Dispersal and Quality in Populations of Cicadulina Species (Cicadellidae)

Abstract

Cicadulina species are found in grasses in many parts of the tropical and warm temperate parts of the world. Ruppel (1965) recognizes thirteen different species and most of these are of economic importance, either confirmed or suspected of being vectors of plant diseases. Five species have been shown to be vectors of miaize streak, a disease which is a cause of crop losses in eastern and southern Africa (Nielson 1968). Three of these, C. mbila (Naude), C. parazeae Ghauri, and C. storeyi China are the principal vectors of the disease in Rhodesia (Rose 1963). Field observations have indicated that Cicadulina leafhoppers fly two very different kinds of distances. Many fly only a few metres and there is a sharp fall in numbers captured on trap plants with distance from the edge of a breeding source. During the main flight season, however, Cicadulina are captured in suction traps 20 m above the ground and there is widespread invasion of cereal crops by leafhoppers dispersing from low density populations in drying grasses. These two levels of dispersal are also indicated by the differences in the patterns of streak disease which are found during the main flight period in the dry season and on irrigation areas in the summer (Rose 1971). Although these patterns are partly explained by increases in numbers of Cicadulina emerging from grasses at the end of the summer, they may also be due to changes in quality of populations within the grasses. Changes in qualities such as size, wing length, colour and activity are known to be caused in populations of many different insects as responses to effects produced by the environment (Uvarov 1921; Faure 1932, 1943). The mass of information that has been collected on morphological and behavioural polymorphism is thoroughly discussed by Johnson (1969) who stresses its significance as it affects the dispersal of insects. He concludes that it 'seems that the proportion of migrants in a population may be determined not only by ontogenetic effects produced by adverse changes or associated token stimuli in the environment but also by selection from particularly vigorous females from one generation to another'. A laboratory method was required in order to find out more about the conditions that affect flight and flight duration in the three Cicadulina species. Flying Cicadulina tethered to pins proved to be satisfactory.