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Lepidopteran stemborers are major limiting factors in the production of maize and sorghum in Africa and Asia. Host-plant resistance and biological control are important components of an IPM programme and can effectively maintain pest populations below economic threshold levels, especially in low-input subsistence farming systems. When damaged by herbivores, plants produce allelochemicals which can attract natural enemies of the pest. These volatiles occur systematically throughout the plant and are different from those emitted in response to mechanical damage. We report evidence of plant volatiles-mediated differential reaction of parasitoid activity to sorghum genotypes. These volatiles could have a positive impact on the efficacy of stemborer parasitoids, leading to increased host finding or increased searching efficiency, thereby enhancing the suppression of stemborer populations. In a single-season case study, parasitoid activity was higher in stemborer-resistant genotypes than in susceptible ones and varied with crop age. Knowledge of such interactions should be explored and intensified by plant breeders and chemical ecologiste aiming at producing plant materials possessing appreciable levels of resistance to stemborers and able to encourage natural enemy activity in cereal-based ecosystems.
Regular field surveys were carried out in 2 week intervals in 1986 and 1987 to determine population density, species composition, migration activity of Cicadulina leafhoppers and their infectivity with maize streak virus (MSV) in different ecological locations of southeastern Nigeria: mangrove swamp forest (IITA and RIART station at Onne), deltaic swamp forest (RSUST, Port Harcourt), seasonal swamp forest (Iriebe) and high forest zone (NCRI, Amakama near Umuahia).
Population density was always low during dry season, significantly lower in the rain forest zone and higher in the swamp forest, rapidly rising in the second rainy season in all ecological zones and reaching its maximum before the rains finally stopped. Sharp increase in the population density in Nov. and Dec. may indicate mass migration activity of Cicadulina leafhoppers from already dry areas of adjacent Guinea savanna at that period.
Six species of Cicadulina were collected from most of sampled areas. C. ghaurii was the predominant species in 1986 and 1987, averaging 55.9% for all locations and periods. Its portion was higher in the mangrove swamp forest (Onne) than in the high forest zone (Amakama). The predominance of other species declined in the order: C. triangula (18.2%), C. mbila (17.2%), C. arachidis (5.9%), C. similis (2.8%) and C. hartmansi (0.1%). In absolute values, the portion of individuals out of the total number of leafhoppers collected was for C. triangula 53.7%, C. ghaurii 25.5%, C. mbila 17.3%, C. arachidis 2.3%, C. similis 1.1% and C. hartmansi0.1%. A high population of C. triangula occurred only in the relatively short period in Nov.-Dec, especially in Iriebe and was correlated with the rapid increase of Cicadulina densities in all sampled areas; a high migration activity of C. triangula and C. mbila was confirmed.
Incidence of MSV in the surveyed farmers' field varied between 0–22% in the first rainy season (1987) and 12–25% in the second rainy season (1986). The portion of Cicadulina adults already infected with MSV under field conditions and capable of transmitting MSV to the susceptible maize seedlings varied from 3.3 to 23.3%, with higher values for leafhoppers collected at the end of the second rainy season.
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