Assessment of Infestation levels of Chilo partellus swinhoe(Lepidoptera:Crambidae)on maize and the Impact of its parasitoid cotesia flavipes cameron(Hymenoptera:braconidae)in Sinazongwe District of Zambia
Moonga, Nzala Miyanda
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The spotted stem borer, Chilo partellus (Swinhoe) is one of the most destructive pests of maize in the warm, low altitude regions of Zambia. A study was carried out in Sinazongwe district, which is located at about 600 metres above sea level in the Southern Province to determine the incidence of C. partellus, damage to maize plants and the abundance of its natural enemy, Cotesia flavipes. This study was carried out during the winter cropping seasons of 2005 and 2006 in four locations where previous releases of C. flavipes had been made. The locations described by intensity of C. flavipes released were: locality 1; 500-1000 parasitoids, locality 2; 50,000 parasitoids, locality 3; 100,000 parasitoids and locality 4; non-release. These were classified as low, medium, high and non-release locations respectively. In each location, maize plants were sampled from sixteen farmers' fields at three different growth stages, namely; knee height, tasseling and cob maturity. Ten plants were also randomly collected from each farmer's fields by destructive sampling to assess severity of C. partellus damage. Analysis of variance (ANOVA) was carried out to determine if there were any significant differences across the locations and between plant growth stages. The results showed significant differences (F=7.74; d.f.=3; PO.001) in incidence of C. partellus across the locations. Similar significant differences were also obtained among the three plant growth stages (F=63.59; d.f.=2; PO.001). In addition, significant differences were obtained in respect to the following damage variables; tunnel length (F=12.04; d.f.-3; PO.001), damage index (F=16.39; d.f.=3; PO.001) and number of exit holes (F=100.83; d.f.=3; P<0.001) across the locations. Similarly, significant differences were also obtained at the three plant growth stages in tunnel length (F=115.54; d.f=2; PO.001), damage index (F=12.04; d.f.=2; P<0.001) and number of exit holes (F=7.96; d.f.=2; P<0.001). There were however, no significant differences in leaf damage when a t- test was carried out to compare damage at knee height and tasseling stages across all the locations. Similarly, there were no significant differences in the number of parasitized larvae across the locations and at the three maize growth stages. C. partellus constituted 94.5% and 95.6% of the total stem borer larvae collected from maize plants, and the remaining 5.9% and 4.4% were Sesamia calamistis in the 2005 and 2006 winter cropping seasons, respectively. Results of the present study confirm that C. partellus is the dominant cereal stem borer in Sinazongwe district and clearly show that C. flavipes is fairly well established in this agroecosystem. The results indicate however, that at this level of establishment, C. flavipes did not adequately suppress stem borer populations possibly due to low parasitism levels at peak stem borer incidence. Follow up studies might be necessary to clarify this phenomenon.