Ecology of maize stemborers in irrigated subsistence farming systems in the Limpopo Province
Stemborer ecology in South Africa has been studied well but largely in maize in commercial monoculture production systems. Stemborers are important pests of maize in resource-poor farming systems at the Tshiombo irrigation scheme in the Limpopo Province, especially since crops are available throughout the year. Both irrigation and the subtropical climate make crop production possible throughout the year. Before this study no information existed on Sesamia calamistis (Lepidoptera: Noctuidae) moth flight patterns in South Africa and limited information on flight patterns of Busseola fusca (Lepidoptera: Noctuidae) in small-farming areas was available. In this study the moth flight patterns of B. fusca and S. calamistis were determined on small-scale, irrigated farms in Venda using pheromone traps. The B. fusca flight pattern showed two distinct peaks, the first during October (2005) and the second during December/January (2004/05). Periods of no moth flight activity occurred during December (2004). During December and January increased numbers were observed showing two peaks for S. calamistis. Sesamia calamistis moths were however also active between October and January with no activity being recorded during February. During the winter months of June and July high numbers of S. calamistis moths were captured. Also prior to this study no information existed on the relative abundance and natural enemies of stemborers in maize production systems such as that at Tshiombo. The incidence of damaged plants was determined on fields at monthly intervals between June 2005 and March 2006. Species distribution and population dynamics of stemborers were determined by dissecting plants at monthly intervals. Data showed that the incidence of stemborer damaged plants was highest during the months of July (2005) to February (2006) during the pre-flowering period and from June to November during the post-flowering period. The incidence of damaged plants ranged between 7 and 30 %. The stemborers that occurred were B. fusca, Chilo partellus (Lepidoptera: Pyralidae) and S. calamistis. Chilo partellus, B. fusca and S. calamistis made up 85, 5 and 7 % of the total population of stemborers, respectively. Percentage parasitism of stemborer larvae by Cotesia sesamiae (Hymenoptera: Braconidae) ranged between 0 and 34 % during the 15-month sampling period. This is low compared to observations in another study on B. fusca in maize in commercial farming systems where Co. sesamiae caused mortality of 90 % in diapause larvae of B. fusca. A study was done to determine if Napier grass would be effective as a trap crop for stemborers when planted as contour strips along two sides of maize fields. The incidence of damaged plants and stemborer species composition on fields with Napier grass as trap crop was compared to fields without the trap crop. Fields during the whorl stage had lower incidences of damaged plants in trap crop fields compared to control fields, but only six of these were significantly lower (P < 0.05). The lower incidence of infestation in blocks with Napier grass as trap crop showed that this method of pest control could be effective under certain conditions. During the pre-flowering period C. partellus was the dominant species with proportions of between 67 and 100 % of the population in the trap fields and 88 to 100 % in the control fields. High proportions of C. partellus were always present in control fields with statistically significant differences between trap and control fields. Plants at different growth stages were always present and made it difficult to measure infestations compared to mono-culture systems. Data showed a strong association between moth flight peaks and high larval infestations, which indicate that timing of pest management activities e.g. insecticide application could be based on the moth flight pattern. The potential for biological control of stemborers is huge and Cotesia flavipes (Hymenoptera: Braconidae) may be recommended for release. An integrated pest management (IPM) strategy will however only be viable if adequate advisory services exist to monitor moth flights and to assist farmers in dealing with stemborers on a sustainable level.