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The feeding activities of C. mbila produce negligible direct damage, and the resulting salivary sheaths are visible only under microscopic examination (Mesfin et al., 1995). The presence of the insect, as with other Cicadulina species, is revealed by the symptoms of the disease it transmits. Species of Cicadulina transmit a related group of geminiviruses infecting maize, sugarcane and Panicum spp. (Bock et al., 1974; Pinner et al., 1988; Peterschmitt et al., 1991). Of these, only Maize streak virus (MSV) is of importance (Rose, 1978; Efron et al., 1989), and species of Cicadulina are the only known vectors. C. mbila acquires MSV particles by feeding from leaf mesophyll tissues of diseased plants and, following a period of at least 12 h, can infect healthy plants by feeding in phloem tissues for a minimum of 5 min (Storey, 1925, 1928).
Maize is susceptible to MSV disease from emergence to tasseling (flowering), but symptoms are quicker to appear in younger plants (3 to 5 days in 1-week-old plants, 7 to 19 days in 9-week-old plants; Gibson and Page, 1997). The first symptoms are the appearance of pale spots 0.5 to 2.0 mm in diameter on the youngest leaves of the plant. As the plant grows these coalesce into almost continuous, longitudinal chlorotic streaks. The streaks result from the failure of chloroplasts to develop in tissues surrounding vascular bundles. The secondary and tertiary veins are usually most affected, so that leaves typically develop several parallel streaks (Rose, 1978; Efron et al., 1989). In severely affected plants, chlorotic stripes may merge and the plants appear pale green, yellow or white from a distance, and are stunted and produce small cobs with open husks. If infected at an early stage, susceptible varieties may die (Efron et al., 1989). In species other than maize, infections with Cicadulina-transmitted geminiviruses are generally comparatively mild, with streaking reduced or absent (Efron et al., 1989; Mesfin et al., 1992; Briddon et al., 1996).
The number of infective leafhoppers is positively associated with the speed and severity of MSV disease symptom expression, and low temperatures may delay symptom appearance (Damsteegt, 1984; Van Rensburg et al., 1991a; Gibson and Page, 1997). Because leaves produced before infection are symptomless (Storey, 1925), and virus activity occurs only in tissues produced after infection (Peterschmitt et al., 1992), the stage at which a plant was infected can be estimated from the position of the lowest infected leaf (Gibson and Page, 1997).
The economic impact of C. mbila results largely from its ability to transmit Maize streak virus (MSV) in maize.This is probably the most important insect-vectored plant disease in sub-Saharan Africa (Geddes, 1990; Shepherd et al., 2010). There are currently 11 known MSV strains (MSV-A to MSV-K), of which MSV-A is the most severe and economically important strain that attacks maize (Martin and Shepherd, 2009; Karavina, 2014). If infection occurs at an early stage, plants may die and 100% yield loss results (Thottappilly, 1992; Vogel et al., 1993), but the severity of infection declines with the age of the plant, and infections later than 8 weeks after emergence have little or no effect on yield (Guthrie, 1978; Van Rensburg, 1981; Mzira, 1984). MSV epidemiology is primarily governed by environmental influences on its Cicadulina vectors (mainly C. mbila and Cicadulina storeyi), resulting in erratic epidemics every 3 to 10 years. Disease incidences can vary even in epidemic years, ranging from a few infected plants per field with very low yield loss, to 100% infection rates and complete yield loss (Shepherd et al., 2010). Studies in northern Nigeria over a 3-year period (2000-2002) showed that the number of leafhoppers (Cicadulina spp.) caught was positively correlated with MSV incidence and mean temperature, and negatively correlated with mean rainfall and with age of plants. MSV incidence was positively correlated with age of plants and with mean temperature and negatively correlated with mean relative humidity (Alegbejo and Banwo, 2005).As a result of its wide geographic distribution and high efficiency of transmission of MSV, C. mbila must be regarded as a major pest of maize.
MSV was first observed on sugarcane in KwaZulu-Natal, South Africa, in 2007 on variety N44 (McFarlane et al., 2008). Phylogenetic analysis determined that the virus closely resembled the South African maize-adapted MSV strain, MSV-A, subtype MSV-A4. C. mbila was one among 13 species of leafhoppers collected from sugarcane during the first season of monitoring (van Antwerpen et al., 2014).