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Ecological aspects of cereal chlorotic mottle virus

The ecology of CCMV has been studied during the past 2 years. Nesoclutha pallida (Evans), which is a vector of CCMV has a wide distribution in Australia (3). In Queensland, this species can readily be found in grassland at most times of the year and numbers increase through the spring and summer months. However, populations are never very large and N. pallida comprised only about 1% of leafhoppers collected in maize crops in mid and late summer. Delphacid leafhoppers comprised about 5%, but most leafhoppers ( 90%) were Cicadulina bimaculate (Evans) which was found breeding on grass spp. such as Echinochloa colona (L.) Link and Chloris gayana Kunth. C. bimaculata also bred on maize, especially during the period when its population was largest. The spread of CCMV is susceptible maize hybrids and lines during the 1980-81 season seemed to be too rapid to be reconciled with the observed numbers of N. pallida and consequently C. bimaculata was tested as a possible vector. Because the incubation period in the insect averages 17 days (1) and C. bimaculata has a comparatively short life-span, very young nymphs from colonies were used for acquisition feeding on infected maize and Dinebra retroflexa (Vahl.) Panz.. In single insect tests, 25% of adults from these nymphs transmitted CCMV to D. retroflexa and maize seedlings. Positive transmissions were checked by electron microscopy and serology of sap from the infected plants (2) and the virus particles were readily found in thin sections of brain tissue of individual insects which had transmitted CCMV. N. pallida is an efficient vector of CCMV (1) and in this work was found to be present in grass and infected cereal plantings in late winter and early spring when its numbers were still like those of C. bimaculata. At this time of the year N. pallida is probably the major vector of CCMV. It has a much longer life-span than C. bimaculata and hence has a far longer period for transmission after completion of the incubation period of the virus. N. pallida was found on young maize in early summer, when a few plants became infected with CCMV. Incidence increased to 60% in commercial hybrids and 95% in susceptible breeding lines in late summer of 1979-80. These high levels are probably due to secondary spread by C. bimaculata which breeds prolifically on maize during the mid and late summer. Nymphs of C. bimaculata, taken directly from infected field maize, transmitted the virus as adults to D. retroflexa and maize in the glasshouse. Populations of this leafhopper were very labile. During a 4-week period which corresponded with the development of an epidemic of both CCMV and wallaby ear disease, the numbers of C. bimaculata increased from 15 to 50 and then decreased to 5 per maize plant.