tissue and stomach epithelial tissue. Virions were detected predominantly in the cytoplasm but were also found in the nucleus. ALPV antigen was not detected in muscle fibres or mitochondria. ALPV and Rhopalosiphum padi virus (RhPV) are transmitted transovarially. Different incidences of transmissions of ALPV were obtained for R. padi (2996) and Sitobion avenae (1696) and ALPV infections dramatically reduced the longevity and fecundity of these aphids. Infected apterous R. padi aphids were more fecund than alate aphids of the same clone. The percentage of viral infections in different aphid species (R. padi, S. avenae and Diuraphis noxia) was positively associated with temperature; higher temperatures dramatically increased the incidence of ALPV and RhPV and vice versa. The influence of ALPV on a natural R. padi aphid population was found to reduce the population size by 4996. This reduction coincided with a high death factor (70) of aphids per plant. A dramatic decline in R. padi aphid numbers and a high incidence of ALPV present in this aphid population was experienced. Parasitic fungal infections peaked at a later stage than ALPV, and a level of 21 parasitized aphids per plant was reached during this period. This appears to indicate that the presence of ALPV contributes to limit population development in R. padi aphids. Similar results were obtained with S. avenae aphids. Based on this data, ALPV could be considered as a major growth limiting factor in the development of small grain aphid populations in the western Cape. If the presence of the virus is taken into consideration, it could influence pest management strategies directly.